Experimental studies of two-dimensional complex plasma crystals: waves and instabilities
a Aix-Marseille Université, Aix-en-Provence, France
b Institut für Materialphysik im Weltraum, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Köln, Germany
c Joint Institute for High Temperatures, Russian Academy of Sciences, ul. Izhorskaya 13/19, Moscow, 127412, Russian Federation
d Bauman Moscow State Technical University, ul. 2-ya Baumanskaya 5/1, Moscow, 105005, Russian Federation
A review of experimental studies on waves, phonon dispersion relations and mode coupling instability in two-dimensional complex plasma crystals is presented. An improved imaging method allowing the simultaneous measurements of the three waves modes (compression in-plane, shear in-plane and out-of-plane modes) is exposed. This method is used to evidence the formation of hybrid modes and the triggering of the mode coupling instability due to wake-mediated interactions. The main stages of the mode coupling instability are analysed. In the early stage, synchronisation of the microparticle motion at the hybrid mode frequency is reported. The spatial orientation of the observed synchronisation pattern correlates well with the directions of the maximal increment of the shear-free hybrid mode. When the instability is fully developed, a melting front is formed. The propagation of the melting front has similarities with flame propagation in ordinary reactive matter. Finally, it is experimentally demonstrated that an external mechanical excitation of a stable 2D complex plasma crystal can trigger the mode coupling instability and lead to the full melting of the two-dimensional complex plasma crystal.