Dynamical transition induced by large bubbles in two-dimensional foam flows.

We study the dynamical behavior of a large bubble embedded in the plug flow of an ordered two-dimensional foam. At a critical velocity, the foam structure becomes instable and the large bubble migrates through the foam faster than the mean flow. This size segregation is due to viscous effects and happens only for flow velocities larger than a given threshold. We present analytical and numerical predictions for the pressure field, the velocity threshold, and the relative velocity of the large bubble. We show that the phenomenon can induce flow destabilization with dramatic effects on foam transport.