  We present a simple solution to enhance the separation ability of
deterministic lateral displacement (DLD) systems by expanding the
two-dimensional nature of these devices and driving the particles into
size-dependent, fully three-dimensional trajectories. Specifically, we drive
the particles through an array of long cylindrical posts, such that they not
only move in the plane perpendicular to the posts as in traditional
two-dimensional DLD systems (in-plane motion), but also along the axial
direction of the solid posts (out-of-plane motion). We show that the
(projected) in-plane motion of the particles is completely analogous to that
observed in 2D-DLD systems. In fact, a theoretical model originally developed
for force-driven, two-dimensional DLD systems accurately describes the
experimental results. More importantly, we analyze the particles out-of-plane
motion and observe that, for certain orientations of the driving force,
significant differences in the out-of-plane displacement depending on particle
size. Therefore, taking advantage of both the in-plane and out-of-plane motion
of the particles, it is possible to achieve the simultaneous fractionation of a
polydisperse suspension into multiple streams.
