The present invention relates to an apparatus and methods of separating particles or cells according to their sizes, wherein the size of each of the particles or cells is characterized by a corresponding
diameter. In one embodiment, the method includes the steps of providing a microchannel structure having at least one channel that is defined by a first sidewall and a second, opposite sidewall and has an insulating protrusion formed on one of the first sidewall and the second, opposite sidewall, introducing a plurality of particles or cells in a
liquid medium into the at least one channel, and generating a non-uniform electrical field in the at least one channel such that when the plurality of particles or cells passes by the insulating protrusion, the plurality of particles or cells each receives a
dielectrophoretic force proportional to its diameters, thereby being separable according to their sizes. The method further has the step of collecting particles or cells after the separation of particles or cells.