CHARACTERISATION OF THE THINNING OF SPHERICAL LIQUID FILMS BY THE EVANESCENT WAVE EFFECT

Techniques for the measurement of the thickness of liquid films are important for the basic understanding of their stability, in relation to the science and the technology of foams and emulsions. Recently [Gabrieli, 2012] a new methodology has been developed, based on an application of the optical evanescent wave phenomena, useful to evaluate the thickness at different points of a spherical liquid film. The methodology has been validated on aqueous films stabilised by SDS in a decane matrix. Such films, relevant for emulsions, remain stable for several hours. During this time the thinning behaviour of the film has been accurately characterised, showing a slow evolution of the film thickness, from micrometric dimensions at the film formation, to a few nanometres at the film breakage. Measuring the thicknesses along the azimuthal direction provides in addition a tool to investigate the gravitational drainage. The methodology offers several advantages in terms of simplicity and speed of the optical data elaboration and could be fruitfully adopted as an alternative or a complement to the classical interferometric techniques. Here we provide an overview of the working principle of the method illustrating its application with some examples. References Gabrieli R., Loglio G. et al., Spherical Cap-Shaped Emulsion Films: Thickness Evaluation at the Nanoscale Level by the Optical Evanescent Wave Effect. 2012. Submitted to Colloids and Surfaces A.