L. Keal, V. Lapeyre, V. Ravaine, V. Schmitt and C. Monteux
We investigate the drainage dynamics of thin liquid foam films containing PNiPAM microgels suspensions with two cross-linking densities (1.5 % or 5% mol BIS) and at two microgel concentrations (0.1 and 1% wt). For this purpose we use a thin film pressure balance apparatus that can apply a controlled and sudden hydrostatic pressure on the film and record the subsequent film thinning as a function of time. Once the film thickness has reached a stationary value, we test the adhesion between the interfaces of the film by reducing the pressure and measuring the angle between the film and the meniscus. This angle increases on reduction of pressure for adhesive films, which resist the separation of their interfaces. Non-adhesive films separate easily, and the meniscus angle stays constant. At low microgel concentration, the more densely cross-linked microgels (5% mol BIS) tend to drain into more adhesive films than the more loosely cross-linked particles (1.5% mol BIS). The adhesion results from particles that bridges the two air-water interfaces constituting the film, i.e. particles being shared between both interfaces of the films. In those cases, the film, which is initially stabilized by a bilayer of microgel particles rearrange to a state where the particles are shared by them. These results are discussed and compared with previous studies at low concentration of microgels, which have shown that emulsions stabilized with densely crosslinked microgels are more adhesive and less resistant to mechanical stresses than those obtained with lower cross-linking densities. In addition micron-scale depleted zones with no microgels are observed in the films stabilized with the 5% mol BIS particles, which eventually lead to the rupture of the film. At 1% wt, the films drain slowly, are not adhesive and have the thickness of a bilayer of microgels while at 0.1% wt, the films have the thickness of a monolayer of microgels, are adhesive and show bridging. From the thin liquid foam film thicknesses we extract a rough estimation of the radii of adsorbed particles in the thick-films before applying the pressure. Our results are consistent with particles being adsorbed in a spread conformation for the 0,1% wt sample and in a compressed conformation for the 1% wt sample. In line with previous studies on emulsions, we conclude that a larger surface coverage may reduce rearrangements, thus preventing bridging.