Exploring the origin of self-induced vertical oscillations of dust particles in a plasma

Micron-size charged particles can be easily levitated in a low-density plasma environment. Such "dusty plasmas" are often used to investigate traditional condensed matter and statistical physics at the single-particle level. We have recently observed a novel phenomenon where hundreds of particles can switch between crystalline and gas-like states over minutes-long time scales (Gogia et al., PRL, 2017). The constituent-level source of energy for this "active matter" system is sustained, large-amplitude vertical oscillations of the individual particles. Delayed charging, charge fluctuations, and variations in the plasma number density have been previously invoked to explain such behavior, however, we show that these mechanisms are unlikely to drive the oscillations we observe. Langmuir probe measurements suggest the plasma environment around the grains is time-invariant. Furthermore, grains carry charges on the order of 10^4 electrons, suggesting that square-root N fluctuations would be too small to drive such large amplitude oscillations. We hypothesize that particle oscillations arise not from electrostatic effects, but from a complicated interaction between the ion wakes streaming out in the lee of the particles and the underlying electrode at low pressures.