Rate Dependence and Role of Disorder in Linearly Sheared Two-Dimensional Foams

The shear flow of two dimensional foams is probed as a function of shear rate and disorder. Disordered foams exhibit strongly rate dependent velocity profiles, whereas ordered foams show rate independence. Both behaviors are captured quantitatively in a simple model based on the balance of the time-averaged drag forces in the foam, which are found to exhibit power-law scaling with the foam velocity and strain rate. Disorder modifies the scaling of the averaged inter-bubble drag forces, which in turn causes the observed rate dependence in disordered foams.