Interactions between small oil droplets and solid surfaces in aqueous solutions

Introduction of a fiber of appropriate material into an aqueous solution containing dispersed oil droplets may lead to deposition of small oil droplets onto the fiber surface, depending on their interactions. Such deposition process is governed by at least two types of colloidal interactions between the oil droplets and the fiber surfaces across the aqueous phase, based on the well-known Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, and possibly external force fields as well. The colloidal interactions represent the van der Waals (vdW) and the electrical double layer (EDL) interactions. This thesis focuses on understanding and experimentally determining the colloidal interactions and the deposition process.;A general theoretical method is presented to determine the retarded vdW interaction between a spherical particle and a cylinder. The detailed numerical predictions indicate that the widely used flat plate approximation may seriously overestimate the vdW interaction for the sphere-cylinder system. An approximate integral solution is also obtained for the EDL interaction between a spherical particle and a cylindrical surface. This numerical solution shows that the curvature effect of cylinder on the EDL interaction can not be neglected at smaller separation distances.;In order to determine the Hamaker constant involved in the vdW interaction, a novel contact angle technique has been developed. This experimental method determines the contact angle by analysis of the capillary profile around a cylinder (ACPAC). The contact angles as measured by the ACPAC technique agree very well with those measured by the Wilhelmy plate technique mid the axisymmetric drop shape analysis (ADSA) technique for the sessile drop case, respectively. To determine the EDL interaction, a new electrical suspension method is devised to measure the zeta-potentials of small liquid droplets dispersed in another immiscible liquid and the streaming potential technique is applied by using a parallel-plate microchannel to measure the zeta-potentials of glass surfaces in contact with an aqueous phase.;Both the analytical and numerical solutions of 1-D mass transfer equation have been obtained. The 2-D numerical model is also developed, in which both the colloidal interactions and the external force fields can be accounted for. In particular, the effects of the gravitational field and the electric field on the deposition process have been examined using the 2-D model. A systematic deposition experiment was conducted to examine the effects of the colloidal interactions on the deposition processes of silicone oil droplets onto two kinds of glass surfaces in a variety of aqueous solutions. The deposition data for the bare glass surface (hydrophilic) is in an excellent agreement with the numerical predictions of the 1-D model. However, the measured deposition rates for the FC725-precoated glass surface (hydrophobic) are found to be significantly higher than the numerical predictions. The latter differences are attributed to the attractive hydrophobic interaction.