Effect of Surface Hydrophobicity on the Hydrodynamic Detachment of Particles from Surfaces Alexandre M. Freitas and Mukul M. Sharma* Department of Petroleum and Geosystems Engineering, University of Texas at Austin, Austin, Texas 78712 Received June 16, 1998. In Final Form: December 17, 1998 Experiments were performed to evaluate the hydrodynamic force required to detach colloidal particles from substrates by fluids in simple shear flow. The role of DLVO and acid-base interactions on the process of adhesion and removal of colloidal particles, from model surfaces, in various fluid environments was investigated. Particles and substrates with varying degrees of hydrophobicity were used. The free energy of interaction between these surfaces in a number of fluid media was calculated using the acid-base approach. Good qualitative agreement was obtained between calculated values of free energy of interaction using the acid-base approach and the critical hydrodynamic force required to detach the particles if long range electrostatic repulsion is properly accounted for. Results for a range of particles and substrates of varying hydrophobicity and for a range of solvents can be consistently explained using this approach. Introduction Dussan and Lee 1 studied the behavior of a small spherical particle initially at rest on the surface of a flat plate in a laminar boundary layer. The adhesion of carbon black particles to a glass substrate in an aqueous media and their subsequent removal by ionic surfactant solutions were studied by Clayfield and Smith 2 using a powder-bed technique. It was shown that, at certain surfactant and electrolyte concentrations, about 60% of the adherent particles could be removed with no appreciable hydro- dynamic force. The adhesion force between carbon-black particles and a cellulose film in aqueous solution was studied by Visser. 34 He also studied the adhesion of colloidal polystyrene particles to the same substrate 35 as a function of pH and ion concentration and concluded that the adhesion of the particles could be described in terms of the DLVO theory. Pokusayev et al. 3 investigated models for particle detachment from flat substrates by laminar flow and Grishin 4 studied the mechanisms of detachment of particles in turbulent flow. Hubbe 5 presented models for the detachment of colloidal particles from solid surfaces exposed to shear flow. The models are most relevant for hard, spherical particles. It was concluded that the component of hydrodynamic force acting parallel to a sheared wall is usually much larger than the lifting force. 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