Dynamical transition in a model for dry foams

We propose a particular percolation model for the coalescence of 2-dimensio nal dry foams. Growth takes place by successive wall ruptures according to a probability proportional to the length of interface between two cells. We show that there exists a critical time t(c) where the systems undergo a ph ase transition. Analyses of the fluctuations of the order parameter and of Binder's cumulant suggest that this transition is first order. [S0031-9007( 99)08936-X].