Frequency dependent structures in an electrostatically driven powder medium

We report the existence of different phases when a composite inorganic powder salt, made up of irregularly shaped particles, is under the influence of a high electric field with variable amplitude and frequency. We show that the charged granular medium undergoes a fingering growth resulting in consequent coarsening of the fingers. The size, velocity and stability of the fingers vary with the frequency and amplitude of the electric field. We also found phases that we called a granular solid and a granular gas. We propose a phase diagram of a granular system under the influence of a high electric field.