MONTE-CARLO SIMULATION OF INLAYER STRUCTURE FORMATION IN THIN LIQUID-FILMS

The structure formation in free liquid films containing surfactant micelles or other colloidal particles is of fundamental importance in colloid and interface science and its applications. Experiments on thinning of single flat or curved symmetrical liquid films and asymmetrical or pseudoemulsion films formed from colloidal dispersions show that these films thin in a stepwise manner. We present here results of our grand-canonical-ensemble Monte Carlo simulations using both the hard sphere and Leonard-Jones potentials, which not only verify the presence of particle layering inside the free thinning films but also reveal for the first time that, depending on the film thickness and particle volume fraction, there exists within the particle layers, in the direction parallel to the film surfaces, an ordered 2-D hexagonal structure. The calculated, in-layer radial distribution functions show that the in-layer structure transition depends on the film thickness and the position of the layer inside the film. The sequence of the formation of microstructures in a thin film of fixed thickness with increasing particle concentration is found to be disorder --> layering --> inlayer ordering for the surface layers --> inlayer ordering for middle layers --> bulk type ordering. This ordering phenomenon in free liquid film is new. The formation of ordered structures inside the layers has practical implications for both rheology of such films and the film stability and thereby the dispersion stability.