  We present a computational investigation of the mechanism governing
size-based particle separation in microfluidic pinched flow fractionation. We
study the behavior of particles moving through a pinching gap (i.e., a
constriction in the aperture of a channel) in the Stokes regime as a function
of particle size. The constriction aperture is created by a plane wall and
spherical obstacle, and emulates the pinching segment in pinched flow
fractionation devices. The simulation results show that the distance of closest
approach between the particle and obstacle surfaces (along a trajectory)
decreases with increasing particle size. We then use the distance of closest
approach to investigate the effect of short-range repulsive non-hydrodynamic
interactions (e.g., solid-solid contact). We define a critical trajectory as
the one in which the minimum particle-obstacle separation is equal to the range
of the non-hydrodynamic interactions. The results further show that the initial
offset of the critical trajectory (defined as the critical offset) increases
with particle size. We interpret the variation of the critical offset with
particle size as the basis for size-based microfluidic separation in pinched
flow fractionation. We also compare the effect of different driving fields on
the particle trajectories; we simulate a constant force driving the particles
in a quiescent fluid as well as a freely suspended particles in a
pressure-driven flow. We observe that the particles driven by a constant force
approach closer to the obstacle than those suspended in a flow (for the same
initial offset). On the other hand, the increment in the critical offset (as a
function of particle size) is larger in the pressure-driven case than in the
force-driven case. Thus, pressure-driven particle separation using pinched flow
fractionation would prove more effective than its force-driven counterpart.
