Viscosity quantified in 2D dusty plasma experiment

Summary form only given. Polymer microparticles are introduced into a glow discharge plasma, yielding a dusty plasma. In the experiment reported here, the dusty plasma was a four-component mixture of negatively-charged microspheres, positive argon ions, electrons, and neutral argon gas. The microparticles are electrically suspended in a single layer in the sheath above a lower electrode. Due to their large electric charge, the microparticles can self-organize in a Wigner crystal. The thermal motion of these microparticles can be augmented by giving them random kicks by moving laser beams. This laser-heating method can increase the kinetic temperature for the microparticles motion so that the crystal melts, and the microparticle component of the dusty plasma then has the structure of a liquid. Here, the viscosity η of this liquid is determined experimentally using a method that, unlike in previous experiments, does not require a macroscopic shear flow. Instead, time series for the relative positions of individual particles and their velocities are used to compute a fluctuating shear stress, and then an autocorrelation of this fluctuating shear stress is used to calculate the viscosity η. This computation relies on an application of the fluctuation-dissipation theorem, called the Green-Kubo relation, which previously was used to determine viscosity only for numerical simulations, not for experiments. The experiment was performed with no macroscopic shear flow. Video microscopy allowed the tracking of individual microparticles, yielding time series for their positions and velocities, which are inputs for the Green-Kubo relation of viscosity. The resulting viscosity is in agreement with a previous experiment using a steady macroscopic shear flow.