The disclosure relates to obstacle array devices (also known as bump array devices) for separating populations of particles by size. Improvements over previous obstacle array devices are realized by causing the fluid velocity profile across gaps between obstacles to be asymmetrical with respect to the plane that bisects the gap and is parallel to the direction of bulk fluid flow. Such asymmetry can be achieved by selecting the shape(s) of the obstacles bounding the gap such that the portions of the obstacles upstream from, downstream from or bridging the narrowest portion of the gap are asymmetrical with respect to that plane.Improvements are also realized by using obstacles that have sharp edges bounding the gaps. Other improvements are realized by selecting obstacle shapes such that the critical particle dimensions defined by the gaps in two different fluid flow directions differ.