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

In this contribution, we report on the experimental observation of a new type of collective excitation in 2D plasma crystals, a spinning pair of particles, which we call a “torsion”. A torsion is a pair of identical particles oriented along the flow of ions (orthogonal to the plane of the plasma crystal), which is spinning around its axis. It is a dissipative structure where energy input is provided by the flow of ions and energy dissipation is through the neutral gas friction on particles. The experiment was carried out in a modified Gaseous Electronics Conference (GEC) rf reference cell. A single layer of melamine-formaldehyde microspheres with a diameter of 8.89±0.09 μm was suspended in the plasma sheath of a capacitively coupled discharge in argon at the pressure of 157 mtorr; the discharge power was varied in the range of 5–31 W. Upon a sudden reduction of the discharge power, spontaneous pairing of particles was observed. The pairs were oriented vertically with the upper particle above the crystal layer and the lower particle beneath it, the vertical 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. They disturbed the plasma crystal through interaction with their neighbours. Upon further reduction of the discharge power, torsions proliferated in the plasma crystal and eventually it melted. We propose a theoretical model of a torsion based on the plasma wake effect. Torsions can be used as a diagnostic tool for plasma wakes or as a generic model of a 2D system of vortices