Complex plasma - Overview of strong coupling effects in 2-dimensional and 3-dimensional systems

Abstract

Complex plasma is a state of soft matter where microparticles are immersed in a weakly ionized gas. The particles acquire charges in the plasma that scales with the surface potential and the particle size and ranges to 10^3–10^4 elementary charges for micrometer sized particles. This provides a strong Coulomb interaction and therefore strong coupling between the microparticles and allows studying gaseous, liquid and crystalline states of the particle arrangements as well as transitions between them on the individual particle - the kinetic - level. The microparticles’ high mass - compared to electrons, ions and neutrals of the surrounding medium - on the one hand has the positive effect that the microparticle motion is slowed down and can be easily tracked with standard optical diagnostics. On the other hand, gravity starts to dominate over all other forces, and the microparticles sediment in the gravitational field. The particles can be levitated by a strong electric field in the sheath of an electrode system or through thermophoretic force in a strong temperature gradient. The first allows the formation of two-dimensional (2D) and compressed three-dimensional (3D) complex plasmas in the liquid and crystalline state - the plasma crystal - and the investigation of many interesting phenomena on the kinetic level like melting and crystallization, defect motion, or laser induced shear-melting. The latter allows the formation of large 3D structures in the bulk of the plasma under stress produced mainly through the gas convection within the strong temperature gradient necessary to counterbalance gravity. Only under microgravity conditions large 3D, homogeneous and isotropic complex plasma systems can be formed and investigated. Therefore, for a full understanding of complex plasmas, the complementary research under microgravity conditions, e.g. on parabolic flights, sounding rockets or the International Space Station is necessary. A longstanding program exists for this microgravity research. Especially the long-term projects PKE-Nefedov (2001–2005), PK-3 Plus (2006–2013) and PK-4 (operational since 2014) on the International Space Station (ISS) allow a deep understanding of the physics and opened up new research topics in the field, like electrorheological effects in complex plasmas, and phase separation in binary (regarding the microparticle component) mixtures. In this lecture I will present an overview of research in complex plasmas using highlight topics investigated in 2D and 3D systems.