Spontaneous formation and spin of particle pairs in a single-layer complex plasma crystal

Kurzfassung

In an experiment with a single-layer plasma crystal, spontaneous pairing of particles was observed upon a sudden reduction of the discharge power. The pairs were oriented vertically with the upper particle above the crystal layer and the lower particle beneath it, the pair size was about 0.2 mm. The pairs were spinning around their vertical axis with the upper particle leading and the lower one following it; the rotation speed was 10–13 Hz. Spinning particle pairs disturbed the plasma crystal through interaction with their neighbors. Upon further reduction of the discharge power, the spinning pairs proliferated in the plasma crystal and eventually it melted. The experiment was performed with micron-size polymer particles suspended in the radio-frequency (rf) argon plasma at a pressure of 157 mtorr. We propose a theoretical model of a spinning particle pair based on the plasma wake effect. Spinning particle pairs can be used as a diagnostic tool for plasma wakes or as a generic model of a 2D system of vortices.