Sorting particles by shape
Crystals of micron-sized particles underlie the iridescence of opal and structural color of butterfly wings. Researchers can artificially tune the properties of similar structures by using particles with just the right shape and size. Unfortunately, many bulk particle syntheses produce populations that are heterogeneous. Techniques already exist to separate micron-scale particles by density, volume, or charge. Now Peter Mage, Tom Soh, and collaborators at Stanford University have developed a practical approach for sorting particles purely by shape.
The new sorting technique uses a fluorescence-activated cell sorter, which is usually used for separating biological cells, to send particles single file down a channel and through a focused laser beam. Two detectors—one in line with and one perpendicular to the beam, as shown in the figure—measure the intensity of light scattered by each particle. Using those two measurements, the device identifies the particle shape based on user-defined criteria and directs the particle to the appropriate outlet.
To make different particle shapes without varying mass, volume, or refractive index, the researchers started with 6-µm-diameter polystyrene spheres and stretched some of them. The resulting ellipsoids had aspect ratios of 2.0, 3.0, and 4.5. Each type of particle had a unique scattering signature that the researchers used to define sorting criteria. If a particle’s scattering signal closely matched that for a particular shape, the particle was directed toward the appropriate outlet; otherwise, it was discarded.
The researchers increased the specificity of their sorting criteria by incorporating measurements of the particles’ transit times across the laser beam. The enhanced technique produced output with greater than 90% purity for all four shapes. Because the method relies on only the particles’ shape-dependent scattering and uses a widely available technology, it can be readily applied by other researchers to their colloidal systems. (P. L. Mage et al., Nat. Mater., 2018, doi:10.1038/s41563-018-0244-9 .)
