A new approach to separation of a variety of microscopic and mesoscopic
objects in dilute solution is presented. The approach takes advantage of unique
properties of a specially designed separation device (sieve), which can be
readily built using already developed microlithographic techniques. Due to the
broken reflection symmetry in its design, the direction of motion of an object
in the sieve varies as a function of its self-diffusion constant, causing
separation transverse to its direction of motion. This gives the device some
significant and unique advantages over existing fractionation methods based on
centrifugation and electrophoresis.Comment: 4 pages with 3 eps figures, needs RevTeX 3.0 and epsf, also available
in postscript form http://cmtw.harvard.edu/~deniz