Filter-free separation of particles by shape

Filter-free separation of particles by shape. Mahdokht Masaeli of Dino Di Carlo’s group at UCLA and her collaborators have devised a new microfluidic method to separate micron-scale particles of the same size but different shape. The method’s novelty lies in its simplicity. Whereas other shape-separation methods involve complex geometries, arrays of baffles, or some sort of symmetry-breaking external field, the UCLA method relies on differences in how particles tumble freely as they flow through a rectangular channel. Both pairs of channel walls are close enough (a few tens of micrometers) that the flow’s transverse velocity gradient extends to the center of the channel. As the particles are carried by the flow, the velocity gradient pushes them toward the walls, while a force induced by the walls pushes them away. For rods and other particles with high aspect ratios, the outward force predominates far more than it does for spheres and...