Dynamic analysis of deformation and start-up process of residual-oil droplet on wall under shear flow

Abstract

This study focuses on the dynamic process of deformation and start-up of an oil droplet on a wall driven by an external shear flow. A mesoscopic numerical method is developed based on the lattice Boltzmann framework for multiphase coupled with the color-gradient model. After validations of the numerical method, we performed force analyses based on the flow field, considering the pressure difference and the shear stress inside the droplet simultaneously. It was intriguing to find that the shear stress is a driving force instead of a resistance due to the interesting phenomenon that a vortex was formed inside the droplet and the direction of reaction shear stress acted by solid wall accorded with the direction of the start-up. A force-balance model was established on the basis of the force analyses to predict the yield criteria in terms of the critical capillary number and pressure difference. Finally, by comparing the flow field before and after the start-up of the droplet, it was found that the proportion of the wall applying shear driving force decreases but the values of the shear stress increase significantly after the start-up compared to before the start-up.