Opposing Shear Induced Forces Dominate Inertial Focusing in High Reynolds Numbers

In this work we describe a novel mechanism dominating inertial focusing in high fluid velocity. Inertial focusing is currently described as the migration of particles in fluid toward equilibrium where shear and wall-induced lift forces are balanced. Recent work demonstrated focusing at Reynolds (Re) numbers > 300 in miniaturized devices that cannot be explained by current theory. We show that entry effect derived opposing shear-induced lift forces create stable traps due to local velocity minima near the center of the channel. We provide a novel framework to calculate the conditions for this new equilibrium, showing it is responsible for recently reported high velocity focusing in rectangular channels, as well as inertial focusing in curved geometries. Furthermore, we show that the new focusing mechanism in the developing flow regime enables a 10-fold miniaturization of inertial focusing devices.