Microphysics of liquid complex plasmas in equilibrium and non-equilibrium systems

The dynamic evolution of the microscopic structure of solid and liquid phases of complex plasmas is studied experimentally and by means of molecular dynamics (MD) simulations. In small finite systems, the cooperative motion can be described in terms of discrete modes. These modes are studied with different experimental approaches. Using diffuse scattered laser light, applying laser tweezer forces to individual particles, and periodic laser pulses, the excitation of modes is investigated. The instantaneous normal mode analysis of experimental data from two-dimensional liquid clusters gives access to the local dynamics of the liquid phase. Our investigations shed light on the role of compressional and shear modes as well as the determination of diffusion constants and melting temperatures in finite systems. Special attention is paid to hydrodynamic situations with a stationary inhomogeneous dust flow. MD simulations allow to study the collective motion in the shell of nearest neighbors, which can be linked to smooth and sudden changes of the macroscopic flow. Finally, the observed micro-motion in all situations above allows to shed light on the preference of shear-like over compressional motion in terms of a minimized potential energy and a dynamic incompressibility

String structures in driven 3D complex-plasma clusters

The structure of driven three-dimensional complex-plasma clusters was studied experimentally. The clusters consisted of around 60 glass microspheres that were suspended in a plasma of rf discharge in argon. The particles were confined in a glass box with conductive yet transparent coating on its four side walls. This allowed manipulating the particle cluster by biasing the confining walls in a certain sequence and direct imaging of the cluster. In this work, a rotating electric field was used to drive the clusters. Depending on the field frequency, the clusters rotated (104–107 times slower than the rotating field) or remained stationary. The cluster structure was neither that of nested spherical shells nor a simple chain structure. Strings of various lengths were found consisting of 2 to 5 particles, their spatial and temporal correlations were studied. The results are compared to recent simulations