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UBC Theses and Dissertations

Capillary menisci between particles absorbed at a liquid-fluid interface Hou, Linda


One aspect of the stability of solids-stabilized emulsions was indicated by Denkov et al. (8) who reported that the capillary pressure required for an emulsion droplet’s liquid to squeeze between the interfacial particles and come into contact with another droplet’s free liquid interface must be overcome and provides a barrier to the thinning of the film between droplets. In light of the work done by Denkov et al., the objective of the thesis has been to determine the effect several factors such as particle size, separation distance, wettability, fluid properties, and contact angle hysteresis have on emulsion stability, represented in terms of capillary pressures, using a two-layer particle model for coalescence. Two general models were developed, one which is based on a uniform layer of spherical particles adsorbed on a fluid-fluid interface, and the other, a two dimensional analogue, in which parallel, horizontal cylinders are situated on the interface. Numerical techniques were applied in the solution of both models since no simplification was made by the neglect of gravity as is found in similar models (8, 11, 12). As a consequence, the models can be used to describe macroscopic systems whose characteristic dimensions are well above the micron scale. A corresponding experimental system employing parallel cylinders was then constructed and used to generate capillary pressures and meniscus profiles for comparison with the computed model results. The model results agree with the experimental findings in the literature that the smaller the particles, the closer the packing of particles, or the rougher the solids are, the more stable the emulsions. Furthermore, a decrease in the Bond number or a decrease in wettability of the particles to the disperse phase would increase stability based on capillary pressure considerations. Overall trends for the cylinders model and the spheres model were similar. The contact angles which yield optimal stability from capillary and thermodynamic considerations lie in the range of 90°≤θ≤180°. The study revealed that good agreement between the model and the experimental measurements was obtained when apparent contact angles, which take into account hysteresis, were used to generate the model profiles for each meniscus.

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