Filter-free separation of particles by shape Free

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 low-aspect-ratio particles. The upshot is a shape gradient across the flow that can be exploited by sending the central part of the flow down one outlet and the outer part down another. In one run, Masaeli and her collaborators extracted up to 96% of spheres from a 50–50 mix of spheres and rods. Possible applications for the method include separating healthy red blood cells from diseased, misshapen ones; sorting manmade nanoparticles; and segregating single-celled organisms in various stages of cell division. Indeed, the UCLA team has confirmed that its method works on budding and nonbudding yeast cells. (M. Masaeli et al., Phys. Rev. X 2, 031017, 2012.)—Charles Day