Structural transitions in crystalline foams

In this letter, the structural stability of foam is investigated experimentally. We use crystalline foams (equi-sized gas bubbles confined in a cylindrical glass tube) under conditions of a steady-state drainage where soap solution is added onto the top of the foam at a constant rate. By varying this how rate, we observe: 1) Reproducible structural transitions that are induced when drainage exceeds a critical liquid flow rate. The transition leads to a new ordered foam stable under the flow. A structural diagram is defined, showing the boundary between the two structures for the whole range of existence of the initial structure. 2) A deviation from the classical Poiseuille behavior of the draining liquid, exhibited at low flow rates, is observed. 3) We propose a simple model for the how into the foam, explaining both the deviation from the Poiseuille behavior and the stability boundary shown in the structural diagram.