Observations of singularity formation during the capillary collapse and bubble pinch-off of a soap film bridge

Observations of capillary-driven collapse of a soap film bridge and consequ ent bubble pinch-off are reported. According to a recent mathematical surfa ce model, in which the surface tension force is resisted only by the fluid inertia, a neck starts on the midplane between the solid supports and split s into two mirror-symmetric necks that straddle the midplane, and these nec ks "turn over" (the interface becomes multivalued), after which self-simila rity develops as the pinch-off exhibits finite time blow-up of both princip al curvatures. Singular curvatures imply an unbounded pressure and divergin g axial and, radial velocities. It is therefore anticipated that the model breaks down at a length scale near pinch-off where neglected physics become s important in the experiment. This paper examines where and how the model breaks down. Observations on length scales from 200 mm. down to 0.2 mm (tim e scales from 1.0 s down to 10(-4) s) show that the model quantitatively pr edicts behavior down to the millimeter scale below which deviations occur. These deviations are attributed to the finite thickness of the soap film wh ose inertia can be considerable in motions approaching 10 m/s. (C) 2001 Aca demic Press.