  We use a macromodel of a flow-driven deterministic lateral displacement (DLD)
microfluidic system to investigate conditions leading to size-separation of
suspended particles. This model system can be easily reconfigured to establish
an arbitrary orientation between the average flow field and the array of
obstacles comprising the stationary phase (forcing angle). We also investigate
the effect of obstacle size using two arrays with different obstacles but same
surface-to-surface distance between them. In all cases, we observe the presence
of a locked mode at small forcing angles, in which particles move along a
principal direction in the lattice until a locked-to-zigzag transition takes
place when the driving force reaches a critical angle. We show that the
transition occurs at increasing angles for larger particles, thus enabling
particle separation at specific forcing angles. Moreover, we observe a linear
correlation between the critical angle and the size of the particles that could
be used in the design of microfluidic systems with a fixed orientation of the
flow field. Finally, we present a simple model, based on the presence of
irreversible interactions between the suspended particles and the obstacles,
which describes the observed dependence of the migration angle on the
orientation of the average flow.
