Pairing of particles in two-dimensional complex plasma crystals

Two-dimensional (2D) plasma crystals are popular model systems to study various phenomena such as waves, phase transitions, and transport phenomena, in real time and at the level of individual particles. However, careful attention is required to clearly differentiate between the generic and plasma-related phenomena which are peculiar to plasma crystals. The latter, for instance, may include channeling of upstream extra particles, mode-coupling instability (MCI), or particle pairing. Vertical pairs of particles can form in a 2D plasma crystal spontaneously or after a sudden drop of the neutral gas pressure. Pairs of particles which were spinning around their axis were recently observed. Such spinning pairs, or torsions are driven by the vertical flow of ions in the plasma sheath where the particles are suspended. Their formation can be stimulated by reducing the discharge power at constant neutral gas pressure. At higher gas pressures, torsions spontaneously form below a certain power threshold. However, at lower gas pressures their formation is preceded by MCI