Flow-Induced Segregation in Confined Multicomponent Suspensions.

摘要

Flow-induced segregation is present in confined multicomponent suspensions such as blood, which is our focus here. During blood flow white blood cells and platelets segregate near the vessel walls, a phenomenon known as margination, while red blood cells tend to be depleted in the near-wall region, leading to a nonuniform distribution in flow and the formation of a depletion layer. We developed a mechanistic theory to describe these segregation phenomena in confined multicomponent suspensions. It incorporates the two key phenomena arising in these systems at low Reynolds number: hydrodynamic pair collisions and hydrodynamic migration. Two flow profiles are considered: plane Couette and plane Poiseuille flows. The model captures important phenomena such as depletion layer formation and margination. The depletion layer thickness of the primary component is predicted to follow a master curve relating it in a specific way to confinement ratio and volume fraction. In a binary suspension, several regimes of segregation arise, depending on the value of a "margination parameter" M. Most importantly, in both Couette and Poiseuille flows there is a critical value of M below which a sharp "drainage transition" occurs: one component is completely depleted from the bulk flow to the vicinity of the walls. Direct simulations corroborate this prediction.;We also investigate the effect of drag-reducing polymers (DRPs) on the drift mechanism of a single particle due to the presence of a wall. DRPs add viscoelastic properties to the flow and show significant beneficial therapeutic effects on blood circulation by increasing blood flow and tissue oxygenation. They also show promise in particle separation in medical microfluidic devices. A model is developed to describe flowing DRPs and included in direct hydrodynamic simulations of deformable capsules and polymers. Under shear flow, the presence of the wall induces a drift velocity that causes the particle to exhibit a different velocity than that of the imposed flow. A connection between this difference in velocity and the effect of DRPs is also investigated.;The presented theory and direct simulations provide substantial insights into the mechanisms behind margination and segregation phenomena in confined multicomponent suspensions.