Active Janus particles in a complex plasma

Abstract

A complex plasma is a suspension of micron-size solid particles in regular plasma. Complex plasmas are excellent model systems which are used to study various plasma-specific and generic phenomena at the level of individual particles. Their advantages include the possibility of direct and real-time observation of virtually undamped dynamics of the particle suspended in a rarefied gas. Active matter is a collection of active particles, which can convert the energy coming from their environment into directed motion, therefore driving the whole system far from equilibrium. Active matter has some intriguing physical properties and potentially a number of applications in nanotechnologies, biology, and medicine. It has recently become a hot topic of multiple interdisciplinary studies. A prominent example of particles that can be active in various environments is the so-called Janus particles, which have two sides with different properties. Janus particles where two sides were made of (or coated with) different metals were previously found to be active in certain aqueous solutions. In this presentation, we report on our experiments with Janus particles - polymer microspheres where one half was plasma-coated with a thin layer of platinum - suspended in a gas discharge plasma. Our experimental setup was a modified Gaseous Electronics Conference (GEC) radio-frequency (rf) reference cell. Plasma was produced by an rf capacitively coupled discharge at 13.56 MHz in argon. The gas pressure was varied in the range of 0.66-13.3 Pa, the rf discharge power was in the range of 1-20 W. When suspended in plasma, the Janus particles moved in characteristic looped trajectories suggesting a combination of spinning and circling motion; their interactions led to the emergence of rich dynamics characterized by non-Maxwellian velocity distribution. The particle propulsion mechanism is discussed, the force driving the particle motion is identified as a combination of photophoretic force and asymmetric ion-drag force due to the plasma-particle surface interaction.