Suppression of coalescence and of wetting: The shape of the interstitial film

The shape of the interstitial air film between noncoalescing liquids and between a drop and a solid surface is detected by means of laser interferometry. Alternative optical techniques are exploited to reveal the film profile and absolute thickness and its dynamical behavior. In particular, the thickness of the film is measured, by means of an angle shift method, in the case of a drop resting on a solid surface; further experiments, based upon reflectometry, show that stable noncoalescence may be realized both under steady and time-dependent conditions of the interfaces. A light deflection technique, coupled with image analysis, is exploited to detect the features of the film unsteadiness. The related Fourier spectra are explicitly calculated. The procedure for the determination of the film thickness is explained with the aid of a specific numerical code that is able to simulate the interference fringe patterns generated by thin films. The results presented herewith are explained in terms of theory of lubrication, exploiting the experimental data as boundary conditions that allow one to uncouple the stress–balance equations at the interfaces and the lubrication equations and solve only the latter ones.