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 patterns with ever-increasing diameters, the spheres were transported radially outward, evacuating the central region. Reversing the sequence concentrates the spheres in the middle. Grier says that their HOT technique can create arbitrary configurations of optical traps in three dimensions and move all the traps independently under computer control. Optical peristalsis is a special case. (B. A. Koss, D. G. Grier, Appl. Phys. Lett. 82 , 3985, 2003 http://dx.doi.org/10.1063/1.1579859 .)