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We present a computational investigation of the mechanism governingsize-based particle separation in microfluidic pinched flow fractionation. Westudy the behavior of particles moving through a pinching gap (i.e., aconstriction in the aperture of a channel) in the Stokes regime as a functionof particle size. The constriction aperture is created by a plane wall andspherical obstacle, and emulates the pinching segment in pinched flowfractionation devices. The simulation results show that the distance of closestapproach between the particle and obstacle surfaces (along a trajectory)decreases with increasing particle size. We then use the distance of closestapproach to investigate the effect of short-range repulsive non-hydrodynamicinteractions (e.g., solid-solid contact). We define a critical trajectory asthe one in which the minimum particle-obstacle separation is equal to the rangeof the non-hydrodynamic interactions. The results further show that the initialoffset of the critical trajectory (defined as the critical offset) increaseswith particle size. We interpret the variation of the critical offset withparticle size as the basis for size-based microfluidic separation in pinchedflow fractionation. We also compare the effect of different driving fields onthe particle trajectories; we simulate a constant force driving the particlesin a quiescent fluid as well as a freely suspended particles in apressure-driven flow. We observe that the particles driven by a constant forceapproach closer to the obstacle than those suspended in a flow (for the sameinitial offset). On the other hand, the increment in the critical offset (as afunction of particle size) is larger in the pressure-driven case than in theforce-driven case. Thus, pressure-driven particle separation using pinched flowfractionation would prove more effective than its force-driven counterpart.

Mechanism governing separation in microfluidic pinched flow fractionation devices