Deformation and Flow of a Two-Dimensional Foam under Continuous Shear

We investigate the flow properties of a 2D foam (a confined monolayer of jammed bubbles) submitted to a continuous shear in a Couette geometry. A strong localization of the flow at the moving inner wall is evidenced. Moreover, velocity fluctuations measurements reveal self-similar dynamical structures consisting of clusters of bubbles moving coherently. A stochastic model is proposed where bubbles rearrangements are activated by local stress fluctuations produced by the shearing wheel. This model gives a complete description of our observations and is also consistent with available data on granular shear bands.