No abstract

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.

PACS. 68.45.-v Solid-fluid interfaces - 82.70.-y Disperse systems,

We study both experimentally, with an atomic force microscope (AFM), and theoretically, using scaling arguments, the stretching of a single polyelectrolyte chain adsorbed at a planar charged surface. The main result is that the force needed to pull a monomer of the chain at a distance z from the surface reaches a plateau at distances larger than the Debye screening length of the solution.