Dynamics of yielding observed in a three-dimensional aqueous dry foam

We study the onset of yielding in stable three-dimensional dry foams following the start up of steady shear flow. By means of a charge-coupled device camera equipped with a small depth-of-field objective, we visualize the Plateau border network in the bulk of the foam. The onset of yielding is identified with the deformation ${\gamma }_c$ for which shear induced rearrangements start occurring. We show that ${\gamma }_c$ is independent of shear rate $\dot{\gamma }$ in a quasistatic regime whereas at high strain rates, a rapid increase of ${\gamma }_c$ with $\dot{\gamma }$ is observed, in qualitative agreement with theoretical models. Moreover, spatiotemporal image analyses are used to determine the velocity profile in the gap. We find that this profile remains linear up to strains far beyond ${\gamma }_c.$ Moreover, we have studied the strain history dependence of ${\gamma }_c.$