Theory of Margination in Confined Multicomponent Suspensions

In blood flow, leukocytes and platelets tend to segregate near the vessel walls; this phenomenon is known as margination. Margination of leukocytes and platelets is important, for example, in physiological processes (e.g., hemostasis, response to inflammation), medical diagnostics (e.g., microfluidic cell separation) and drug delivery (the margination properties of a drug delivery particle influences its distribution in the vessel and thus its effectiveness). A mechanistic theory is developed to describe flow-induced segregation in confined multicomponent suspensions of deformable particles such as blood. The theory captures the essential features of margination by describing it in terms of two key competing processes in these systems at low Reynolds number: (1) wall-induced migration and (2) hydrodynamic pair collisions. The theory also includes the effect of physical properties of the deformable particles as well as the effect of molecular diffusion. Several regimes of segregation are identified, depending on the value of a “margination parameter”, M. Moreover, 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 hydrodynamic simulations also display this transition in suspensions where the components differ in size or flexibility. The developed mechanistic theory leads to substantial insight into the origins of margination and will help in guiding development of new technologies involving blood and other multicomponent suspensions.