Intermittent dynamics in rotating Coulomb clusters: from 2D crystals to liquid dust drops

Complex systems often experience intermittent oscillations between quiescent and highly dynamic states. This phenomenon is also observed in dusty plasmas, where micron-sized charged dust particles form crystalline structures due to the competition between a confining potential and screened Coulomb interactions between particles. Here we investigate clusters of 2-100 particles which are made to rotate applying a non-uniform magnetic field perpendicular to the plasma sheath above an electrode in an argon rf plasma. By tuning the pressure and rf power, the clusters can transition between chaotic liquid-like droplet states and ordered rotating crystals. The intermittent dynamics we observe can depend sensitively on the number of particles, and hence, the normal mode frequencies of the particle clusters. We characterize these dynamics by tracking each particle using 3D tomographic imaging, revealing information about their rotation speed, oscillation frequency, kinetic energy, and structural asymmetry. Interestingly, for certain cluster structures, intermittent switching can occur without changing the plasma environment. For small crystalline clusters (less than 20 particles), the melting often occurs at the onset of the symmetric, radial "breathing" mode oscillation (if one exists), which can parametrically couple to the cluster's vertical oscillation. For larger clusters, melting can also occur, but the breathing mode isn't necessary. We further analyze the normal mode spectra of these clusters and the stability of their structure when intermittent melting is observed.