Excitation and evolution of spiral structures in two-dimensional dusty plasmas

Spiral waves are ubiquitous structures found in the vast area of natural and laboratory scenario. For the first time in the dusty plasma medium, we have excited nonlinear spiral waves using rotating electric fields (REFs). Both fluid and molecular-dynamics (MD) simulations have been carried out to study these waves. Characteristics of spiral waves with varying strength and frequency of REFs and neutral drags have been studied. It is found that the spiral structure gets determined by an interplay between the frequency of REF and acoustic speed of the dust medium. Comparative study of fluid and MD simulations reveal that single particle dynamical effects also play a crucial role. Interestingly in the crystalline state of dusty plasma, the spiral wavefront becomes hexagonal in shape which is understood by the difference in the phase velocity in directions associated with the crystal lattice (viz., lattice axis and lattice diagonal). Analytical modeling of the spiral wave structure has also been provided. The study carried out here has relevance in other systems such as cardiac tissue, epidemics, Belousov-Zhabotinsky reaction, coupled oscillators, and spiral galaxy.