Pattern formation in wet granular matter under vertical vibrations

Experiments on a thin layer of cohesive wet granular matter under vertical vibrations reveal kink separated domains that collide with the container at different phases. Due to the strong cohesion arising from the formation of liquid bridges between adjacent particles, the domains move collectively upon vibrations. Depending on the periodicity of this collective motion, the kink fronts may propagate, couple with each other and form rotating spiral patterns in the case of period tripling, or stay as standing wave patterns in the case of period doubling. Moreover, both patterns may coexist with granular `gas bubbles' - phase separation into a liquidlike and a gaslike state. Stability diagrams for the instabilities measured with various granular layer mass $m$ and container height $H$ are presented. The onsets for both types of patterns and their dependency on $m$ and $H$ can be quantitatively captured with a model considering the granular layer as a single particle colliding completely inelastically with the container.