Doctor of Philosophy (PhD)
A liquid film of thickness h < 100 nm is subject to additional intermolecular forces, which are collectively called disjoining pressure Pi. Since Pi dominates at small film thicknesses, it determines the stability and wettability of thin films. Current theory for uniform films gives Pi = Pi(h). It becomes unbounded as h -> 0. We present a theory of curvature-dependent disjoining pressure. The new Pi depends on the curvature hxx, slope hx, and h. When this theory is implemented for Lennard-Jones liquid films, the new Pi is bounded as h -> 0. We show that this Pi captures three regimes of drop behavior (complete wetting, partial wetting, and pseudo partial wetting) without altering the signs of the long and short-range interactions. We also find that a drop with a uniform film is linearly stable, whereas a drop without a uniform film is unstable. Evaporating thin films is important in solvent coating and thin-film heat transfer. In some experiments, satellite liquid drops were observed when an evaporating film retracts. We model the evolution of a two-dimensional evaporating thin film on a heated substrate. The results show that the film thins and the film edge retracts. The thinning film forms a ridge at the edge followed by a thin neck before it returns to the uniform thickness. This profile is maintained till dry-out. The ridge breaks away to form a droplet and the remaining film keeps evolve. In this way, a series of droplets is formed with decreasing volumes. We find that the drop volume depends on the evaporation parameters and the drops have similar profiles. We explore the reasons for the similar profiles of satellite droplets. We study self-similar retraction of a step liquid film pinned at the contact line. We find a self-similar solution in which the x-coordinate is scaled with time t. We also simulate the evolution of a pinned step film and find that the film profile always approaches the self-similar solution as t -> infinity. We study the linear stability of the self-similar solution and find that the self-similar solution is stable.
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Zhang, Jin, "Theory of Curvature-Dependent Disjoining Pressure and Its Application to Liquid Films" (2006). LSU Doctoral Dissertations. 3428.