Complex plasma with active Janus particles

Abstract

A two-dimensional complex plasma containing active Janus particles was studied experimentally. A single layer of micrometer-sized plastic microspheres was suspended in the plasma sheath of a radio-frequency (rf) discharge in argon at low pressure. The particle sample used was a mixture of regular particles and Janus particles, which were coated on one side with a thin layer of platinum. The experiments were carried out in a modified Gaseous Electronics Conference (GEC) rf reference cell. Plasma was produced by a capacitively coupled rf discharge in argon at 13.56 MHz. The gas pressure was 1.66 Pa, the discharge rf power was 20 W. As expected, the regular melamine-formaldehyde (MF) particles formed a two-dimensional triangular lattice (plasma crystal) in our experimental conditions. The lattice consisted of ≈ 1900 particles and was highly ordered, as evidenced by the pair correlation function for particles g(r) with the high first and split second peaks. On the contrary, the suspension of the mixed Janus particles did not crystallize. Instead, the particles energetically moved around colliding with each other. The pair correlation function for the mixed Janus particles g(r) indicates a highly disordered (gas-like) state. The mean-squared displacement MSD(t) of the mixed Janus particles scales as MSD(t) ∝ t^α with α = 2 at small times t ≪ γ^−1, where γ is the neutral-gas damping rate for the particles, indicating ballistic motion. Here, the particle inertia is important. At later times, the dynamical exponent α(t) declines, finally reaching the value of ≈ 0.56. We ascribe this to the combined effect of the Janus particle propensity to move in circular trajectories due to a combination of the photophoretic force from the illumination laser and the oppositely directed ion drag force and external confinement.