Experimental evidence of flow destabilization in a two-dimensional bidisperse foam

Liquid foam flows in a Hele-Shaw cell were investigated. The plug flow obtained for a monodisperse foam is strongly perturbed in the presence of bubbles whose sizes are larger than the average bubble size by an order of magnitude, at least. Above a velocity threshold, the large bubbles migrate faster than the mean flow. We evidence experimentally this instability and, in the case of a single large bubble, we compare the large bubble velocity to the prediction deduced from scaling arguments. In the case of a bidisperse foam, an attractive interaction between large bubbles induces segregation and the large bubbles organize themselves in columns oriented along the flow.