Thin Liquid Films - Structure and Stability

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ABSTRACT CTS9423584 Wasan, D.T. Illinois Institute of Technology The objective of this research is to examine, both experimentally and theoretically, the phenomenon of multilayered microstructuring or stratification in sub-micron thin liquid films with fluid surfaces stabilized by surfactant micelles of Brownian particles; further to explore its use as a probe for investigating order inducing long-range structural forces in colloidal dispersions. A unique feature of this continuing study lies in the use of its newly instrumented surface force balance for curved films with fluid interfaces for investigating the influence of multilayered microstructuring on film thickness stability. Surface force balance will be used in conjunction with differential interference microscopy to study symmetric foam and emulsion films as well as asymmetric films such as those between approaching small oil droplets and an air-water surface or water droplets and an air-oil surface. X-ray diffraction using synchrotron radiation from the advanced Photon Source at Argonne National Laboratory will be used to directly observe the intrinsic, in-layer structure within thin liquid films with fluid surfaces. Experiments using differential and common microinterferometric methods will be performed to obtain the local mean curvature of the film meniscus and the film/meniscus contact angles and, thereby, the film excess energy and the disjoining pressure attributable to the presence of micellar structure inside the film. It is proposed to investigate the micellar/particle structure in the region between the thin liquid film and the meniscus, i.e., inside the wedged film by using Confocal Scanning Laser Fluorescence Microscopy as well as low-angle, transmitted light diffraction technique. The experiments will be compared with our Monte Carlo and Molecular Dynamics simulations. Important technological factors affecting the film structure and stability, such as effective micelle/particle volume fraction, film size and i ts thickness, particle size, shape and polydiserpisity, will be evaluated. Systems to be investigated will include both spherical and non-spherical micelles, mixed micelles (such as mixtures of anionic and nonionic micellar solution), proteins, and differentially charged colloidal particles such as latexes and silica, and hydrocolloids such as gums and starches. This study on thin liquid film phenomena is aimed at understanding the role of surface-induced structural/depletion forces in stabilizing or destabilizing foams, emulsions, and collodical dispersions.