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Complex plasma research under microgravity conditions

Complex plasma research under microgravity conditions www.nature.com/npjmgrav REVIEW ARTICLE OPEN 1✉ 2 3 3 4 Markus. H. Thoma , Hubertus M. Thomas , Christina A. Knapek , Andre Melzer and Uwe Konopka The future of complex plasma research under microgravity condition, in particular on the International Space Station ISS, is discussed. First, the importance of this research and the benefit of microgravity investigations are summarized. Next, the key knowledge gaps, which could be topics of future microgravity research are identified. Here not only fundamental aspects are proposed but also important applications for lunar exploration as well as artificial intelligence technology are discussed. Finally, short, middle and long-term recommendations for complex plasma research under microgravity are given. npj Microgravity (2023) 9:13 ; https://doi.org/10.1038/s41526-023-00261-8 INTRODUCTION space stations, on planetary surfaces, etc. Extending complex plasma experiments to lunar-like dust will yield Complex plasma is a state of soft matter where microparticles are results of high importance to future space missions. immersed in a weakly ionized gas. The particles acquire a charge 5. The presence of dust plays an important role in many in the plasma that scales with the surface potential and the dust 3 4 technological processes (such as plasma deposition, micro- size and ranges to 10 –10 elementary charges for micrometer- electronic production, etching, where dust is formed during sized particles. This provides a strong Coulomb interaction and the production process), as well as in thermonuclear fusion therefore strong coupling between the microparticles and allows (where formation of radioactive and toxic dust is critical for studying gaseous, liquid and crystalline states of the particle the design of the facilities). These applications can profit arrangements as well as transitions between them on the 1–5 considerably from the fundamental knowledge gained in individual particle—the kinetic – level . For example, a plasma complex plasma experiments. crystal and the corresponding pair correlation function are shown in Fig. 1 . These unique features make complex plasmas a strongly The importance of complex plasma research is based on several interdisciplinary research field comprising, among others, con- 1–5 aspects: densed matter physics, many body physics or astrophysics . Gravity strongly affects the behavior of complex plasmas due to 1. Physical processes in complex plasmas can be studied at the the high mass of the microparticles. It forces the microparticles kinetic level: the behavior of individual microparticles can be into 2-D, quasi-2-D and stressed 3-D systems. Already this allows observed in real time using rather simple optical means. This fundamental studies of complex plasmas and the list of results is makes complex plasmas an ideal model system for the long concerning basic properties (particles charging, pair interac- investigation of statistical processes in many-particle sys- tion, waves, etc.), kinetic studies of liquids and solids (liquid-solid tems. Moreover, an analysis of the three-dimensional phase transitions in 2D and stressed 3D, 2D crystals and dynamics is accessible. crystallization dynamics, defect propagation, etc.), driven systems 2. The particle-plasma and the particle-particle interaction can (hydrodynamic instabilities, shear flow and heat transport in 2D be tuned, controlled and manipulated in various ways (e.g., systems, etc.) and anisotropic interactions (active and anisotropic by changing plasma parameters, applying external electric 1–30 particles) . However, to reveal the underlying interactions, or magnetic fields, radiation fields, optical tweezers and homogeneous and isotropic 3D arrangements of the microparti- many more). cles in the bulk plasma are required. This makes experiments in # Corresponding author (markus.h.thoma@physik.jlug.de) microgravity mandatory to explore this very special state of matter 3. Complex plasma offers the opportunity to extend the in its entirety. In Fig. 2 complex plasmas in the stressed state regime of soft matter research to a virtually undamped under gravity are compared with homogeneous, extended clouds system—complementary to the strongly damped colloidal under microgravity. systems. Due to the low charge-to-mass ratio and neutral The research on complex plasmas under microgravity condi- gas density, characteristic relaxation times in the micro- tions performed with the ISS-based laboratories PKE-Nefedov particle component are considerably stretched with respect (2001–2005), PK-3 Plus (2006–2013) and PK-4 (since 2014, see Figs. to normal condensed matter, but still far shorter than in 31–33 3 and 4) significantly contributed and still contributes to a colloids, yielding reasonable observation times. better understanding of these systems. In total about 120 scientists 4. Due to the widespread occurrence of dusty plasma media in from all over the world participated in these experiments leading nature, increasing the knowledge about it is itself of great to more than 130 peer reviewed publications. As examples, interest. Dust and dusty plasmas are ubiquitous in the microgravity research on complex plasmas has opened up new Universe. They can be found in planetary rings, cometary topics like driven complex plasmas and phase separation in binary tails, interplanetary and interstellar clouds, Earth meso- mixtures, phase transition from liquid to solid, electrorheological sphere, thunderclouds, in the vicinity of spacecrafts and 31–43 plasmas, etc. . Besides these condensed matter topics, the 1 2 I. Physics Institute, University Giessen, Heinrich-Buff-Ring 16, 35392 Giessen, Germany. DLR Institute of Materials Physics in Space, German Aerospace Center (DLR), Linder 3 4 Höhe, 51147 Köln, Germany. Institute of Physics, University Greifswald, Felix-Hausdorff-Straße 6, 17489 Greifswald, Germany. Auburn University, 380 Duncan Drive, Auburn, AL 36849, USA. email: markus.h.thoma@physik.jlug.de Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA 1234567890():,; Markus.H. Thoma et al. research also provided great progress in the understanding of This experiment is possible only under microgravity where the complex/dusty plasma specific topics, like waves, shock waves and microparticles are located in the center of the glass tube and not Mach cones produced by projectiles, the charging/decharging of close to the bottom where the electric field of the plasma sheath microparticles, the ion drag force, gelation and agglomeration of prevents the string formation. Using ISS experiments the 44–60 microparticles due to opposite charging, and many more . 3-dimensional structure of a string fluid and the propagation of Presently the complex plasma facility PK-4 (“Plasmakristallex- dust waves in an electrorheological plasma were investigated . periment #4”) is operated onboard the ISS. In contrast to its In the following, we will review future research opportunities precursors, in which an rf discharge in a cubic plasma chamber with complex plasmas under microgravity. This review is based on a white paper prepared for the European Space Agency (ESA) . was used to ignite the plasma, the plasma is produced by a dc discharge in an elongated glass tube (see Fig. 3). In addition, the PK-4 facility is equipped with various manipulation and diagnostic KEY KNOWLEDGE GAPS devices for performing many different experiments and providing New imaging technologies, current machine-learning based a profound analysis. This configuration allows in particular the (image) analysis tools as well as newly developed techniques for investigation of the liquid phase of the microparticle system, e.g., generating near-equilibrium dust clouds allow to address new, streaming of the microparticles, waves, electrorheology, turbu- relevant questions in the field of complex plasmas. The detailed lence, and viscosity. So far, 14 experiment campaigns have been investigations of these topics in 3-dimensional, homogeneous and conducted on the ISS leading already to numerous results e.g., isotropic complex plasmas are only possible under microgravity 33,41–43,49–51,61 refs. . A prominent example is the formation of an conditions. electrorheological plasma by applying an external electric field Here, we list some hot topics as examples for key knowledge 41–43 leading to the formation of microparticle strings (see Fig. 5) . gaps in the field of complex plasmas as representative (model) system for other fields of physics: 1. Thermodynamics and Statistical physics: How does “break- ing” Newton’s 3rd law affect the energy transport and temperature distribution in the system? In the presence of externally applied electric fields in complex plasmas the ions start to drift against the slow and heavy microparticles. This causes a wake potential with an interaction force that apparently leads to breaking Newton’s third law (actio = reactio) between the particles in this open system. This effect can lead to structural changes such as string formation in electrorheological plasmas (see Fig. 5). However, it is also of great interest to study the fundamental dynamical and thermodynamic properties of such a system, for which complex plasma provides an outstanding possibility. In laboratory experiments on quasi-2d systems some aspects like self-excited instabilities, non-equilibrium statistical mechanics or non-equilibrium thermodynamics have been observed and studied . Under long-term and high-quality microgravity conditions, new aspects of statis- tical physics can be studied in 3D complex plasma systems, such as the measurement of the minimum value of the shear viscosity in the strongly coupled system, the validity of the work fluctuation theorem, or the equation of state that could be derived from measured 3D velocity distributions. Fig. 1 The plasma crystal. Snapshot of a 2D complex plasma crystal 2. Phase transitions: How is the long-time dynamics in an showing about 400 particles (upper panel) and the pair correlation undercooled liquid related to structural changes taking function g(r) as a function of the inter-particle distance r (lower place close to the glass transition? The physics of under- panel). The image is inverted and its brightness is adjusted for better cooled liquids, especially near the glass transition, is one of viewing . the most controversially discussed topics in fluid Fig. 2 Complex plasma under gravity (left) and microgravity (right) conditions . The microparticle cloud is compressed by gravity (left), whereas the microparticles under microgravity (right) are distributed in the entire plasma chamber apart from a central void. npj Microgravity (2023) 13 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA 1234567890():,; Markus.H. Thoma et al. 65–67 physics . There exist a number of mutually contradictory individual particle level of thousands of particles in a 3D interpretations of different aspects of the complex behavior volume allows a unique view on these questions. of undercooled liquids. For example, at least two different 3. Hydrodynamics and Nonlinear dynamic: How do micro- scenarios of dynamical heterogeneity are known, which lead scopic interactions lead to the development of large-scale to a stretched exponential relaxation at longer time scales. nonlinear (turbulent) motion? Viscoelastic fluids, e.g., active One theory links this to the spatial heterogeneity, another to fluids, polymer solutions with high viscosity, can exhibit fluctuations in the stochastic activation process. Another turbulent behavior at very low Reynolds numbers. Hydro- very important question is: what is the dependence of the dynamic models describe the large-scale turbulent fluid structural glass transition on the spatial dimensionality (that motion, but cannot explain the underlying microscopic is whether the system is 2D or 3D)? Especially what is the origin. Complex plasmas are a powerful tool for the role of the geometrical frustration, which – as many believe investigation of fluids at “nanoscales”. Especially the – is essential for the glass transition ? Here, the ability of investigation of the transition from the collective hydro- complex plasmas to resolve slow and fast dynamics on the dynamic behavior to the dynamics of individual particles is of special broader interest. The study of shear flow, vortex 21,22,68 formation, viscoelastic properties of a wide range of coupling strength needs more fundamental investigation in the future and opens up one more interesting field of fundamental research in complex plasmas of basic interest (see Fig. 6). Additionally, a range of nonlinear wave phenomena can be studied in large 3D systems only under microgravity conditions, e.g., self-excited waves that appear when the dust density is increased above a critical value, the reflection behavior of dissipative solitary waves, or the propagation of shock waves. 4. Active and non-spherical particles in complex plasmas: How does the collective behavior of self-propelled particles connect to the energy input on the single-particle level? Anisotropic interaction can be forced due to the special shape of the particles. Rod-like particles, Janus particles with two different sides, platelets, cubes, pyramids, ellipsoids, etc. Fig. 3 The plasma chamber of the PK-4 experiment. The orange plasma glow in the glass tube of the plasma chamber and the green reflections of the illumination laser are visible. Fig. 5 String formation. The micropartcles arrange in strings in the presence of an external electric field. Fig. 4 The PK-4 container onboard the ISS. PK-4 was installed in the Columbus module by Cosmonaut Elena Serova in November Fig. 6 Turbulence in a complex plasma . Turbulent microparticle 2014 (Photo: DLR, CC BY-NC-ND 3.0). flow has been observed in a complex plasma experiment. Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA npj Microgravity (2023) 13 Markus.H. Thoma et al. 24,25 shall be mentioned here as examples . In the context of PRIORITIES FOR THE SPACE PROGRAM (MICROGRAVITY AND/ active matter, complex plasmas may reveal the formation of OR EXPLORATION RELEVANCE) various liquid and crystalline structures as well as dynamics The priority of our research program in space is a better characterized e.g., by non-Maxwellian velocity distribu- understanding of fundamental aspects of many-body physics as 69,70 tions . In complex plasmas, where the interaction described above (topics 1–4) for which complex plasma is an ideal between the particles is long-range and the damping is model system. In particular, the investigation of homogeneous 3D very low (even the particle motion can be virtually particle systems, which cannot be realized on Earth, will provide undamped), active matter might exhibit new phenomena, important new insights in fundamental questions concerning non- which again can be studied on the most fundamental—the equilibrium thermodynamics, phase transitions, the origin of kinetic level. Active matter research is in need of experi- turbulence, active matter and others. Long-term investigations on mental methods to provide realizations of large, dense, and platforms in Low-Earth-Orbit are the optimal means of choice for tuneable 3D systems, which can be achieved with a complex this research, whereas parabolic flights are useful for preparing the plasma microgravity facility. experiments. 5. Natural dusty plasmas and planetary physics: Although The second priority is natural dusty plasmas (topic 5) as mankind visited the Moon long time ago there are still a lot encountered in the lunar environment. Here parabolic flights of questions about the dust of the lunar regolith: what is under Moon gravity conditions should be considered, but also their charge in the local space plasma environment, are the fundamental dusty plasma measurements on the Moon are local electric fields building up on the surface strong necessary for detailed planning of dust mitigation. enough to transport, loft or even levitate dust, are the dust particles attracted by the surfaces of the space craft or space suits, etc.? Some of these questions need local in-situ BENEFIT FOR EARTH AND INDUSTRIAL RELEVANCE measurements on the Moon but some aspects could be The knowledge gaps to be filled are current hot topics in the studied in a plasma chamber under lunar gravity parabolic respective scientific fields, and great efforts are undertaken to flight conditions, like the charging of lunar dust simulants in answer them. The results are most relevant not only in science, but a plasma, the lofting from a surface, their levitation and in industrial (e.g., turbulent flows in pipe constructions) and collective effects of larger dust clouds. This could give medical (e.g., self-propelled motion of living cells or driven protein indications on the behavior of the dust on the Moon and filaments) applications as well, which would greatly profit from the strongly support the in-situ measurements. The investiga- expected knowledge gain. Lunar exploration missions are about to tions could be extended to dust on other planetary bodies start and knowledge on the behavior and interactions of the or in planet forming regions. Further, plasma-based electro- charged dust component on the Moon’s surface is, therefore, static methods to remove dust from surfaces, which are important and timely. relevant for human exploration missions, show great promise and might require testing in microgravity environ- ments. RECOMMENDATIONS IN SHORT, MIDDLE AND LONG TERM 6. Artificial intelligence and big data: Predictive science is in Short term the forefront of aerodynamics, fluid dynamics, solid The existing ISS facility PK-4 will be used for preliminary studies of materials under extreme conditions, geology, biology and the scientific program outlined above. The knowledge gained an increasing number of disciplines where complex from these investigations concerning thermodynamics, phase phenomena are a common theme, in spite of the fact that transitions and hydrodynamics will be the starting point for the underlying processes and physics are simple and well further dedicated experiments in future microgravity facilities. PK- understood. Data-driven and deep-learning methods are 4 is the current laboratory installed in the Columbus module of the thriving in many of these areas. Such methods have already ISS and is planned to be operated at least until the beginning of been applied to complex plasmas for structure recognition 2023. It offers certain flexible possibilities for future research in 71–76 and stereoscopy and will be used much more in data complex plasma physics, but it is, due to its special design using a analysis, classification and interpretation. The large video dc discharge plasma, not perfectly suited for the gross of topics data sets of PK-4 and successors will require new efficient mentioned above. Nevertheless, it offers a broad range of science data processing, such as machine learning. They also will topics to be studied as long as the main resources of open completely new pathways, which are quite different microparticles and gas are available and the facility is operational. from more established approaches such as human-intuition driven, first-physics-principle driven, and computationally Middle/long term driven methods. For example, machine learning models can be developed purely based on large experimental data The scientific program described above shall be considered by the 77–79 sets . Opportunities exist for predictive complex plasma development of a new facility “COMPACT” and its use on the ISS. A science in the near future. This can be important also for plasma laboratory for dedicated research topics is by definition automatic experiment control on the ISS to help obtain even always a multi-purpose and therefore a multi-user facility. The better results. This basic knowledge would be easily “COMPLEX PLASMAFACILITY” (COMPACT) could fulfill the transferable to other systems and to applications. requirements for the above-mentioned scientific topics, and even more. Its design builds upon the developments of the former These questions have not been addressed in former ISS 80,81 Ekoplasma project and should be finalized in the near future. experiments either because it was not possible, e.g., because of At the moment, a Phase A feasibility study is ongoing. In contrast missing active or non-spherical particles, or because other aspects to former microgravity experiments with complex plasmas the have been studied. Whereas PKE-Nefedov and PK-3 Plus focussed multi-purpose and multi-user facility COMPACT shall provide a on the study of the plasma crystal and related questions, PK-4 is much larger parameter space and much more flexible design used mainly to investigate the liquid phase of complex plasmas. allowing to address the key knowledge points mentioned in The design of the plasma chambers in these experiments was chapter 2 in great detail. adopted to these special applications. For the future a new The heart of the facility COMPACT will be a cylindrical capacitive RF experiment facility, called COMPACT (see below), will be designed discharge plasma chamber, called Zyflex . Capacitive RF discharges in a way to tackle the important open problems listed above. have already been used in PKE-Nefedov and PK-3 Plus. However, the npj Microgravity (2023) 13 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA Markus.H. Thoma et al. Table 1. Open fundamental scientific question for complex plasma research under microgravity. Open fundamental Future space experiments and Space relevance (importance of microgravity and/ Timeline (short, medium, long) scientific question suitable environment (LEO, or relevance for space exploration) Moon, Mars, BLEO) Thermodynamics LEO (ISS) Sedimentation - Study of large 3D systems only Medium to long (new hardware possible in micro-g necessary) Phase transitions LEO (ISS) Sedimentation - Study of large 3D systems only Medium to long (new hardware possible in micro-g necessary) Hydrodynamics LEO (ISS) Sedimentation - Study of large 3D systems only First experiments with PK-4: short; possible in micro-g Medium to long (new hardware necessary) Active particles in LEO (ISS) Sedimentation - Study of large 3D systems only Medium to long (new hardware complex plasmas possible in micro-g necessary) Natural dusty plasmas Parabolic flights (luna-g), Moon Especially dust lofting/levitation in a plasma needs Parabolic flights: short; Moon: to be investigated under lunar-g conditions on medium, long parabolic flights and directly on the Moon Zyflex design will be quite different allowing to address new 7. 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Plasmas 25, 083705 (2018). space station. Microgravity Sci. Technol. 16, 317–321 (2005). 50. Yaroshenko, V. V. et al. Excitation of low-frequency dust density waves in flowing 83. Morfill, G. E. et al. “Liquid plasmas” at the kinetic level. Phys. Rev. Lett. 92, 175004 complex plasmas. Phys. Plasmas 26, 053702 (2019). (2004). 51. Antonova, T. et al. Particle charge in PK-4 dc discharge from ground-based and microgravity experiments. Phys. Plasmas 26, 113703 (2019). 52. Ivlev, A. V. et al. Decharging of complex plasmas: First kinetic observations. Phys. ACKNOWLEDGEMENTS Rev. Lett. 90, 055003 (2003). The authors gratefully acknowledge the joint ESA-Roscosmos experiment “Plasmak- 53. Khrapak, S. A., Ivlev, A. V., Morfill & Thomas, H. M. Ion drag force in complex ristall-4” on board the International Space Station. This work was supported in part by plasmas. Phys. Rev. E 66, 046414 (2002). DLR/BMWi Grant Numbers 50WM1441, 50WM1962, 50WM2044, 50WM2161 and 54. Ivlev, A. V., Morfill, G. E. & Konopka, U. Coagulation of charged microparticles in 50WM2162. U. Konopka further thanks NASA and NSF for their support via the grants neutral gas and charge-induced gel transitions. Phys. Rev. Lett. 89, 195502 (2002). JPL-RSA-1667433 and NSF-PHY-1740784. The authors thank ESA for the opportunity 55. Konopka, U. et al. Charge-induced gelation of microparticles. N. J. Phys. 7, 227 to contribute to the white papers and this special issue. (2005). 56. Bin, L. et al. Nonlinear wave synchronization in a dusty plasma under micro- gravity on the International Space Station (ISS). IEEE Trans. Plasma Sci. 49, 3958–3962 (2021). AUTHOR CONTRIBUTIONS 57. Himpel, M. et al. Stereoscopy of dust density waves under microgravity: Velocity C.A.K., U.K., A.M., M.H.T., and H.M.T. provided contributions to the ESA White paper on distributions and phase-resolved single-particle analysis. Phys. Plasmas 21, which this review is based. M.H.T. and H.M.T. led the composition and edition of the 033703 (2014). manuscript and are considered as co-first authors. 58. Himpel, M., Killer, C., Buttenschön, B. & Melzer, A. Three-dimensional single par- ticle tracking in dense dust clouds by stereoscopy of fluorescent particles. Phys. Plasmas 19, 123704 (2012). COMPETING INTERESTS 59. Buttenschön, B., Himpel, M. & Melzer, A. Spatially resolved three-dimensional The authors declare no competing interests. particle dynamics in the void of dusty plasmas under microgravity using ste- reoscopy. N. J. Phys. 13, 023042 (2011). 60. Wolter, M., Melzer, A., Arp, O., Klindworth, M. & Piel, A. Force measurements in dusty plasmas under microgravity by means of laser manipulation. Phys. Plasmas ADDITIONAL INFORMATION 14, 123707 (2007). Correspondence and requests for materials should be addressed to Markus. H. 61. Kretschmer, M., Antonova, T., Zhdanov, S. & Thoma, M. H. Wave phenomena in a Thoma. stratified complex plasma. IEEE Trans. Plasma Sci. 44, 458–462 (2016). 62. Vailati, A. et al. ROADMAP: Soft Matter and Biophysics, https:// Reprints and permission information is available at http://www.nature.com/ esamultimedia.esa.int/docs/HRE/04_Physical_Sciences_Soft-Matter- reprints Biophysics.pdf (2021). 63. Ivlev, A. V. et al. Statistical mechanics where newton’s third law is broken. Phys. Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims Rev. X 5, 011035 (2015). in published maps and institutional affiliations. npj Microgravity (2023) 13 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA Markus.H. Thoma et al. 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Complex plasma research under microgravity conditions

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www.nature.com/npjmgrav REVIEW ARTICLE OPEN 1✉ 2 3 3 4 Markus. H. Thoma , Hubertus M. Thomas , Christina A. Knapek , Andre Melzer and Uwe Konopka The future of complex plasma research under microgravity condition, in particular on the International Space Station ISS, is discussed. First, the importance of this research and the benefit of microgravity investigations are summarized. Next, the key knowledge gaps, which could be topics of future microgravity research are identified. Here not only fundamental aspects are proposed but also important applications for lunar exploration as well as artificial intelligence technology are discussed. Finally, short, middle and long-term recommendations for complex plasma research under microgravity are given. npj Microgravity (2023) 9:13 ; https://doi.org/10.1038/s41526-023-00261-8 INTRODUCTION space stations, on planetary surfaces, etc. Extending complex plasma experiments to lunar-like dust will yield Complex plasma is a state of soft matter where microparticles are results of high importance to future space missions. immersed in a weakly ionized gas. The particles acquire a charge 5. The presence of dust plays an important role in many in the plasma that scales with the surface potential and the dust 3 4 technological processes (such as plasma deposition, micro- size and ranges to 10 –10 elementary charges for micrometer- electronic production, etching, where dust is formed during sized particles. This provides a strong Coulomb interaction and the production process), as well as in thermonuclear fusion therefore strong coupling between the microparticles and allows (where formation of radioactive and toxic dust is critical for studying gaseous, liquid and crystalline states of the particle the design of the facilities). These applications can profit arrangements as well as transitions between them on the 1–5 considerably from the fundamental knowledge gained in individual particle—the kinetic – level . For example, a plasma complex plasma experiments. crystal and the corresponding pair correlation function are shown in Fig. 1 . These unique features make complex plasmas a strongly The importance of complex plasma research is based on several interdisciplinary research field comprising, among others, con- 1–5 aspects: densed matter physics, many body physics or astrophysics . Gravity strongly affects the behavior of complex plasmas due to 1. Physical processes in complex plasmas can be studied at the the high mass of the microparticles. It forces the microparticles kinetic level: the behavior of individual microparticles can be into 2-D, quasi-2-D and stressed 3-D systems. Already this allows observed in real time using rather simple optical means. This fundamental studies of complex plasmas and the list of results is makes complex plasmas an ideal model system for the long concerning basic properties (particles charging, pair interac- investigation of statistical processes in many-particle sys- tion, waves, etc.), kinetic studies of liquids and solids (liquid-solid tems. Moreover, an analysis of the three-dimensional phase transitions in 2D and stressed 3D, 2D crystals and dynamics is accessible. crystallization dynamics, defect propagation, etc.), driven systems 2. The particle-plasma and the particle-particle interaction can (hydrodynamic instabilities, shear flow and heat transport in 2D be tuned, controlled and manipulated in various ways (e.g., systems, etc.) and anisotropic interactions (active and anisotropic by changing plasma parameters, applying external electric 1–30 particles) . However, to reveal the underlying interactions, or magnetic fields, radiation fields, optical tweezers and homogeneous and isotropic 3D arrangements of the microparti- many more). cles in the bulk plasma are required. This makes experiments in # Corresponding author (markus.h.thoma@physik.jlug.de) microgravity mandatory to explore this very special state of matter 3. Complex plasma offers the opportunity to extend the in its entirety. In Fig. 2 complex plasmas in the stressed state regime of soft matter research to a virtually undamped under gravity are compared with homogeneous, extended clouds system—complementary to the strongly damped colloidal under microgravity. systems. Due to the low charge-to-mass ratio and neutral The research on complex plasmas under microgravity condi- gas density, characteristic relaxation times in the micro- tions performed with the ISS-based laboratories PKE-Nefedov particle component are considerably stretched with respect (2001–2005), PK-3 Plus (2006–2013) and PK-4 (since 2014, see Figs. to normal condensed matter, but still far shorter than in 31–33 3 and 4) significantly contributed and still contributes to a colloids, yielding reasonable observation times. better understanding of these systems. In total about 120 scientists 4. Due to the widespread occurrence of dusty plasma media in from all over the world participated in these experiments leading nature, increasing the knowledge about it is itself of great to more than 130 peer reviewed publications. As examples, interest. Dust and dusty plasmas are ubiquitous in the microgravity research on complex plasmas has opened up new Universe. They can be found in planetary rings, cometary topics like driven complex plasmas and phase separation in binary tails, interplanetary and interstellar clouds, Earth meso- mixtures, phase transition from liquid to solid, electrorheological sphere, thunderclouds, in the vicinity of spacecrafts and 31–43 plasmas, etc. . Besides these condensed matter topics, the 1 2 I. Physics Institute, University Giessen, Heinrich-Buff-Ring 16, 35392 Giessen, Germany. DLR Institute of Materials Physics in Space, German Aerospace Center (DLR), Linder 3 4 Höhe, 51147 Köln, Germany. Institute of Physics, University Greifswald, Felix-Hausdorff-Straße 6, 17489 Greifswald, Germany. Auburn University, 380 Duncan Drive, Auburn, AL 36849, USA. email: markus.h.thoma@physik.jlug.de Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA 1234567890():,; Markus.H. Thoma et al. research also provided great progress in the understanding of This experiment is possible only under microgravity where the complex/dusty plasma specific topics, like waves, shock waves and microparticles are located in the center of the glass tube and not Mach cones produced by projectiles, the charging/decharging of close to the bottom where the electric field of the plasma sheath microparticles, the ion drag force, gelation and agglomeration of prevents the string formation. Using ISS experiments the 44–60 microparticles due to opposite charging, and many more . 3-dimensional structure of a string fluid and the propagation of Presently the complex plasma facility PK-4 (“Plasmakristallex- dust waves in an electrorheological plasma were investigated . periment #4”) is operated onboard the ISS. In contrast to its In the following, we will review future research opportunities precursors, in which an rf discharge in a cubic plasma chamber with complex plasmas under microgravity. This review is based on a white paper prepared for the European Space Agency (ESA) . was used to ignite the plasma, the plasma is produced by a dc discharge in an elongated glass tube (see Fig. 3). In addition, the PK-4 facility is equipped with various manipulation and diagnostic KEY KNOWLEDGE GAPS devices for performing many different experiments and providing New imaging technologies, current machine-learning based a profound analysis. This configuration allows in particular the (image) analysis tools as well as newly developed techniques for investigation of the liquid phase of the microparticle system, e.g., generating near-equilibrium dust clouds allow to address new, streaming of the microparticles, waves, electrorheology, turbu- relevant questions in the field of complex plasmas. The detailed lence, and viscosity. So far, 14 experiment campaigns have been investigations of these topics in 3-dimensional, homogeneous and conducted on the ISS leading already to numerous results e.g., isotropic complex plasmas are only possible under microgravity 33,41–43,49–51,61 refs. . A prominent example is the formation of an conditions. electrorheological plasma by applying an external electric field Here, we list some hot topics as examples for key knowledge 41–43 leading to the formation of microparticle strings (see Fig. 5) . gaps in the field of complex plasmas as representative (model) system for other fields of physics: 1. Thermodynamics and Statistical physics: How does “break- ing” Newton’s 3rd law affect the energy transport and temperature distribution in the system? In the presence of externally applied electric fields in complex plasmas the ions start to drift against the slow and heavy microparticles. This causes a wake potential with an interaction force that apparently leads to breaking Newton’s third law (actio = reactio) between the particles in this open system. This effect can lead to structural changes such as string formation in electrorheological plasmas (see Fig. 5). However, it is also of great interest to study the fundamental dynamical and thermodynamic properties of such a system, for which complex plasma provides an outstanding possibility. In laboratory experiments on quasi-2d systems some aspects like self-excited instabilities, non-equilibrium statistical mechanics or non-equilibrium thermodynamics have been observed and studied . Under long-term and high-quality microgravity conditions, new aspects of statis- tical physics can be studied in 3D complex plasma systems, such as the measurement of the minimum value of the shear viscosity in the strongly coupled system, the validity of the work fluctuation theorem, or the equation of state that could be derived from measured 3D velocity distributions. Fig. 1 The plasma crystal. Snapshot of a 2D complex plasma crystal 2. Phase transitions: How is the long-time dynamics in an showing about 400 particles (upper panel) and the pair correlation undercooled liquid related to structural changes taking function g(r) as a function of the inter-particle distance r (lower place close to the glass transition? The physics of under- panel). The image is inverted and its brightness is adjusted for better cooled liquids, especially near the glass transition, is one of viewing . the most controversially discussed topics in fluid Fig. 2 Complex plasma under gravity (left) and microgravity (right) conditions . The microparticle cloud is compressed by gravity (left), whereas the microparticles under microgravity (right) are distributed in the entire plasma chamber apart from a central void. npj Microgravity (2023) 13 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA 1234567890():,; Markus.H. Thoma et al. 65–67 physics . There exist a number of mutually contradictory individual particle level of thousands of particles in a 3D interpretations of different aspects of the complex behavior volume allows a unique view on these questions. of undercooled liquids. For example, at least two different 3. Hydrodynamics and Nonlinear dynamic: How do micro- scenarios of dynamical heterogeneity are known, which lead scopic interactions lead to the development of large-scale to a stretched exponential relaxation at longer time scales. nonlinear (turbulent) motion? Viscoelastic fluids, e.g., active One theory links this to the spatial heterogeneity, another to fluids, polymer solutions with high viscosity, can exhibit fluctuations in the stochastic activation process. Another turbulent behavior at very low Reynolds numbers. Hydro- very important question is: what is the dependence of the dynamic models describe the large-scale turbulent fluid structural glass transition on the spatial dimensionality (that motion, but cannot explain the underlying microscopic is whether the system is 2D or 3D)? Especially what is the origin. Complex plasmas are a powerful tool for the role of the geometrical frustration, which – as many believe investigation of fluids at “nanoscales”. Especially the – is essential for the glass transition ? Here, the ability of investigation of the transition from the collective hydro- complex plasmas to resolve slow and fast dynamics on the dynamic behavior to the dynamics of individual particles is of special broader interest. The study of shear flow, vortex 21,22,68 formation, viscoelastic properties of a wide range of coupling strength needs more fundamental investigation in the future and opens up one more interesting field of fundamental research in complex plasmas of basic interest (see Fig. 6). Additionally, a range of nonlinear wave phenomena can be studied in large 3D systems only under microgravity conditions, e.g., self-excited waves that appear when the dust density is increased above a critical value, the reflection behavior of dissipative solitary waves, or the propagation of shock waves. 4. Active and non-spherical particles in complex plasmas: How does the collective behavior of self-propelled particles connect to the energy input on the single-particle level? Anisotropic interaction can be forced due to the special shape of the particles. Rod-like particles, Janus particles with two different sides, platelets, cubes, pyramids, ellipsoids, etc. Fig. 3 The plasma chamber of the PK-4 experiment. The orange plasma glow in the glass tube of the plasma chamber and the green reflections of the illumination laser are visible. Fig. 5 String formation. The micropartcles arrange in strings in the presence of an external electric field. Fig. 4 The PK-4 container onboard the ISS. PK-4 was installed in the Columbus module by Cosmonaut Elena Serova in November Fig. 6 Turbulence in a complex plasma . Turbulent microparticle 2014 (Photo: DLR, CC BY-NC-ND 3.0). flow has been observed in a complex plasma experiment. Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA npj Microgravity (2023) 13 Markus.H. Thoma et al. 24,25 shall be mentioned here as examples . In the context of PRIORITIES FOR THE SPACE PROGRAM (MICROGRAVITY AND/ active matter, complex plasmas may reveal the formation of OR EXPLORATION RELEVANCE) various liquid and crystalline structures as well as dynamics The priority of our research program in space is a better characterized e.g., by non-Maxwellian velocity distribu- understanding of fundamental aspects of many-body physics as 69,70 tions . In complex plasmas, where the interaction described above (topics 1–4) for which complex plasma is an ideal between the particles is long-range and the damping is model system. In particular, the investigation of homogeneous 3D very low (even the particle motion can be virtually particle systems, which cannot be realized on Earth, will provide undamped), active matter might exhibit new phenomena, important new insights in fundamental questions concerning non- which again can be studied on the most fundamental—the equilibrium thermodynamics, phase transitions, the origin of kinetic level. Active matter research is in need of experi- turbulence, active matter and others. Long-term investigations on mental methods to provide realizations of large, dense, and platforms in Low-Earth-Orbit are the optimal means of choice for tuneable 3D systems, which can be achieved with a complex this research, whereas parabolic flights are useful for preparing the plasma microgravity facility. experiments. 5. Natural dusty plasmas and planetary physics: Although The second priority is natural dusty plasmas (topic 5) as mankind visited the Moon long time ago there are still a lot encountered in the lunar environment. Here parabolic flights of questions about the dust of the lunar regolith: what is under Moon gravity conditions should be considered, but also their charge in the local space plasma environment, are the fundamental dusty plasma measurements on the Moon are local electric fields building up on the surface strong necessary for detailed planning of dust mitigation. enough to transport, loft or even levitate dust, are the dust particles attracted by the surfaces of the space craft or space suits, etc.? Some of these questions need local in-situ BENEFIT FOR EARTH AND INDUSTRIAL RELEVANCE measurements on the Moon but some aspects could be The knowledge gaps to be filled are current hot topics in the studied in a plasma chamber under lunar gravity parabolic respective scientific fields, and great efforts are undertaken to flight conditions, like the charging of lunar dust simulants in answer them. The results are most relevant not only in science, but a plasma, the lofting from a surface, their levitation and in industrial (e.g., turbulent flows in pipe constructions) and collective effects of larger dust clouds. This could give medical (e.g., self-propelled motion of living cells or driven protein indications on the behavior of the dust on the Moon and filaments) applications as well, which would greatly profit from the strongly support the in-situ measurements. The investiga- expected knowledge gain. Lunar exploration missions are about to tions could be extended to dust on other planetary bodies start and knowledge on the behavior and interactions of the or in planet forming regions. Further, plasma-based electro- charged dust component on the Moon’s surface is, therefore, static methods to remove dust from surfaces, which are important and timely. relevant for human exploration missions, show great promise and might require testing in microgravity environ- ments. RECOMMENDATIONS IN SHORT, MIDDLE AND LONG TERM 6. Artificial intelligence and big data: Predictive science is in Short term the forefront of aerodynamics, fluid dynamics, solid The existing ISS facility PK-4 will be used for preliminary studies of materials under extreme conditions, geology, biology and the scientific program outlined above. The knowledge gained an increasing number of disciplines where complex from these investigations concerning thermodynamics, phase phenomena are a common theme, in spite of the fact that transitions and hydrodynamics will be the starting point for the underlying processes and physics are simple and well further dedicated experiments in future microgravity facilities. PK- understood. Data-driven and deep-learning methods are 4 is the current laboratory installed in the Columbus module of the thriving in many of these areas. Such methods have already ISS and is planned to be operated at least until the beginning of been applied to complex plasmas for structure recognition 2023. It offers certain flexible possibilities for future research in 71–76 and stereoscopy and will be used much more in data complex plasma physics, but it is, due to its special design using a analysis, classification and interpretation. The large video dc discharge plasma, not perfectly suited for the gross of topics data sets of PK-4 and successors will require new efficient mentioned above. Nevertheless, it offers a broad range of science data processing, such as machine learning. They also will topics to be studied as long as the main resources of open completely new pathways, which are quite different microparticles and gas are available and the facility is operational. from more established approaches such as human-intuition driven, first-physics-principle driven, and computationally Middle/long term driven methods. For example, machine learning models can be developed purely based on large experimental data The scientific program described above shall be considered by the 77–79 sets . Opportunities exist for predictive complex plasma development of a new facility “COMPACT” and its use on the ISS. A science in the near future. This can be important also for plasma laboratory for dedicated research topics is by definition automatic experiment control on the ISS to help obtain even always a multi-purpose and therefore a multi-user facility. The better results. This basic knowledge would be easily “COMPLEX PLASMAFACILITY” (COMPACT) could fulfill the transferable to other systems and to applications. requirements for the above-mentioned scientific topics, and even more. Its design builds upon the developments of the former These questions have not been addressed in former ISS 80,81 Ekoplasma project and should be finalized in the near future. experiments either because it was not possible, e.g., because of At the moment, a Phase A feasibility study is ongoing. In contrast missing active or non-spherical particles, or because other aspects to former microgravity experiments with complex plasmas the have been studied. Whereas PKE-Nefedov and PK-3 Plus focussed multi-purpose and multi-user facility COMPACT shall provide a on the study of the plasma crystal and related questions, PK-4 is much larger parameter space and much more flexible design used mainly to investigate the liquid phase of complex plasmas. allowing to address the key knowledge points mentioned in The design of the plasma chambers in these experiments was chapter 2 in great detail. adopted to these special applications. For the future a new The heart of the facility COMPACT will be a cylindrical capacitive RF experiment facility, called COMPACT (see below), will be designed discharge plasma chamber, called Zyflex . Capacitive RF discharges in a way to tackle the important open problems listed above. have already been used in PKE-Nefedov and PK-3 Plus. However, the npj Microgravity (2023) 13 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA Markus.H. Thoma et al. Table 1. Open fundamental scientific question for complex plasma research under microgravity. Open fundamental Future space experiments and Space relevance (importance of microgravity and/ Timeline (short, medium, long) scientific question suitable environment (LEO, or relevance for space exploration) Moon, Mars, BLEO) Thermodynamics LEO (ISS) Sedimentation - Study of large 3D systems only Medium to long (new hardware possible in micro-g necessary) Phase transitions LEO (ISS) Sedimentation - Study of large 3D systems only Medium to long (new hardware possible in micro-g necessary) Hydrodynamics LEO (ISS) Sedimentation - Study of large 3D systems only First experiments with PK-4: short; possible in micro-g Medium to long (new hardware necessary) Active particles in LEO (ISS) Sedimentation - Study of large 3D systems only Medium to long (new hardware complex plasmas possible in micro-g necessary) Natural dusty plasmas Parabolic flights (luna-g), Moon Especially dust lofting/levitation in a plasma needs Parabolic flights: short; Moon: to be investigated under lunar-g conditions on medium, long parabolic flights and directly on the Moon Zyflex design will be quite different allowing to address new 7. Homann, A., Melzer, A. & Piel, A. Measuring the charge on single particles by laser-excited resonances in plasma crystals. Phys. Rev. E 59, R3835 (1999). questionsin complex plasma physicsasthe keyknowledge gaps 8. Ratynskaia, S. et al. Experimental determination of dust-particle charge in a dis- discussed above. First of all, the accessible parameter space will be charge plasma at elevated pressures. Phys. Rev. Lett. 93, 085001 (2004). enlarged greatly concerning electron temperature (0.1–6eV com- 13 15 9. Konopka, U., Morfill, G. E. & Ratke, L. Measurement of the interaction potential of pared to 2–3 eV in former experiments), plasma density (10 –10 microsphere in the sheath of a rf discharge. Phys. Rev. Lett. 84, 891–894 (2000). −3 14 16 −3 m compared to 10 –10 m ), and neutral gas pressure 10. Zuzic, M., Thomas, H. M. & Morfill, G. E. Wave propagation and damping in plasma (0.1–100 Pa instead of 10–200 Pa). In addition, segmented electrodes crystals. J. Vac. Sci. Tech. 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E. & Konopka, U. Coagulation of charged microparticles in 50WM2162. U. Konopka further thanks NASA and NSF for their support via the grants neutral gas and charge-induced gel transitions. Phys. Rev. Lett. 89, 195502 (2002). JPL-RSA-1667433 and NSF-PHY-1740784. The authors thank ESA for the opportunity 55. Konopka, U. et al. Charge-induced gelation of microparticles. N. J. Phys. 7, 227 to contribute to the white papers and this special issue. (2005). 56. Bin, L. et al. Nonlinear wave synchronization in a dusty plasma under micro- gravity on the International Space Station (ISS). IEEE Trans. Plasma Sci. 49, 3958–3962 (2021). AUTHOR CONTRIBUTIONS 57. Himpel, M. et al. Stereoscopy of dust density waves under microgravity: Velocity C.A.K., U.K., A.M., M.H.T., and H.M.T. provided contributions to the ESA White paper on distributions and phase-resolved single-particle analysis. Phys. Plasmas 21, which this review is based. M.H.T. and H.M.T. led the composition and edition of the 033703 (2014). manuscript and are considered as co-first authors. 58. Himpel, M., Killer, C., Buttenschön, B. & Melzer, A. Three-dimensional single par- ticle tracking in dense dust clouds by stereoscopy of fluorescent particles. Phys. Plasmas 19, 123704 (2012). COMPETING INTERESTS 59. Buttenschön, B., Himpel, M. & Melzer, A. Spatially resolved three-dimensional The authors declare no competing interests. particle dynamics in the void of dusty plasmas under microgravity using ste- reoscopy. N. J. Phys. 13, 023042 (2011). 60. Wolter, M., Melzer, A., Arp, O., Klindworth, M. & Piel, A. Force measurements in dusty plasmas under microgravity by means of laser manipulation. Phys. Plasmas ADDITIONAL INFORMATION 14, 123707 (2007). Correspondence and requests for materials should be addressed to Markus. H. 61. Kretschmer, M., Antonova, T., Zhdanov, S. & Thoma, M. H. Wave phenomena in a Thoma. stratified complex plasma. IEEE Trans. Plasma Sci. 44, 458–462 (2016). 62. Vailati, A. et al. ROADMAP: Soft Matter and Biophysics, https:// Reprints and permission information is available at http://www.nature.com/ esamultimedia.esa.int/docs/HRE/04_Physical_Sciences_Soft-Matter- reprints Biophysics.pdf (2021). 63. Ivlev, A. V. et al. Statistical mechanics where newton’s third law is broken. Phys. Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims Rev. X 5, 011035 (2015). in published maps and institutional affiliations. npj Microgravity (2023) 13 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA Markus.H. Thoma et al. 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