Optical peristalsis

Part of the digestion process consists of peristalsis—the wavelike movement of powerful esophageal muscles urging food particles along the alimentary tract. Now, a similar sort of particle transport has been carried out at the nanoscopic level, using a holographic optical trapping (HOT) technique. David Grier and Brian Koss at the University of Chicago use computer-designed holograms to project up to several hundred optical traps into a volume of about 3 × 10⁵ µm³. Each trap is a symmetric potential well that can hold a small amount of matter. By repeatedly changing to different holograms whose traps are displaced but overlapping with the previous ones, the matter can be shuttled deterministically, like a bucket brigade, along preordained paths of potential wells. Parallelism is one of the technique’s strengths, as shown in this image of 1.6-µm silicon spheres dispersed in water. When subjected to a sequence of hologram...