Dynamics of rotating spirals in agitated wet granular matter

Pattern formation of a thin layer of vertically agitated wet granular matter is investigated experimentally. Due to the strong cohesion arising from the capillary bridges formed between adjacent particles, agitated wet granular matter exhibits a different scenario compared with cohesionless dry particles. Rotating spirals with three arms, which correspond to the kinks between regions with different colliding phases with the vibrating plate, have been found to be the dominating pattern. From both top view snapshots and laser profilometry methods, the rotation frequency of the spiral arms is characterized with image processing procedures. Both methods reveal that there exists a finite rotation frequency $\nu_{\rm r}$ at a threshold vibration acceleration, above which $\nu_{\rm r}$ increases linearly with the peak vibration acceleration with a slope strongly dependent on the vibration frequency.