Two-dimensional foam fragmentation produced by traversing pins

We report an experimental study of the interaction of a two-dimensional liq uid foam with a round pin that moves across it. A disturbed film bends and eventually snaps, creating an extra film (tail) connected to the pin. When the pin with a bent film or tail reaches another film, bubble division or f ragmentation takes place. The deformation of the foam caused by the pin may result in a T1 topological transformation involving the tail (tail switchi ng). The foam may also just relax locally, with or without topological chan ges but without bubble division, when disturbed by the moving pin. The rela tive dimensions of the pin and bubbles, the size of the Plateau borders and the velocity of pin displacement affect foam fragmentation. A map of 'comp etitive mechanisms' displays these effects. We also show foam fragmentation curves relating the number of bubbles in a foam to the length traversed by a pin.