Crookes Radiometer - All-black Light Mill

All-black Light Mill

To rotate, a light mill does not have to be coated with different colors across each vane. In 2009, researchers at the University of Texas, Austin created a monocolored light mill which has four curved vanes; each vane forms a convex and a concave surface. The light mill is uniformly coated by gold nanocrystals, which are a strong light absorber. Upon exposure, due to geometric effect, the convex side of the vane receives more photon energy than the concave side does, and subsequently the gas molecules receive more heat from the convex side than from the concave side. At rough vacuum, this asymmetric heating effect generates a net gas movement across each vane, from the concave side to the convex side, as shown by the researchers' Direct Simulation Monte Carlo (DSMC) modeling. The gas movement causes the light mill to rotate with the concave side moving forward, due to Newton's Third Law. This monocolored design promotes the fabrication of micrometer- or nanometer- scaled light mills, for it is difficult to pattern materials of distinct optical properties within a very narrow, three-dimensional space.