Using electrically connected metal-coated posts in a deterministic lateral displacement (DLD) device and applying electric fields, electrokinetics is used to tune separations, significantly decrease the critical size for separation, and increase the dynamic range with switching times on the order of seconds. The strength of DLD stems from its binary behavior. To first approximation, particles move in one out of two trajectories based on their effective size. For particles that are close to the threshold size, a small external force is sufficient to nudge the particles from one trajectory to another. The devices consist of arrays of cylindrical metal-coated SU-8 posts connected by an underlying metal layer. This allows the application of voltages at the post surfaces and the generation of electric field gradients between neighboring posts, causing polarizable particles to experience a dielectrophoretic (DEP) force. This force, which depends on the volume and polarizability of the particle, can be made sufficient to push particles from one trajectory into another. In this way, the critical size in a device, normally fixed by the geometry, can be tuned. What's more, adding DEP in this way allows for the simultaneous creation of multiple size fractions.