Active turbulence in a 2D complex plasma with Janus particles

Abstract

Janus particles are polymer microspheres with hemispherical metal coating. When dispersed in plasma, they acquire self-propulsion and act as microswimmers. Such dusty plasmas with Janus particles are useful model systems of more complex active matter systems such as living organisms. A two-dimensional complex plasma containing active Janus particles was studied experimentally. The experiments were carried out in a modified Gaseous Electronics Conference (GEC) rf reference cell. Plasma was produced by a capacitively coupled radio-frequency (rf) discharge in argon at 13.56 MHz. The gas pressure was 0.66 Pa, the discharge rf power was 20 W. A single layer of micrometer-sized plastic microspheres coated on one side with a 40-nm layer of gold was suspended in the plasma sheath above the lower rf electrode. 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 (gas-like) state. Below, we address the important question of whether this apparently chaotic state has characteristics of active turbulence. A key feature of turbulence is the energy cascade with inertial interval where a power-law scaling indicates self-similarity of the velocity field. In the classical Kolmogorov theory of turbulence, the universal power-law scaling exponent of -5/3 is predicted. In our situation, the energy cascade is quantified by the power spectral density of the particle velocity as a function of the wave number PSD(k). We calculated PSD(k) as the Fourier transform of the Eulerian correlation function S2 of the Janus particles velocity. The observed power-law scaling of PSD(k) with a non-universal scaling exponent of -1.2 indicates possible presence of active turbulence in the suspension of Janus particles in plasma. Further research is necessary to find out whether it belongs to the same universality class as the inertial turbulence.