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Sessile volatile drop evaporation under microgravity

Sessile volatile drop evaporation under microgravity www.nature.com/npjmgrav ARTICLE OPEN 1✉ 1 2 1,3✉ Sanjeev Kumar , Marc Medale , Paolo Di Marco and David Brutin The evaporation of sessile drops of various volatile and non-volatile liquids, and their internal flow patterns with or without instabilities have been the subject of many investigations. The current experiment is a preparatory one for a space experiment planned to be installed in the European Drawer Rack 2 (EDR-2) of the International Space Station (ISS), to investigate drop evaporation in weightlessness. In this work, we concentrate on preliminary experimental results for the evaporation of hydrofluoroether (HFE-7100) sessile drops in a sounding rocket that has been performed in the frame of the MASER-14 Sounding Rocket Campaign, providing the science team with the opportunity to test the module and perform the experiment in microgravity for six consecutive minutes. The focus is on the evaporation rate, experimentally observed thermo-capillary instabilities, and the de- pinning process. The experimental results provide evidence for the relationship between thermo-capillary instabilities and the measured critical height of the sessile drop interface. There is also evidence of the effects of microgravity and Earth conditions on the sessile drop evaporation rate, and the shape of the sessile drop interface and its influence on the de-pinning process. npj Microgravity (2020) 6:37 ; https://doi.org/10.1038/s41526-020-00128-2 INTRODUCTION preparation for an experiment that is to be performed in the near 1–3 future at the European Drawer Rack 2 of the International Space Drops have been fascinating researchers for centuries . Topics of Station under the EVAPORATION project of the ESA. The intent is interest include water falling onto a hot cooking plate, which is a typical example of Leidenfrost drops , the evaporation of sessile to study evaporating drops of pure fluids as well as drops of fluids drops with nanoparticle deposition in coffee rings , inkjet that contain a low concentration of metallic nanoparticles. The 5,6 7 8,9 printing , pesticides sprayed onto leaves , and blood analysis . influence of an electric field is also of interest. The application of Although sessile drops are simple in geometry, the physics an external electrostatic field induces electric stress at the involved in the evaporation process is complex due to the vapor–liquid interface, deforming it and altering the contact numerous intricate interactions with the substrate and ambient angle. The resulting electric forces press the drop against the environment, and the fluid nature of the sessile drop itself. An surface and elongate it in the vertical direction; in addition, accurate quantitative model of the evaporation process can lead electroconvection is induced in the liquid and in the surrounding to greater understanding of the evaporation rate and control over vapor atmosphere, resulting in a possible enhancement of the pattern formation or the deposition of particles after the evaporation rate, which may result useful when gravity-driven evaporation of a sessile drop. This knowledge can then enhance convection is suppressed. The scientific objectives include dealing the efficiency of several applications. The physically rich and with the flow motion and the thermo-capillary instabilities complex evaporation of sessile drops is thus of interest to both the occurring in the drop, at the drop interface, and in the vapor academic and industry communities. phase, and investigating the pattern formation on the substrate Parabolic flight experiments on drops of various fluids have after the evaporation phase. been performed multiple times by The National Centre for Space ARLES was a collaborative experiment among various teams. Studies (CNES), France, and The European Space Agency (ESA) Each team focused on different aspects of the experiment to 10–15 parabolic flight campaigns . The existence of thermo-capillary contribute to the overall scientific objectives of the experiment, 14,16 instabilities and the effect of the reduced gravity environment such as flow motion and thermo-capillary instabilities occurring in 11,17 10,18,19 on evaporation and the drop interface have already the drop, at the drop interface, and in the vapor phase, the pattern been demonstrated. Parabolic flights have enabled these observa- formation on the substrate after evaporation of the volatile phase, tions, but such flights are not sufficient in terms of duration or the deposition of nanoparticles, and the eventual heat transfer residual acceleration for accurate measurements to be taken. enhancement. Our team primarily focused on the analysis of the Furthermore, the drop interface is highly sensitive to aircraft flow motion and thermo-capillary instabilities occurring in the vibrations. A better level of microgravity and a longer evaporation drop using data from the infrared (IR) (top view) camera and on time are therefore needed. the evaporation rate and interface evolution of the sessile drop The Advanced Research on Liquid Evaporation in Space (ARLES) using data from the side-view camera. The experimental results experiment module (see Figs. 1 and 2) was designed to support presented here address the effect of microgravity and Earth the investigation of the evaporation process in a controlled conditions on the evaporation, thermo-capillary instabilities, drop environment. ARLES was part of the payload in a SubOrbital interface, and de-pinning of a forced sessile drop of hydrofluor- Express rocket (MASER 14) and it successfully took place on oether (HFE-7100) liquid on a heated substrate. The experimental Monday, 24 June 2019 from the Esrange Space Center in northern Sweden under the collaboration of the ESA and Swedish Space results allow for a comparison of data from both ground and Corporation (SSC). The ARLES experiment was conducted as a space experiments, thereby providing firm conclusions. 1 2 3 Aix-Marseille Universite, CNRS, IUSTI UMR 7343, Marseille 13013, France. DESTEC, University of Pisa, Largo Lazzarino 1, Pisa 56122, Italy. Institut Universitaire de France, Paris 75231, France. email: sanjeev.kumar@univ-amu.fr; david.brutin@univ-amu.fr Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA 1234567890():,; S. Kumar et al. Fig. 1 Overview of the ARLES experimental setup. a Experiment module on-board the MASER 14 rocket, divided into two parts: the main evaporating and multi-evaporating cells. b, c Main evaporating cell (MEC) with a detailed schematic. d Platinum layered surface crystal silicon wafer substrate (top view) with grooves. Images a, b, and c are credited to the European Space Agency (ESA) and Swedish Space Corporation (SSC). Fig. 2 Schematic of the main evaporating cell (MEC) of the ARLES experiment (left). Axisymmetric electric field around sessile drop (right). Readers are advised to refer to web version of this figure for better display. RESULTS injection liquid volume with the current injection system and hardware. Even though the actual injected volume of the drops Experimental setup and conditions during the ground experiment is lower than the target theoretical In Fig. 1a, a complete setup of the ALRES experiment has been nominal value but the actual injected volume of the drops during shown. It consists of two parts, namely the main evaporating cell the microgravity conditions is higher than the target nominal one (MEC; bottom) and multi-evaporating cells (top). Our current focus (see Fig. 3). The temperature of the main test cell was set at 26 °C is on the MEC experiment. The detailed schematic of the MEC is and the temperature of the substrate was set at 28 °C with an presented in the Fig. 2 (left) along with its chamber shown in imposed electric field 8 kV for all drops with electric field, except Fig. 1b (top view) and 1c (cut view) with injection system, for drop 8DPμgEF under microgravity, for which the field was set substrate, and electric field electrode (substrate is connected to at 5.7 kV. Due to the grooves on the substrate, the base diameter the negative (−) terminal and the electrode to the positive (+) of all the sessile drops remained constant (4 mm) during terminal). Figure 2 (right) shows the electric field distribution evaporation until the drops de-pinned. around the sessile drop for the axisymmetric case. For more details, please refer to MEC schematic in Fig. 2. Experimental results The ideal experimental conditions for the MEC are as follows: target theoretical nominal parameters for microgravity and Earth For Earth gravity, the experimental data from the sensor are as conditions were set to be similar for the purposes of comparison. follows: the main cell pressure (inside chamber) P , ambient amb The injection velocity of liquid HFE-7100 for sessile drop creation temperature (inside chamber) T , and substrate center tem- amb −1 on the heated substrate was 4 μLs and the nominal volume of perature T , and the difference between the substrate center sc each sessile drop was set at 6 μL. However, multiple ground and ambient temperatures (T − T ) were in the sc amb experiments have shown that it is difficult to precisely control the ranges 1053–1058 mbar, 26.16–25.87 °C, 27.93–28.00 °C, and npj Microgravity (2020) 37 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA 1234567890():,; S. Kumar et al. Fig. 3 Volume of sessile drops on the heated substrate vs. time. a Earth’s gravity. b Microgravity conditions. The bar l denotes the de- pinning stage of the sessile drops. For the better interpretation, please also refer Table 1 along with figure. Readers are advised to refer to the web version of the figure. 1.84–2.14 °C, respectively, for all drops. Furthermore, the substrate ambient temperature T in the MEC, M is the molecular weight of amb l edge temperature T was in the range 28.11–28.24 °C. Thus, the liquid (HFE-7100), and D is the diffusion coefficient of HFE-7100 se eff (T − T ) was in the range 0.16–0.22 °C and (T − T ) was in the in a nitrogen gas environment. The diffusion coefficient D was se sc se amb eff range 2.04–2.31 °C for all drops. calculated according to the Fuller–Schettler–Giddings equation and Similarly, for the microgravity experiment, the sensor data are as F(θ) is a function of the contact angle of the sessile drop, derived by follows: the main cell pressure (inside chamber) P , ambient Picknett and Bexon . amb temperature (inside chamber) T , and substrate center tempera- A comparison of experimental and theoretical evaporation rates amb ture T , and the difference between the substrate center and is presented in Fig. 4 for drop 7DPμg under microgravity sc ambient temperatures (T − T ) were in the range conditions at time t = 30 s (see Figs. 3 and 5b for a side view). sc amb 1050–1057 mbar, 25.65–25.21 °C, 27.95–28.08 °C, and 2.39–2.79 °C, The parameters for the analytical calculation are the base radius −6 2 −1 respectively, for all drops. The substrate edge temperature T was L = 2 mm, contact angle θ = 45. 6°, D = 5.4 × 10 m s , P = eff sat se −1 in the range 28.13–28.18 °C. Thus, (T − T ) was in the range 27,268 Pa, M = 0.25 Kg mol , P = 105,100 Pa, and T = l amb amb se sc 0.15–0.21 °C and (T − T ) was in the range 2.58–2.99 °C for all 25.36 °C. The calculated theoretical value of the diffusion-limited se amb evaporation rate for this drop (7DPμg) under microgravity drops. The data and results from the Earth gravity and microgravity −1 conditions at time t = 30 s is 0.095 μLs . The experimental value experiment are summarized in Table 1. for the time evolution of the sessile drop volume is calculated A comparison of the sessile drop volume with respect to time during from post-processing the side view of the drop shape (see Fig. 5e). evaporation is presented in Fig. 3. We can see that in the Earth’s gravity The experimental values under Earth and microgravity conditions experiment, all drops evaporated from the heated substrate before −1 without electric field are 0.198 and 0.087 μLs , respectively. This flushing started, whereas in the microgravity experiment flushing technique is more accurate in the constant contact area started before evaporation was complete(seethe sudden fall in the evaporation mode with an uncertainty maximum up to ±0.05 μL drop volume). In the latter, only drop 6DPμgEF de-pinned, conversely for the volume and of ±0.015 μL for the evaporation rate. to Earth’s gravity experiment, where all drops did. Drop shapes result from body forces equilibrium during the To compare the evaporation rates of sessile drops measured in evaporation process. Fig. 5 shows the comparison of the sessile microgravity experiment, one can refer to the analytical model for drop under gravity only (see Fig. 5a), microgravity only (see evaporation limited by diffusion, first derived by Picknett and Fig 5b), both gravity and electric field (see Fig. 5c), and finally Bexon for a constant contact area (up to de-pinning) and a microgravity and electric field (see Fig. 5d). The combination of spherical cap shape. In our experiments, the wetted area between body forces results in changes of interface curvature, contact the liquid HFE-7100 and heated substrate was constant with a angle, and thus in the de-pining stage. Figure 5e is only intended base diameter of 4 mm (owing to the groove in the substrate). The to show the comparison between raw images from experiments analytical evaporation rate is thus: (top) and clean ones (bottom) after post-processing. The cleaned dV images have been later used to calculate the time evolution of (1) ¼ 2πD C LFðθÞ eff sat dt drop volumes reported in Fig. 3. To better understand the overall evaporation process, it could P M sat l be interesting to address the related coupled fluid-flow problem C ¼ (2) sat R T gas amb that is induced. For that purpose, Fig. 6 displays top view IR and side-view images of drop 6DPμgEF in the microgravity experi- 5 2 3 FðθÞ¼ð8:957 10 þ 0:633 θ þ 0:116 θ  0:08878 θ ment and drop 4DP1gEF in the ground experiment subjected to (3) an 8 kV electric field, as these were the only two drops of similar þ 0:01033 θ Þ= sin θ for π=18  θ  π; initial volume (see Fig. 3). The drop evaporation time series is where L is the drop base radius, C is the saturated vapor divided in five sections, starting the sequence from the liquid sat concentration, T is the ambient temperature in Kelvin, R is the injection to flushing. Next to the injection phase, surface amb gas universal gas constant, P is the saturation pressure based on the temperature was almost uniforminbothexperiments,until sat Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA npj Microgravity (2020) 37 S. Kumar et al. Fig. 4 Evaporation rates over the constant contact angle mode. Earth conditions: average evaporation rates of sessile drops 1DP1g and 3DP1g (without an electric field), and 2DP1gEF and 4DP1gEF (with an electric field). Microgravity conditions: evaporation rates of sessile drops 7DPμg (without an electric field) and 8DPμgEF (with an electric field). The parameters of sessile drop 7DPμg were used for the calculation of the analytical diffusion-limited evaporation rate without an electric field . Error bars are calculated estimating the minimum and maximum evaporation rate experimentally measured. thermo-capillary instabilities take place for drop 6DPμgEF at t = 18.3 s in the microgravity experiment and drop 4DP1gEF at t = 12 s in the ground experiment. The pattern of thermo-capillary instabilities shows several cells coming from bottom to surface of sessile drop and then moving toward the contact line. It clearly appears that these thermo-capillary instabilities only occur once the drop volume gets below a critical value (see horizontal lines in Fig. 3 and detailed values in Table 1). It is noteworthy from Fig. 3 that these thermo-convective instabil- ities do not significantly modify the evaporation rates, whatever been under Earth or microgravity conditions. The last two sections of Fig. 6 display the initiation stage of de-pinning and that of flushing, respectively. DISCUSSION Owing to unrepeatable injection drop volumes, one was faced with very different initial evaporation conditions between Earth and microgravity experiments (see Fig. 3). Moreover, the time plot for drop 5DPμg under microgravity conditions (see Fig. 3) exhibits some oscillations until de-pinning occurs. The detailed reasons for this strange behavior are under investigations, but the oscillations in volume may be related to higher mechanical coupling to the rocket vibrations due to its initial volume being larger than that of the other drops (see Table 1). It might also have resulted from the release of gas bubbles inside the drop during evaporation, as can be observed from the side-view images of the drop. The global evaporation rate of drop 5DPμg (microgravity) is thus excluded in the subsequent analysis. The effect of gravity on the evaporation rate clearly appears in Fig. 4: its value is roughly halved under microgravity conditions as compared to Earth conditions; this is in agreement with npj Microgravity (2020) 37 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA Table 1. Summary of experiments with internal notation. −1 Drops no. Drops code Injected volume Evaporation rate during constant contact area (μLs ) Instabilities appeared At de-pinning Volumetric force condition (μL) at volume Evaporation Uncertainty Uncertainty (μL) Volume (μL) θ (deg.) rate (plus) (minus) 1 1DP1g 2.91 at t = 2 s 0.205 0.011 0.013 1.6 0.43 7.6 Gravity 2 2DP1gEF 2.81 at t = 2.2 s 0.191 0.005 0.007 1.59 0.81 14.6 Gravity with elecrtic field 3 3DP1g 4.67 at t = 2.6 s 0.192 0.006 0.004 1.54 0.43 8.6 Gravity 4 4DP1gEF 3.88 at t = 2.4 s 0.181 0.012 0.005 1.67 0.76 13.4 Gravity with elecrtic field 5 5DPμg 9.45 at t = 5 s 0.053 0.003 0.003 NA NA NA Microgravity 6 6DPμgEF 3.28 at t = 4.8 s 0.097 0.014 0.001 2.01 1.83 18.7 Microgravity with electric field 7 7DPμg 7.75 at t = 4.6 s 0.087 0.007 0.006 NA NA NA Microgravity 8 8DPμgEF 6.10 at t = 4.5 s 0.109 0.009 0.005 NA NA NA Microgravity with electric field 9 9DPμg 6 0.095 NA NA NA NA NA Microgravity without electric field (Analytical) S. Kumar et al. Fig. 5 Comparison of the sessile drop interface under the effect of gravitational and electrical field forces. a Drop 1DP1g on the ground at t = 2.3 s. b Drop 7DPμg under microgravity at t = 30 s. c Drop 2DP1gEF on the ground with an electric field at t = 2.4 s. d Drop 6DPμgEF under microgravity with an electric field at t = 6.8 s. e Image from a side-view camera with interferometry lines (top) and after cleaning (bottom). The cleaned images are used to measure volume over the time (see Fig. 3). Fig. 6 Time series of infrared images (top view) during the evaporation of liquid HFE-7100 sessile drops on a heated substrate under microgravity and Earth conditions with electric field (EF). The frames illustrate the injection, instability pattern, and de-pinning stages, respectively, for drops 6DPμgEF and 4DP1gEF under microgravity (top) and Earth’s gravitational conditions (bottom) (see Table 1 and refer to the Supplementary Materials for complete movies). 11,17,22 previous works . Indeed, the average evaporation rate of interface. Theinterface shapeofthe sessile drops resulted from body the sessile drops of HFE-7100 under microgravity is 56% and 45% and surface forces acting on them. As it clearly appears in Fig. 5a, b, lower than that under Earth conditions without and with the the shape of a sessile drop under microgravity is exactly spherical in electric field, respectively. Interestingly, the analytical diffusion- comparison to that in Fig. 5a. In contrast, sessile drops exhibit clear limited evaporation rate enables us to conclude that the average cone formation under microgravity conditions with an electric field evaporation rate of HFE-7100 sessile drops under microgravity (see Figs. 5d, c and 6). Alongwiththe influence on the interface (see 10,12,18,22,23 conditions in the absence of an electric field seems to be mainly Fig. 5), which is in agreement with other experiments ,the de-pinning process is also associated with the gravitational and controlled by diffusion. Furthermore, note that the average evaporation rate under Earth conditions with an electric field is electrical forces individually or in combination. Based on these 6% lower than the average rate without one, whereas the comparisons, we can see the correlation between the body and average evaporation rate under microgravity conditions with an surface force conditions and the volumes (see Fig. 3) and contact electric field is 19% higher than the average rate without one. angles (contact angles were measured by using the ImageJ software That is to say, the effect of an electric field on the evaporation plugin known as DropSnake, which is based on B-spline snakes (active rate of HFE-7100 is opposite under microgravity and Earth contours)) during de-pinning irrespective of the shape of the sessile conditions, as it is for liquid water drops . drop interface shape. The fact that de-pinning is anticipated in the Figure 5 shows side views of the sessile drops under the four presence of an electric field can be attributed to the fact that the considered conditions. For a fair comparison, compare Fig. 5awith radial electric force is directed inwards, causing striction of the Fig. 5cand Fig. 5d with Fig. 6 (drop 4DP1gEF), as these drops were of interface . Accordingly, the order of de-pinning based on the volume comparable volumes (see Table 1). Also, it is noteworthy that no drop and contact angle and body and surface force conditions was as in microgravity without an electric field had a lower initial injected follows: drop 6DPμgEFwithanelectric field (under microgravity volume (see Fig. 3b). Therefore, the minimum volume for drop 7DPμg conditions) at volume= 1.83 μL and contact angle θ= 18.7° de- under microgravity at t= 30 s can be used for comparison of the pinned at the highest volume and contact angle and did so earlier Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA npj Microgravity (2020) 37 S. Kumar et al. than drop 2DP1gEF with an electric field (under Earth conditions), rates. Through the application of different combinations of which de-pinned at volume= 0.81 μL and contact angle θ= 14.6°, volumetric forces (an electric field and gravity), we also and drop 4DP1gEF with electric field (under Earth conditions), which demonstrate the role of gravity on the shape of the sessile drop de-pinned at volume 0.76 μL and contact angle θ= 13.4°. Drop interface and its influence on the de-pinning of sessile drops. To 3DP1g without an electric field (under Earth conditions) at volume= concrete the above evidence, module will re-fly again (as a 0.43 μL and contact angle θ=8.6° anddrop1DP1g withoutan baseline, in 2022). One of the main objectives of the reflight is to electric field (under Earth conditions) at volume= 0.43 μL and contact better control the actual injected volumes so as to ensure a better angle θ= 7.6° de-pinned with the smallest volumes and contact data comparison among the different testing conditions. angles. According to the above correlations, it can be predicted that for sessile drops 5DPμgand 7DPμg (under microgravity without an METHODS electric field), the volume (and contact angle) should have been either equal to or higher than the volume (and contact angle) of drops Fluid property measurements TM TM 2DP1gEF and 2DP1gEF (under Earth conditions with an electric field) In all cases, the liquid used was 99.9% pure HFE-7100 (3M Novec 7100 at de-pinning. The influence of the substrate grooves in the de- Engineered Fluid, a hydrofluoroether also known as methoxy- nonafluorobutane (C4F9OCH3)). It was chosen for its volatility, semi- pinning dynamics could itself be a subject of investigation . transparency in the IR wavelengths, perfect wetting, non-toxicity, and being The IR images in Fig. 6 reveal some characteristic patterns non-flammable. For more on the properties of HFE-7100, please associated with the thermo-capillary instabilities that occurred refer to https://multimedia.3m.com/mws/media/199818O/3m-novec-7100- for drop 6DPμgEF in microgravity conditions at time t = 18.3 s, engineered-fluid.pdf and https://multimedia.3m.com/mws/media/569860O/ which corresponds to a volume of 2.01 μL, calculated using the 3mtm-thermal-management-fluids-for-military-aerospace-apps.pdf. side-view image (refer Fig. 6) in which the maximum sessile drop height is 0.41 mm. The thermo-capillary instabilities first Hardware description appeared near the periphery of the sessile drop during The ARLES experiment module was designed and manufactured by the evaporation and before de-pinning, and they remained visible SSC under the guidance of the ESA and Science team proposition based on up to complete evaporation (see Fig. 6). In the ground the required scientific objectives. The overall design of the experiment experiment, however, there were instability patterns for drop module is subdivided into two parts (see Fig. 1a): the main evaporation cell 4DP1gEF stating at time t = 15 s and volume = 1.67 μL(max- (MEC), which is for single-drop experiment systems and the multi-drop cell, imum interface height of 0.24 mm); the patterns began to which is for multi-drop experiment systems to be executed in parallel. For appear at time t = 12 s and volume = 1.10 μL. Similarly, safety reasons, a neutral gas nitrogen (N ) atmosphere was used. instability patterns appeared in all the sessile drops in the ground reference experiment (see Fig. 3), for which volume Main evaporation cell. The chamber volume of the main evaporation and time are reported in Fig. 3. The thermo-capillary cell (MEC) is 4 l. It is sized to maintain a low vapor concentration throughout the whole experiment even if the N evacuation fails instabilities appeared as soon as the maximum drop height 2 during the flight. The cell thickness was chosen to withstand the was below a critical value of approximately between 0.2 and expected pressure differences during the filling and emptying of the 0.3 mm for Earth conditions and around 0.4 mm for the gas (N ). Figures 1 and 2 shows the main cell used to perform sessile microgravity conditions, which is associated with thermo- drop evaporation of a pure fluid on a heated substrate with and capillary instabilities referred to as Marangoni instabilities. without an electric field. The substrate was a thin single-crystal silicon Interestingly, the above critical thickness for HFE-7100 under wafer coated with a 50 nm-thick platinum layer, whose surface Earth conditions fully agrees with Chauvet et al. . Therefore, as roughness was less than 1 micron RMS, deposited by atomic layer the injected volume of most of the microgravity drops deposition. The substrate had 50 × 50 μm grooves with 4 ± 0.1 mm in exceeded that of the drops in the Earth reference experiment, diameter to force the pinning of a sessile drop with a diameter of 4 mm (see Fig. 1d). The central hole for the fluid injection was 0.7 mm in longer evaporation times would have been required for the diameter. The substrates were manufactured at MICAS TU Leuwen. An former to reach the critical height at which thermo-capillary IR camera was mounted on the lid of the main evaporation cell, where instabilities are observed. As a result, flushing of the largest a ZnSe window served as the passage for IR wavelengths. The microgravity sessile drops was unfortunately performed before interferometry camera observed the single-drop evaporation process instability patterns could be observed. through the side observation windows of the MEC. In conclusion, it is worth mentioning that it was very difficult to carry out repeatable injection of prescribed sessile drop volume Multi-drop cell. The multi-drop experiment system is for the analysis of both under Earth and microgravity conditions. The exact reasons different fluids with nanoparticle suspensions, and the related pattern for the formation of oversized sessile drops under microgravity formation on the substrates after the evaporation process, and its conditions are still under investigation. In future microgravity consequent functionalisation. As such, it is not part of our analysis. experiments, it would therefore be preferable to perform sessile drop volume injection with real-time feedback control. Under our Heat flux, temperature, and pressure measurements. Two T-type thermo- experimental conditions, the results provide evidence for the couples monitored the substrate temperature. One thermocouple was effect of microgravity conditions on the sessile drop evaporation placed close to the center hole and the other one close to the edge of the substrate. These sensors were incorporated in the heat flux sensor by the rate, indicating that the rate under microgravity conditions is CAPTEC manufacturer. The heat flux sensor with integrated thermocouples nearly half that under Earth conditions for HFE-7100. Furthermore, determined the heat flux to the drops and substrate temperature with a the effect of an electric field on the evaporation rate is opposite −1 2 sensitivity of 2 μVW m ). Along with the substrate temperature, we used under microgravity and Earth conditions. The experimental results a set of PT-100 sensors to monitor the cell wall temperature and the also demonstrate the relationship between thermo-capillary ambient temperature inside the MEC (see Fig. 1 for the position). The instabilities and the measured critical height of the sessile drop temperature measurement rate was 30.4 Hz, with an uncertainty of ±2.1 K interface. For temperature differences between substrate and from the true temperature in the worst case. The passband of the filter was ambient in the range of 2–3 °C with a base diameter of 4 mm, the 4.56 Hz. A dedicated μ-TC interface board performed the readout of the measured critical height for the appearance of thermo-capillary heat flux sensor and the thermocouples. The pressure sensor measured pressure in the range of 0–1.6 bar with an accuracy of ±0.2% inside the instabilities is approximately between 0.2 and 0.3 mm for Earth MEC throughout the experiment. conditions and around 0.4 mm for the microgravity conditions for HFE-7100. It is also noteworthy that meanwhile they strongly Heater. The heaters were custom made and manufactured by NEL change the fluid-flow structure in the sessile drop, these thermo- Technologies Ltd. They are capton patch heaters with an etched resistive capillary instabilities do not significantly influence the evaporation npj Microgravity (2020) 37 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA S. Kumar et al. pattern. For the MEC, the heater was designed to provide 5 W of uniform 6. Schirmer, N. C., Ströhle, S., Tiwari, M. K. & Poulikakos, D. On the principles of heating power at 24 V. 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The cleaned images without fringes were used for of local evaporation rate and interfacial temperature of evaporating droplets. the analysis (see Fig. 5e). The image acquisition rate for all the images was Langmuir 30, 2002–2008 (2014). 25 Hz. 16. Sefiane, K., Moffat, J. R., Matar, O. K. & Craster, R. V. Self-excited hydrothermal waves in evaporating sessile drops. Appl. Phys. Lett. 93, 074103 (2008). 17. Carle, F., Sobac, B. & Brutin, D. Experimental evidence of the atmospheric con- Experimental procedure vective transport contribution to sessile droplet evaporation. Appl. Phys. Lett. 102, The SubOrbital Express rocket (MASER 14) launch took place successfully on 061603 (2013). Monday, 24 June 2019, from the Esrange Space Center in northern Sweden. 18. Kabov, O. A. & Zaitsev, D. V. The effect of wetting hysteresis on drop spreading The atmospheric replacement was executed 60 s after the launch by feeding under gravity. Dokl. Phys. 58, 292–295 (2013). in the N while the experimental cells were connected to an exhaust port in 19. Diana, A., Castillo, M., Brutin, D. & Steinberg, T. Sessile drop wettability in normal the outer structure. At the start of the microgravity phase (100 km level) t= and reduced gravity. Microgravity Sci. Technol. 24, 195–202 (2012). 70.4 s, the experiment liquid was injected to create the first drop of HFE-7100 20. Picknett, R. & Bexon, R. The evaporation of sessile or pendant drops in still air. J. upon the heated substrate. After a delay corresponding to the estimated Colloid Interface Sci. 61, 336–350 (1977). drop evaporation time, the atmosphere in the chamber was flushed. After 21. Fuller, E. N., Ensley, K. & Giddings, J. C. Diffusion of halogenated hydrocarbons in the flushing sequence, another drop was injected, and the evaporation helium. the effect of structure on collision cross sections. J. Phys. Chem. 73, cycle with diagnostics was repeated. The outside pressure was 0 bar during 3679–3685 (1969). the microgravity period. At the bottom of the ARLES experiment module is 22. Almohammadi, H. & Amirfazli, A. Sessile drop evaporation under an electric field. an N pressure vessel for flushing the single-drop cell after each consecutive Colloids Surf. A Physicochem. Eng. Asp. 555, 580–585 (2018). drop. Flushing was performed to prevent the evaporated liquid from 23. Abdella, K., Rasmussen, H. & Inculet, I. Interfacial deformation of liquid drops by condensing in the experiment cell. The ground test experiment were electric fields at zero gravity. Comput. Math. Appl. 31,67–82 (1996). executed in the same way as during the flight. The only difference was the 24. Grishaev, V., Amirfazli, A., Chikov, S., Lyulin, Y. & Kabov, O. Study of edge effect to membrane vacuum pump, which was connected to the exhaust of the stop liquid spillage for microgravity application. Microgravity Sci. Technol. 25, module. 27–33 (2012). 25. Chauvet, F., Dehaeck, S. & Colinet, P. Threshold of bénard-marangoni instability in drying liquid films. Europhys. Lett. 99, 34001 (2012). Reporting summary Further information on research design is available in the Nature Research Reporting Summary linked to this article. DATA AVAILABILITY ACKNOWLEDGEMENTS The data collected during this study is available from the corresponding authors The present work was carried out in the framework of the European Space Agency upon reasonable request. research project AO-1999-110: EVAPORATION. We thank all of the ARLES Science Team Members for their contribution in making possible the Sounding Rocket Received: 16 July 2020; Accepted: 13 November 2020; Experiment in the framework of the ESA MASER 14 Campaign: Science Team Coordinator: Dr. C.S. Iorio, Universite Libre de Bruxelles, University of Pisa, University of Alberta, University of Darmstadt, Universite de Liège, Chinese Academy of Science, University of Edinburgh, University of Loughborough, Trinity College Dublin, and Siberian Branch Russian Academy of Science. We thank Dr. D. Mangini, Dr. B. Toth, REFERENCES and Dr. A. Verga for their interest in and support of the activities linked to ARLES and for the fruitful discussions. The research team of Professor A. Ferrari of the University 1. Leidenfrost, J. G. De aquae communis nonnullis qualitatibus tractatus (Ovenius, 1756). of Cambridge is also gratefully acknowledged for the enormous support in preparing 2. Young, T. III. An essay on the cohesion of fluids. Philos. Trans. R. Soc. Lond. 95, the fluids with nanoparticle suspensions tested both on the ground and in 65–87 (1805). 3. Fuchs, N. A. Evaporation and Droplet Growth in Gaseous Media (Pergamon, 1959). microgravity conditions. We are grateful to A.I. Garivalis from University of Pisa for his 4. Deegan, R. D. et al. Capillary flow as the cause of ring stains from dried liquid contribution in EF calculations and analysis. We thank the Swedish Space drops. Nature 389, 827–829 (1997). Corporation, with particular gratitude to the project manager, Mr. M. Lundin. We 5. Ko, S. H., Chung, J., Hotz, N., Nam, K. H. & Grigoropoulos, C. P. Metal nanoparticle acknowledge the financial support of the French National Space Agency (Centre direct inkjet printing for low-temperature 3d micro metal structure fabrication. J. national d’études spatiales: CNES) research grant for the DROPS experiment on the Micromech. Microeng. 20, 125010 (2010). FLUIDES space mission. Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA npj Microgravity (2020) 37 S. Kumar et al. AUTHOR CONTRIBUTIONS Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. S.K. performed processing of the raw data. S.K., M.M., and D.B. contributed to the analysis of the results and writing the manuscript. P.D.M. contributed on the application of electric field on sessile drop. All authors approved the final manuscript. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give COMPETING INTERESTS appropriate credit to the original author(s) and the source, provide a link to the Creative The authors declare no competing interests. Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the ADDITIONAL INFORMATION article’s Creative Commons license and your intended use is not permitted by statutory Supplementary information is available for this paper at https://doi.org/10.1038/ regulation or exceeds the permitted use, you will need to obtain permission directly s41526-020-00128-2. A supplementary movie for Fig. 6 is available online. from the copyright holder. To view a copy of this license, visit http://creativecommons. org/licenses/by/4.0/. Correspondence and requests for materials should be addressed to S.K. or D.B. 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Sessile volatile drop evaporation under microgravity

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www.nature.com/npjmgrav ARTICLE OPEN 1✉ 1 2 1,3✉ Sanjeev Kumar , Marc Medale , Paolo Di Marco and David Brutin The evaporation of sessile drops of various volatile and non-volatile liquids, and their internal flow patterns with or without instabilities have been the subject of many investigations. The current experiment is a preparatory one for a space experiment planned to be installed in the European Drawer Rack 2 (EDR-2) of the International Space Station (ISS), to investigate drop evaporation in weightlessness. In this work, we concentrate on preliminary experimental results for the evaporation of hydrofluoroether (HFE-7100) sessile drops in a sounding rocket that has been performed in the frame of the MASER-14 Sounding Rocket Campaign, providing the science team with the opportunity to test the module and perform the experiment in microgravity for six consecutive minutes. The focus is on the evaporation rate, experimentally observed thermo-capillary instabilities, and the de- pinning process. The experimental results provide evidence for the relationship between thermo-capillary instabilities and the measured critical height of the sessile drop interface. There is also evidence of the effects of microgravity and Earth conditions on the sessile drop evaporation rate, and the shape of the sessile drop interface and its influence on the de-pinning process. npj Microgravity (2020) 6:37 ; https://doi.org/10.1038/s41526-020-00128-2 INTRODUCTION preparation for an experiment that is to be performed in the near 1–3 future at the European Drawer Rack 2 of the International Space Drops have been fascinating researchers for centuries . Topics of Station under the EVAPORATION project of the ESA. The intent is interest include water falling onto a hot cooking plate, which is a typical example of Leidenfrost drops , the evaporation of sessile to study evaporating drops of pure fluids as well as drops of fluids drops with nanoparticle deposition in coffee rings , inkjet that contain a low concentration of metallic nanoparticles. The 5,6 7 8,9 printing , pesticides sprayed onto leaves , and blood analysis . influence of an electric field is also of interest. The application of Although sessile drops are simple in geometry, the physics an external electrostatic field induces electric stress at the involved in the evaporation process is complex due to the vapor–liquid interface, deforming it and altering the contact numerous intricate interactions with the substrate and ambient angle. The resulting electric forces press the drop against the environment, and the fluid nature of the sessile drop itself. An surface and elongate it in the vertical direction; in addition, accurate quantitative model of the evaporation process can lead electroconvection is induced in the liquid and in the surrounding to greater understanding of the evaporation rate and control over vapor atmosphere, resulting in a possible enhancement of the pattern formation or the deposition of particles after the evaporation rate, which may result useful when gravity-driven evaporation of a sessile drop. This knowledge can then enhance convection is suppressed. The scientific objectives include dealing the efficiency of several applications. The physically rich and with the flow motion and the thermo-capillary instabilities complex evaporation of sessile drops is thus of interest to both the occurring in the drop, at the drop interface, and in the vapor academic and industry communities. phase, and investigating the pattern formation on the substrate Parabolic flight experiments on drops of various fluids have after the evaporation phase. been performed multiple times by The National Centre for Space ARLES was a collaborative experiment among various teams. Studies (CNES), France, and The European Space Agency (ESA) Each team focused on different aspects of the experiment to 10–15 parabolic flight campaigns . The existence of thermo-capillary contribute to the overall scientific objectives of the experiment, 14,16 instabilities and the effect of the reduced gravity environment such as flow motion and thermo-capillary instabilities occurring in 11,17 10,18,19 on evaporation and the drop interface have already the drop, at the drop interface, and in the vapor phase, the pattern been demonstrated. Parabolic flights have enabled these observa- formation on the substrate after evaporation of the volatile phase, tions, but such flights are not sufficient in terms of duration or the deposition of nanoparticles, and the eventual heat transfer residual acceleration for accurate measurements to be taken. enhancement. Our team primarily focused on the analysis of the Furthermore, the drop interface is highly sensitive to aircraft flow motion and thermo-capillary instabilities occurring in the vibrations. A better level of microgravity and a longer evaporation drop using data from the infrared (IR) (top view) camera and on time are therefore needed. the evaporation rate and interface evolution of the sessile drop The Advanced Research on Liquid Evaporation in Space (ARLES) using data from the side-view camera. The experimental results experiment module (see Figs. 1 and 2) was designed to support presented here address the effect of microgravity and Earth the investigation of the evaporation process in a controlled conditions on the evaporation, thermo-capillary instabilities, drop environment. ARLES was part of the payload in a SubOrbital interface, and de-pinning of a forced sessile drop of hydrofluor- Express rocket (MASER 14) and it successfully took place on oether (HFE-7100) liquid on a heated substrate. The experimental Monday, 24 June 2019 from the Esrange Space Center in northern Sweden under the collaboration of the ESA and Swedish Space results allow for a comparison of data from both ground and Corporation (SSC). The ARLES experiment was conducted as a space experiments, thereby providing firm conclusions. 1 2 3 Aix-Marseille Universite, CNRS, IUSTI UMR 7343, Marseille 13013, France. DESTEC, University of Pisa, Largo Lazzarino 1, Pisa 56122, Italy. Institut Universitaire de France, Paris 75231, France. email: sanjeev.kumar@univ-amu.fr; david.brutin@univ-amu.fr Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA 1234567890():,; S. Kumar et al. Fig. 1 Overview of the ARLES experimental setup. a Experiment module on-board the MASER 14 rocket, divided into two parts: the main evaporating and multi-evaporating cells. b, c Main evaporating cell (MEC) with a detailed schematic. d Platinum layered surface crystal silicon wafer substrate (top view) with grooves. Images a, b, and c are credited to the European Space Agency (ESA) and Swedish Space Corporation (SSC). Fig. 2 Schematic of the main evaporating cell (MEC) of the ARLES experiment (left). Axisymmetric electric field around sessile drop (right). Readers are advised to refer to web version of this figure for better display. RESULTS injection liquid volume with the current injection system and hardware. Even though the actual injected volume of the drops Experimental setup and conditions during the ground experiment is lower than the target theoretical In Fig. 1a, a complete setup of the ALRES experiment has been nominal value but the actual injected volume of the drops during shown. It consists of two parts, namely the main evaporating cell the microgravity conditions is higher than the target nominal one (MEC; bottom) and multi-evaporating cells (top). Our current focus (see Fig. 3). The temperature of the main test cell was set at 26 °C is on the MEC experiment. The detailed schematic of the MEC is and the temperature of the substrate was set at 28 °C with an presented in the Fig. 2 (left) along with its chamber shown in imposed electric field 8 kV for all drops with electric field, except Fig. 1b (top view) and 1c (cut view) with injection system, for drop 8DPμgEF under microgravity, for which the field was set substrate, and electric field electrode (substrate is connected to at 5.7 kV. Due to the grooves on the substrate, the base diameter the negative (−) terminal and the electrode to the positive (+) of all the sessile drops remained constant (4 mm) during terminal). Figure 2 (right) shows the electric field distribution evaporation until the drops de-pinned. around the sessile drop for the axisymmetric case. For more details, please refer to MEC schematic in Fig. 2. Experimental results The ideal experimental conditions for the MEC are as follows: target theoretical nominal parameters for microgravity and Earth For Earth gravity, the experimental data from the sensor are as conditions were set to be similar for the purposes of comparison. follows: the main cell pressure (inside chamber) P , ambient amb The injection velocity of liquid HFE-7100 for sessile drop creation temperature (inside chamber) T , and substrate center tem- amb −1 on the heated substrate was 4 μLs and the nominal volume of perature T , and the difference between the substrate center sc each sessile drop was set at 6 μL. However, multiple ground and ambient temperatures (T − T ) were in the sc amb experiments have shown that it is difficult to precisely control the ranges 1053–1058 mbar, 26.16–25.87 °C, 27.93–28.00 °C, and npj Microgravity (2020) 37 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA 1234567890():,; S. Kumar et al. Fig. 3 Volume of sessile drops on the heated substrate vs. time. a Earth’s gravity. b Microgravity conditions. The bar l denotes the de- pinning stage of the sessile drops. For the better interpretation, please also refer Table 1 along with figure. Readers are advised to refer to the web version of the figure. 1.84–2.14 °C, respectively, for all drops. Furthermore, the substrate ambient temperature T in the MEC, M is the molecular weight of amb l edge temperature T was in the range 28.11–28.24 °C. Thus, the liquid (HFE-7100), and D is the diffusion coefficient of HFE-7100 se eff (T − T ) was in the range 0.16–0.22 °C and (T − T ) was in the in a nitrogen gas environment. The diffusion coefficient D was se sc se amb eff range 2.04–2.31 °C for all drops. calculated according to the Fuller–Schettler–Giddings equation and Similarly, for the microgravity experiment, the sensor data are as F(θ) is a function of the contact angle of the sessile drop, derived by follows: the main cell pressure (inside chamber) P , ambient Picknett and Bexon . amb temperature (inside chamber) T , and substrate center tempera- A comparison of experimental and theoretical evaporation rates amb ture T , and the difference between the substrate center and is presented in Fig. 4 for drop 7DPμg under microgravity sc ambient temperatures (T − T ) were in the range conditions at time t = 30 s (see Figs. 3 and 5b for a side view). sc amb 1050–1057 mbar, 25.65–25.21 °C, 27.95–28.08 °C, and 2.39–2.79 °C, The parameters for the analytical calculation are the base radius −6 2 −1 respectively, for all drops. The substrate edge temperature T was L = 2 mm, contact angle θ = 45. 6°, D = 5.4 × 10 m s , P = eff sat se −1 in the range 28.13–28.18 °C. Thus, (T − T ) was in the range 27,268 Pa, M = 0.25 Kg mol , P = 105,100 Pa, and T = l amb amb se sc 0.15–0.21 °C and (T − T ) was in the range 2.58–2.99 °C for all 25.36 °C. The calculated theoretical value of the diffusion-limited se amb evaporation rate for this drop (7DPμg) under microgravity drops. The data and results from the Earth gravity and microgravity −1 conditions at time t = 30 s is 0.095 μLs . The experimental value experiment are summarized in Table 1. for the time evolution of the sessile drop volume is calculated A comparison of the sessile drop volume with respect to time during from post-processing the side view of the drop shape (see Fig. 5e). evaporation is presented in Fig. 3. We can see that in the Earth’s gravity The experimental values under Earth and microgravity conditions experiment, all drops evaporated from the heated substrate before −1 without electric field are 0.198 and 0.087 μLs , respectively. This flushing started, whereas in the microgravity experiment flushing technique is more accurate in the constant contact area started before evaporation was complete(seethe sudden fall in the evaporation mode with an uncertainty maximum up to ±0.05 μL drop volume). In the latter, only drop 6DPμgEF de-pinned, conversely for the volume and of ±0.015 μL for the evaporation rate. to Earth’s gravity experiment, where all drops did. Drop shapes result from body forces equilibrium during the To compare the evaporation rates of sessile drops measured in evaporation process. Fig. 5 shows the comparison of the sessile microgravity experiment, one can refer to the analytical model for drop under gravity only (see Fig. 5a), microgravity only (see evaporation limited by diffusion, first derived by Picknett and Fig 5b), both gravity and electric field (see Fig. 5c), and finally Bexon for a constant contact area (up to de-pinning) and a microgravity and electric field (see Fig. 5d). The combination of spherical cap shape. In our experiments, the wetted area between body forces results in changes of interface curvature, contact the liquid HFE-7100 and heated substrate was constant with a angle, and thus in the de-pining stage. Figure 5e is only intended base diameter of 4 mm (owing to the groove in the substrate). The to show the comparison between raw images from experiments analytical evaporation rate is thus: (top) and clean ones (bottom) after post-processing. The cleaned dV images have been later used to calculate the time evolution of (1) ¼ 2πD C LFðθÞ eff sat dt drop volumes reported in Fig. 3. To better understand the overall evaporation process, it could P M sat l be interesting to address the related coupled fluid-flow problem C ¼ (2) sat R T gas amb that is induced. For that purpose, Fig. 6 displays top view IR and side-view images of drop 6DPμgEF in the microgravity experi- 5 2 3 FðθÞ¼ð8:957 10 þ 0:633 θ þ 0:116 θ  0:08878 θ ment and drop 4DP1gEF in the ground experiment subjected to (3) an 8 kV electric field, as these were the only two drops of similar þ 0:01033 θ Þ= sin θ for π=18  θ  π; initial volume (see Fig. 3). The drop evaporation time series is where L is the drop base radius, C is the saturated vapor divided in five sections, starting the sequence from the liquid sat concentration, T is the ambient temperature in Kelvin, R is the injection to flushing. Next to the injection phase, surface amb gas universal gas constant, P is the saturation pressure based on the temperature was almost uniforminbothexperiments,until sat Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA npj Microgravity (2020) 37 S. Kumar et al. Fig. 4 Evaporation rates over the constant contact angle mode. Earth conditions: average evaporation rates of sessile drops 1DP1g and 3DP1g (without an electric field), and 2DP1gEF and 4DP1gEF (with an electric field). Microgravity conditions: evaporation rates of sessile drops 7DPμg (without an electric field) and 8DPμgEF (with an electric field). The parameters of sessile drop 7DPμg were used for the calculation of the analytical diffusion-limited evaporation rate without an electric field . Error bars are calculated estimating the minimum and maximum evaporation rate experimentally measured. thermo-capillary instabilities take place for drop 6DPμgEF at t = 18.3 s in the microgravity experiment and drop 4DP1gEF at t = 12 s in the ground experiment. The pattern of thermo-capillary instabilities shows several cells coming from bottom to surface of sessile drop and then moving toward the contact line. It clearly appears that these thermo-capillary instabilities only occur once the drop volume gets below a critical value (see horizontal lines in Fig. 3 and detailed values in Table 1). It is noteworthy from Fig. 3 that these thermo-convective instabil- ities do not significantly modify the evaporation rates, whatever been under Earth or microgravity conditions. The last two sections of Fig. 6 display the initiation stage of de-pinning and that of flushing, respectively. DISCUSSION Owing to unrepeatable injection drop volumes, one was faced with very different initial evaporation conditions between Earth and microgravity experiments (see Fig. 3). Moreover, the time plot for drop 5DPμg under microgravity conditions (see Fig. 3) exhibits some oscillations until de-pinning occurs. The detailed reasons for this strange behavior are under investigations, but the oscillations in volume may be related to higher mechanical coupling to the rocket vibrations due to its initial volume being larger than that of the other drops (see Table 1). It might also have resulted from the release of gas bubbles inside the drop during evaporation, as can be observed from the side-view images of the drop. The global evaporation rate of drop 5DPμg (microgravity) is thus excluded in the subsequent analysis. The effect of gravity on the evaporation rate clearly appears in Fig. 4: its value is roughly halved under microgravity conditions as compared to Earth conditions; this is in agreement with npj Microgravity (2020) 37 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA Table 1. Summary of experiments with internal notation. −1 Drops no. Drops code Injected volume Evaporation rate during constant contact area (μLs ) Instabilities appeared At de-pinning Volumetric force condition (μL) at volume Evaporation Uncertainty Uncertainty (μL) Volume (μL) θ (deg.) rate (plus) (minus) 1 1DP1g 2.91 at t = 2 s 0.205 0.011 0.013 1.6 0.43 7.6 Gravity 2 2DP1gEF 2.81 at t = 2.2 s 0.191 0.005 0.007 1.59 0.81 14.6 Gravity with elecrtic field 3 3DP1g 4.67 at t = 2.6 s 0.192 0.006 0.004 1.54 0.43 8.6 Gravity 4 4DP1gEF 3.88 at t = 2.4 s 0.181 0.012 0.005 1.67 0.76 13.4 Gravity with elecrtic field 5 5DPμg 9.45 at t = 5 s 0.053 0.003 0.003 NA NA NA Microgravity 6 6DPμgEF 3.28 at t = 4.8 s 0.097 0.014 0.001 2.01 1.83 18.7 Microgravity with electric field 7 7DPμg 7.75 at t = 4.6 s 0.087 0.007 0.006 NA NA NA Microgravity 8 8DPμgEF 6.10 at t = 4.5 s 0.109 0.009 0.005 NA NA NA Microgravity with electric field 9 9DPμg 6 0.095 NA NA NA NA NA Microgravity without electric field (Analytical) S. Kumar et al. Fig. 5 Comparison of the sessile drop interface under the effect of gravitational and electrical field forces. a Drop 1DP1g on the ground at t = 2.3 s. b Drop 7DPμg under microgravity at t = 30 s. c Drop 2DP1gEF on the ground with an electric field at t = 2.4 s. d Drop 6DPμgEF under microgravity with an electric field at t = 6.8 s. e Image from a side-view camera with interferometry lines (top) and after cleaning (bottom). The cleaned images are used to measure volume over the time (see Fig. 3). Fig. 6 Time series of infrared images (top view) during the evaporation of liquid HFE-7100 sessile drops on a heated substrate under microgravity and Earth conditions with electric field (EF). The frames illustrate the injection, instability pattern, and de-pinning stages, respectively, for drops 6DPμgEF and 4DP1gEF under microgravity (top) and Earth’s gravitational conditions (bottom) (see Table 1 and refer to the Supplementary Materials for complete movies). 11,17,22 previous works . Indeed, the average evaporation rate of interface. Theinterface shapeofthe sessile drops resulted from body the sessile drops of HFE-7100 under microgravity is 56% and 45% and surface forces acting on them. As it clearly appears in Fig. 5a, b, lower than that under Earth conditions without and with the the shape of a sessile drop under microgravity is exactly spherical in electric field, respectively. Interestingly, the analytical diffusion- comparison to that in Fig. 5a. In contrast, sessile drops exhibit clear limited evaporation rate enables us to conclude that the average cone formation under microgravity conditions with an electric field evaporation rate of HFE-7100 sessile drops under microgravity (see Figs. 5d, c and 6). Alongwiththe influence on the interface (see 10,12,18,22,23 conditions in the absence of an electric field seems to be mainly Fig. 5), which is in agreement with other experiments ,the de-pinning process is also associated with the gravitational and controlled by diffusion. Furthermore, note that the average evaporation rate under Earth conditions with an electric field is electrical forces individually or in combination. Based on these 6% lower than the average rate without one, whereas the comparisons, we can see the correlation between the body and average evaporation rate under microgravity conditions with an surface force conditions and the volumes (see Fig. 3) and contact electric field is 19% higher than the average rate without one. angles (contact angles were measured by using the ImageJ software That is to say, the effect of an electric field on the evaporation plugin known as DropSnake, which is based on B-spline snakes (active rate of HFE-7100 is opposite under microgravity and Earth contours)) during de-pinning irrespective of the shape of the sessile conditions, as it is for liquid water drops . drop interface shape. The fact that de-pinning is anticipated in the Figure 5 shows side views of the sessile drops under the four presence of an electric field can be attributed to the fact that the considered conditions. For a fair comparison, compare Fig. 5awith radial electric force is directed inwards, causing striction of the Fig. 5cand Fig. 5d with Fig. 6 (drop 4DP1gEF), as these drops were of interface . Accordingly, the order of de-pinning based on the volume comparable volumes (see Table 1). Also, it is noteworthy that no drop and contact angle and body and surface force conditions was as in microgravity without an electric field had a lower initial injected follows: drop 6DPμgEFwithanelectric field (under microgravity volume (see Fig. 3b). Therefore, the minimum volume for drop 7DPμg conditions) at volume= 1.83 μL and contact angle θ= 18.7° de- under microgravity at t= 30 s can be used for comparison of the pinned at the highest volume and contact angle and did so earlier Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA npj Microgravity (2020) 37 S. Kumar et al. than drop 2DP1gEF with an electric field (under Earth conditions), rates. Through the application of different combinations of which de-pinned at volume= 0.81 μL and contact angle θ= 14.6°, volumetric forces (an electric field and gravity), we also and drop 4DP1gEF with electric field (under Earth conditions), which demonstrate the role of gravity on the shape of the sessile drop de-pinned at volume 0.76 μL and contact angle θ= 13.4°. Drop interface and its influence on the de-pinning of sessile drops. To 3DP1g without an electric field (under Earth conditions) at volume= concrete the above evidence, module will re-fly again (as a 0.43 μL and contact angle θ=8.6° anddrop1DP1g withoutan baseline, in 2022). One of the main objectives of the reflight is to electric field (under Earth conditions) at volume= 0.43 μL and contact better control the actual injected volumes so as to ensure a better angle θ= 7.6° de-pinned with the smallest volumes and contact data comparison among the different testing conditions. angles. According to the above correlations, it can be predicted that for sessile drops 5DPμgand 7DPμg (under microgravity without an METHODS electric field), the volume (and contact angle) should have been either equal to or higher than the volume (and contact angle) of drops Fluid property measurements TM TM 2DP1gEF and 2DP1gEF (under Earth conditions with an electric field) In all cases, the liquid used was 99.9% pure HFE-7100 (3M Novec 7100 at de-pinning. The influence of the substrate grooves in the de- Engineered Fluid, a hydrofluoroether also known as methoxy- nonafluorobutane (C4F9OCH3)). It was chosen for its volatility, semi- pinning dynamics could itself be a subject of investigation . transparency in the IR wavelengths, perfect wetting, non-toxicity, and being The IR images in Fig. 6 reveal some characteristic patterns non-flammable. For more on the properties of HFE-7100, please associated with the thermo-capillary instabilities that occurred refer to https://multimedia.3m.com/mws/media/199818O/3m-novec-7100- for drop 6DPμgEF in microgravity conditions at time t = 18.3 s, engineered-fluid.pdf and https://multimedia.3m.com/mws/media/569860O/ which corresponds to a volume of 2.01 μL, calculated using the 3mtm-thermal-management-fluids-for-military-aerospace-apps.pdf. side-view image (refer Fig. 6) in which the maximum sessile drop height is 0.41 mm. The thermo-capillary instabilities first Hardware description appeared near the periphery of the sessile drop during The ARLES experiment module was designed and manufactured by the evaporation and before de-pinning, and they remained visible SSC under the guidance of the ESA and Science team proposition based on up to complete evaporation (see Fig. 6). In the ground the required scientific objectives. The overall design of the experiment experiment, however, there were instability patterns for drop module is subdivided into two parts (see Fig. 1a): the main evaporation cell 4DP1gEF stating at time t = 15 s and volume = 1.67 μL(max- (MEC), which is for single-drop experiment systems and the multi-drop cell, imum interface height of 0.24 mm); the patterns began to which is for multi-drop experiment systems to be executed in parallel. For appear at time t = 12 s and volume = 1.10 μL. Similarly, safety reasons, a neutral gas nitrogen (N ) atmosphere was used. instability patterns appeared in all the sessile drops in the ground reference experiment (see Fig. 3), for which volume Main evaporation cell. The chamber volume of the main evaporation and time are reported in Fig. 3. The thermo-capillary cell (MEC) is 4 l. It is sized to maintain a low vapor concentration throughout the whole experiment even if the N evacuation fails instabilities appeared as soon as the maximum drop height 2 during the flight. The cell thickness was chosen to withstand the was below a critical value of approximately between 0.2 and expected pressure differences during the filling and emptying of the 0.3 mm for Earth conditions and around 0.4 mm for the gas (N ). Figures 1 and 2 shows the main cell used to perform sessile microgravity conditions, which is associated with thermo- drop evaporation of a pure fluid on a heated substrate with and capillary instabilities referred to as Marangoni instabilities. without an electric field. The substrate was a thin single-crystal silicon Interestingly, the above critical thickness for HFE-7100 under wafer coated with a 50 nm-thick platinum layer, whose surface Earth conditions fully agrees with Chauvet et al. . Therefore, as roughness was less than 1 micron RMS, deposited by atomic layer the injected volume of most of the microgravity drops deposition. The substrate had 50 × 50 μm grooves with 4 ± 0.1 mm in exceeded that of the drops in the Earth reference experiment, diameter to force the pinning of a sessile drop with a diameter of 4 mm (see Fig. 1d). The central hole for the fluid injection was 0.7 mm in longer evaporation times would have been required for the diameter. The substrates were manufactured at MICAS TU Leuwen. An former to reach the critical height at which thermo-capillary IR camera was mounted on the lid of the main evaporation cell, where instabilities are observed. As a result, flushing of the largest a ZnSe window served as the passage for IR wavelengths. The microgravity sessile drops was unfortunately performed before interferometry camera observed the single-drop evaporation process instability patterns could be observed. through the side observation windows of the MEC. In conclusion, it is worth mentioning that it was very difficult to carry out repeatable injection of prescribed sessile drop volume Multi-drop cell. The multi-drop experiment system is for the analysis of both under Earth and microgravity conditions. The exact reasons different fluids with nanoparticle suspensions, and the related pattern for the formation of oversized sessile drops under microgravity formation on the substrates after the evaporation process, and its conditions are still under investigation. In future microgravity consequent functionalisation. As such, it is not part of our analysis. experiments, it would therefore be preferable to perform sessile drop volume injection with real-time feedback control. Under our Heat flux, temperature, and pressure measurements. Two T-type thermo- experimental conditions, the results provide evidence for the couples monitored the substrate temperature. One thermocouple was effect of microgravity conditions on the sessile drop evaporation placed close to the center hole and the other one close to the edge of the substrate. These sensors were incorporated in the heat flux sensor by the rate, indicating that the rate under microgravity conditions is CAPTEC manufacturer. The heat flux sensor with integrated thermocouples nearly half that under Earth conditions for HFE-7100. Furthermore, determined the heat flux to the drops and substrate temperature with a the effect of an electric field on the evaporation rate is opposite −1 2 sensitivity of 2 μVW m ). Along with the substrate temperature, we used under microgravity and Earth conditions. The experimental results a set of PT-100 sensors to monitor the cell wall temperature and the also demonstrate the relationship between thermo-capillary ambient temperature inside the MEC (see Fig. 1 for the position). The instabilities and the measured critical height of the sessile drop temperature measurement rate was 30.4 Hz, with an uncertainty of ±2.1 K interface. For temperature differences between substrate and from the true temperature in the worst case. The passband of the filter was ambient in the range of 2–3 °C with a base diameter of 4 mm, the 4.56 Hz. A dedicated μ-TC interface board performed the readout of the measured critical height for the appearance of thermo-capillary heat flux sensor and the thermocouples. The pressure sensor measured pressure in the range of 0–1.6 bar with an accuracy of ±0.2% inside the instabilities is approximately between 0.2 and 0.3 mm for Earth MEC throughout the experiment. conditions and around 0.4 mm for the microgravity conditions for HFE-7100. It is also noteworthy that meanwhile they strongly Heater. The heaters were custom made and manufactured by NEL change the fluid-flow structure in the sessile drop, these thermo- Technologies Ltd. They are capton patch heaters with an etched resistive capillary instabilities do not significantly influence the evaporation npj Microgravity (2020) 37 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA S. Kumar et al. pattern. For the MEC, the heater was designed to provide 5 W of uniform 6. Schirmer, N. C., Ströhle, S., Tiwari, M. K. & Poulikakos, D. On the principles of heating power at 24 V. The heater was driven from 24 V pulse width printing sub-micrometer 3d structures from dielectric-liquid-based colloids. Adv. modulation (PWM). Funct. Mater. 21, 388–395 (2010). 7. Salyani, M. Droplet size effect on spray deposition efficiency of citrus leaves. Trans. ASAE 31, 1680–1684 (1988). Electrode. The positive high voltage potential was connected to a conical 8. Sobac, B. & Brutin, D. Desiccation of a sessile drop of blood: Cracks, folds shape electrode, which was located above the substrate, concentric with formation and delamination. Colloids Surf. A Physicochem. Eng. Asp. 448,34–44 the substrate grooves and the drop injection inlet hole, at a distance of (2014). 6 mm from the substrate (see Fig. 2). On the other hand, the substrate is 9. Smith, F. & Brutin, D. Wetting and spreading of human blood: Recent advances connected to negative voltage potential. and applications. Curr. Opin. Colloid Interface Sci. 36,78–83 (2018). 10. Brutin, D. et al. 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Phys. 58, 292–295 (2013). in the N while the experimental cells were connected to an exhaust port in 19. Diana, A., Castillo, M., Brutin, D. & Steinberg, T. Sessile drop wettability in normal the outer structure. At the start of the microgravity phase (100 km level) t= and reduced gravity. Microgravity Sci. Technol. 24, 195–202 (2012). 70.4 s, the experiment liquid was injected to create the first drop of HFE-7100 20. Picknett, R. & Bexon, R. The evaporation of sessile or pendant drops in still air. J. upon the heated substrate. After a delay corresponding to the estimated Colloid Interface Sci. 61, 336–350 (1977). drop evaporation time, the atmosphere in the chamber was flushed. After 21. Fuller, E. N., Ensley, K. & Giddings, J. C. Diffusion of halogenated hydrocarbons in the flushing sequence, another drop was injected, and the evaporation helium. the effect of structure on collision cross sections. J. Phys. Chem. 73, cycle with diagnostics was repeated. The outside pressure was 0 bar during 3679–3685 (1969). the microgravity period. At the bottom of the ARLES experiment module is 22. Almohammadi, H. & Amirfazli, A. Sessile drop evaporation under an electric field. an N pressure vessel for flushing the single-drop cell after each consecutive Colloids Surf. A Physicochem. Eng. Asp. 555, 580–585 (2018). drop. Flushing was performed to prevent the evaporated liquid from 23. Abdella, K., Rasmussen, H. & Inculet, I. Interfacial deformation of liquid drops by condensing in the experiment cell. The ground test experiment were electric fields at zero gravity. Comput. Math. Appl. 31,67–82 (1996). executed in the same way as during the flight. The only difference was the 24. Grishaev, V., Amirfazli, A., Chikov, S., Lyulin, Y. & Kabov, O. Study of edge effect to membrane vacuum pump, which was connected to the exhaust of the stop liquid spillage for microgravity application. Microgravity Sci. Technol. 25, module. 27–33 (2012). 25. Chauvet, F., Dehaeck, S. & Colinet, P. Threshold of bénard-marangoni instability in drying liquid films. Europhys. Lett. 99, 34001 (2012). Reporting summary Further information on research design is available in the Nature Research Reporting Summary linked to this article. DATA AVAILABILITY ACKNOWLEDGEMENTS The data collected during this study is available from the corresponding authors The present work was carried out in the framework of the European Space Agency upon reasonable request. research project AO-1999-110: EVAPORATION. We thank all of the ARLES Science Team Members for their contribution in making possible the Sounding Rocket Received: 16 July 2020; Accepted: 13 November 2020; Experiment in the framework of the ESA MASER 14 Campaign: Science Team Coordinator: Dr. C.S. Iorio, Universite Libre de Bruxelles, University of Pisa, University of Alberta, University of Darmstadt, Universite de Liège, Chinese Academy of Science, University of Edinburgh, University of Loughborough, Trinity College Dublin, and Siberian Branch Russian Academy of Science. We thank Dr. D. Mangini, Dr. B. Toth, REFERENCES and Dr. A. Verga for their interest in and support of the activities linked to ARLES and for the fruitful discussions. The research team of Professor A. Ferrari of the University 1. Leidenfrost, J. G. De aquae communis nonnullis qualitatibus tractatus (Ovenius, 1756). of Cambridge is also gratefully acknowledged for the enormous support in preparing 2. Young, T. III. An essay on the cohesion of fluids. Philos. Trans. R. Soc. Lond. 95, the fluids with nanoparticle suspensions tested both on the ground and in 65–87 (1805). 3. Fuchs, N. A. Evaporation and Droplet Growth in Gaseous Media (Pergamon, 1959). microgravity conditions. We are grateful to A.I. 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AUTHOR CONTRIBUTIONS Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. S.K. performed processing of the raw data. S.K., M.M., and D.B. contributed to the analysis of the results and writing the manuscript. P.D.M. contributed on the application of electric field on sessile drop. All authors approved the final manuscript. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give COMPETING INTERESTS appropriate credit to the original author(s) and the source, provide a link to the Creative The authors declare no competing interests. Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the ADDITIONAL INFORMATION article’s Creative Commons license and your intended use is not permitted by statutory Supplementary information is available for this paper at https://doi.org/10.1038/ regulation or exceeds the permitted use, you will need to obtain permission directly s41526-020-00128-2. A supplementary movie for Fig. 6 is available online. from the copyright holder. To view a copy of this license, visit http://creativecommons. org/licenses/by/4.0/. Correspondence and requests for materials should be addressed to S.K. or D.B. Reprints and permission information is available at http://www.nature.com/ © The Author(s) 2020 reprints npj Microgravity (2020) 37 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA

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