Complex plasma with Janus particles as a model active-matter system

Abstract

Active matter is a collection of active particles, each of which can extract energy from their environment and convert it into directed motion, thereby driving the whole system far from equilibrium. Janus particles have two halves of different properties, for example hemispherical metal coating on one side of a plastic particle. When suspended in a plasma, the Janus particles acquire self-propulsion and move in characteristic looped trajectories suggesting a combination of spinning and circling motion; their interactions lead to the emergence of rich dynamics characterized by non-Maxwellian velocity distribution. The particle propulsion mechanism was identified as a combination of photophoretic force and asymmetric ion-drag force [Phys. Rev. Research 2, 033226 (2020)]. A two-dimensional complex plasma containing active Janus particles was studied experimentally [Phys. Plasmas 29, 123701 (2022)]. A single layer of micrometer-sized plastic microspheres coated on one side with a 10-nm layer of platinum was suspended in the plasma sheath above the lower rf electrode in a capacitively coupled radio-frequency (rf) discharge in argon at 13.56 MHz. Unlike a suspension consisting of regular particles only, the suspension with inclusion of Janus particles did not form an ordered lattice in the experimental conditions used. Instead, due to their self-propulsion, the Janus particles moved around with high velocities greatly exceeding the thermal speed and their suspension was in a highly disordered liquid state. We study various spatio-temporal correlations in this homogeneous isotropic active liquid state and discuss whether it has characteristics of active turbulence.