Particle separation in xanthan gum solutions

Label-free separation of particles by an intrinsic property can be implemented in microfluidic devices through either an externally imposed field or an inherent flow-induced force. Among the latter type of passive techniques, elastic or elasto-inertial lift-based particle separation in non-Newtonian fluids has received a rapidly growing interest in the past decade. However, current demonstrations of particle separation in non-Newtonian fluids have all taken place in viscoelastic polymer or biological solutions. We demonstrate for the first time a continuous sheath-free separation of polystyrene particles in the flow of weakly elastic xanthan gum (XG) solution through a simple straight rectangular microchannel. This separation is fundamentally different from that in the flow of viscoelastic solutions. We explain the observed particle migrations in XG solutions using the competition of a strong wall-directed (because of the strong shear thinning effect) and a small center-directed (because of the weak elasticity effect) lateral force induced by normal stresses in a Poiseuille flow.