Viscoelastic Separation of Particles by Size in Straight Rectangular Microchannels: A Parametric Study for a Refined Understanding.

Microfluidic separation of particles has been implemented using a variety of force fields. We demonstrate in this work a continuous sheath-free separation of both a binary and a ternary particle mixture in viscoelastic polymer solutions through straight rectangular microchannels. This label-free separation arises from the flow-induced lift force that directs particles toward size-sensitive focusing positions in a high width/height channel. It is found to be a strong function of multiple experimental parameters, which is systematically investigated in terms of dimensionless numbers. We propose to explain the observed lateral shifting of particle focusing positions as a result of the competing center- (due to fluid elasticity effects) and wall- (due to fluid elasticity and shear thinning effects) directed elastic lift forces. The inertial lift force comes into effect at relatively high flow rates, which appears to reduce the separation efficiency and purity in our experiments.