HYDRODYNAMICS OF THE RUPTURE OF THIN LIQUID-FILMS

Experiments on the rupture of a thin spherical liquid film-formed from a solution of surfactant-are reported. The mean film thickness, prior to rupture, was measured by an electrical conductivity method: initial film thicknesses were of order 0.3-0.9 pm. For an unruptured film, drainage due to gravity reduced the film thickness; the films ruptured naturally at a thickness of order 0.05-0.09 pm. When the spherical film was punctured by a needle, a hole was formed, which grew rapidly, bounded by a liquid rim. As the rim moved, it collected the liquid from the film; but the rim was itself unstable, generating droplets continuously. The rim velocity, of order 10 m/s, was measured by cine photography at 2000 frames/s. Measured rim velocities compared well with a simple theoretical result derived from either (i) a force balance on the rim or (ii) an energy balance, which demonstrates that there is continuous energy dissipation due to collision between the moving rim and the elements of the stationary film. When the moving rim had swept up the whole spherical film, much of the rim had disintegrated into droplets, but the remaining rim finally converged to give an ‘implosion5generating more droplets. These droplets, together with those generated by fragmentation of the rim in flight, were collected: their number, of order 104, was measured by an image analyser, which also measured mean droplet size, of order 102pm. The total droplet area was a few per cent of the area of the original spherical film. © 1990, Cambridge University Press. All rights reserved.