Pattern formation in partially wet granular materials*

From sand dunes to Faraday heaping, driven granular matter, i.e., large agglomeration of macroscopic particles, is rich pattern forming system. When a granular material is partially wet (e.g., wet sand on the beach), a different pattern forming scenario arises due to the cohesive particle-particle interactions: Kink-wave fronts were found to be the dominating pattern. Here, we focus on the formation of density-wave fronts in a driven wet granular layer undergoing intermittent gas-liquid-like transition. Fronts, which are curved into a spiral shape, propagate coherently along the circular rim of the container with leading edges. They are stable beyond a critical distance from the container center. Based on the measurement of the critical distance and the rotation frequency, we propose a model for the pattern formation by considering the competition between the time scale for the collapse of cohesive particles and that of the energy injection resisting this process. Possible connections of this pattern to galaxy formation as well as to traffic jam will be discussed.