THE DRAINAGE OF A THIN AQUEOUS FILM BETWEEN A SOLID-SURFACE AND AN APPROACHING GAS BUBBLE

A new optical technique is used to measure the time-dependent shape of thin aqueous draining films between air bubbles and hydrophilic or hydrophobic quartz surfaces. The thickness is determined to a precision of +/- 0.2 nm to +/- 1 nm from the intensity of the reflected interference beam from a spot of laser light which is focussed on the thinning film and which is scanned radially. The film shape begins as a dimple which then flattens as it thins. Plots of the thickness as a function of time and of radial position are presented for films of pure water and of 0.23 mol m-3 NaCl solution at hydrophilic quartz surfaces. In the ring of closest approach, the pure water film is at all times thicker than that of the salt solution film. In the early phase of the approach, the pure water film thins more slowly at the centre, but after about 40 s it thins more rapidly. Both of these observations are consistent with the expected shorter range of the double layer repulsion in the salt solution. For hydrophobic surfaces, the drainage rate is more rapid, the film eventually collapsing.