Predicting attachment efficiency of colloid deposition under unfavorable attachment conditions

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This study presents an a priori model for predicting attachment efficiency of colloid deposition in porous media under unfavorable conditions. The model takes into account coupled effects of diffusion and hydrodynamic forces on colloid attachment efficiency that result from both primary and secondary minimum deposition. Effect of diffusion was quantified using the Maxwell approach, and influence of hydrodynamic drag was determined by comparing adhesive and hydrodynamic torques that act on the attached colloids. Main findings from this study include (1) the attachment efficiency does not change with increase of flow velocity until it reaches a critical value at which the attachment efficiency decreases as flow velocity further increases and (2) the attachment efficiency increases with increasing collector diameter when the condition that adhesive torque is greater than hydrodynamic torque is not guaranteed over the entire collector surface. Whereas the classic filtration theory only takes into account the effects of system hydrodynamics on single collector contact efficiency, we show additionally their effects on attachment efficiency. Furthermore, results of this study imply that in addition to the important role that collector size plays in the straining process, which has been the focus of many recent studies, it also influences the attachment process thus must be considered when describing colloid retention and transport in porous media under unfavorable attachment conditions.