Lateral Casimir force opens new horizons for micromachines
The Casimir force is an attraction between two bodies in vacuum. It arises because the vacuum is not an absolute emptiness, but is filled with an infinite number of virtual quanta. The usual Casimir force acts perpendicular to the surfaces. In this paper some new features of the lateral Casimir force acting along the boundary surfaces are observed for the first time. It is shown that the lateral Casimir force can be asymmetric, i.e., its profile can deviate from a perfect sine function. The symmetric lateral Casimir force acting between sinusoidally corrugated sphere and a plate was first experimentally demonstrated in 2002 by U. Mohideen and his collaborators, following the 1997 theoretical prediction by R. Golestanian and M. Kardar.
Later it was suggested to use this phenomenon in new generations of micromachines for noncontact transduction of motion without friction. Note that for half a century theorists were incapable of calculating the Casimir force precisely except in a few simplest cases. Only recently, have calculational methods been developed which are applicable to arbitrary bodies. In this work such methods are first applied to experiments and the measurements are found to be in an excellent agreement with the theory. This makes practical applications of the asymmetric lateral Casimir force straightforward.
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LM11808
Superatoms for Cheap Hydrogen Production
In the manuscript, “hydrogen production from water using aluminum-cluster catalysts,” to appear in Physical Review Letters, a multidisciplinary team of physicists, computer scientists and materials scientists introduces a unique nanotechnology-based approach to accelerate hydrogen production from water, which is of great significance for the global energy problem.
Hydrogen production by metal particles in water could provide a renewable energy cycle [A. Seinfeld, Solar Energy 78, 603 (2005); T. Yabe et al., Appl. Phys. Lett. 89, 261107 (2006)] to address the global energy problem [N. S. Lewis, Science 315, 798 (2007)]. Unfortunately, a recent study has concluded that the conventional metal-water reaction kinetics is not fast enough to make it commercially viable [J. Petrovic and G. Thomas, U.S. Department of Energy Report (2008)]. Thus the central question is how to accelerate metal-water reactions?
Here, first-principles molecular dynamics simulation at USC suggests a possible nanotechnology-based solution to this problem, in which chemical reactivity at the nanoscale is enhanced by many orders-of-magnitude compared to its macroscopic counterpart. The simulation results reveal a rapid hydrogen production mechanism by a cluster (or “superatom” pioneered by Professor Castleman’s group at Penn State and Professor Khanna’s group at Virginia Commonwealth University) consisting of a magic number of aluminum atoms, Aln (for instance, n = 12 or 17) [P. J. Roach et al., Science 323, 492 (2009)], in water. The USC-Kumamoto team has discovered a low activation-barrier mechanism, in which a pair of Lewis acid and base sites on the Aln surface preferentially catalyzes hydrogen production at room temperature. This reaction is immensely assisted by rapid proton transport in water [C. J. Wu et al., Nature Chem. 1, 57 (2009); E. Vöhringer-Martinez et al., Science 315, 497 (2007)] via a chain of hydrogen-bond switching events similar to the Grotthuss mechanism [D. Marx et al., Nature 397, 601 (1999)], which converts hydroxide ions to water molecules at the Lewis-acid sites and supplies hydrogen atoms at the Lewis-base sites. Under acidic conditions, these superatoms can continuously produce hydrogen molecules at room temperature. The reaction specificity and efficiency achieved by superatoms and the autocatalytic behavior of water presented here may be applicable to much broader applications, e.g., direct splitting of water using photocatalysts [Z. G. Zhou et al., Nature 414, 625 (2001); K. Maeda et al., Nature 440, 295 (2006)].
