Block to granular - like transition in dense bubble flows

– We have experimentally investigated 2-dimensional dense bubble flows underneath inclined planes. Velocity profiles and velocity fluctuations have been measured. A broad second-order phase transition between two dynamical regimes is observed as a function of the tilt angle θ. For low θ values, a block motion is observed. For high θ values, the velocity profile becomes curved and a shear velocity gradient appears in the flow. Introduction. – The dense granular flow of identical particles is a complex phenomenon which involves multiple processes such as friction, correlated motion and inelastic collisions [1–3]. Various flow regimes have been identified in different experiments. Three major flow regimes are emphasized in Figure 1. Block motion occurs for example when particles form some jammed phases [4–6]. In granular flows on a pile, only the surface grains are in motion. This rolling phase has typically a thickness of 10-20 grains and the velocity v h as a function of the height h is roughly linear for that surface grains [7–9].