Two-bubble instabilities in quasi-two-dimensional foams

We study the effect of the experimental set-up on the structure and rheology of two-dimensional foams. We perform the same experiment, in which a sequence of topological instabilities is realized, in three different set-ups, allowing the relative merits of each system to be discussed. The experiment consists of an in-plane deformation of two bubbles which are confined laterally by bars and wholly or partially confined above and below by glass plates and liquid surfaces. An instability of the bubbles occurs when the bar spacing is increased or decreased (traction or compression) beyond a critical value. The critical values depend strongly on the experimental set-up used, and, because of the finite liquid fraction of the system, do not always agree with predictions based upon a two-dimensional analysis of a dry foam. This behaviour gives information about the discrepancies between reported experimental results on macroscopic two-dimensional foams under shear.