  A mechanistic theory is developed to describe segregation in confined
multicomponent suspensions such as blood. It incorporates the two key phenomena
arising in these systems at low Reynolds number: hydrodynamic pair collisions
and wall-induced migration. In simple shear flow, several regimes of
segregation arise, depending on the value of a "margination parameter" M. Most
importantly, 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 also exhibit this transition as
the size or flexibility ratio of the components changes.
