In the late 1940s, Hendrik Casimir proposed that two perfectly conducting parallel plates should feel a feeble attractive force between them, due to the zero-point energy of the surrounding electromagnetic field and its dependence on the plates' positions. (See the article by Steve Lamoreaux, Physics Today, February 2007, page 40.) About a decade later, Evgeny Lifshitz and colleagues generalized Casimir's work to real conductors and dielectrics and found that the force persisted. In most cases the proposed force was still attractive, but for some configurations—a high-permittivity material and a low-permittivity material separated by a medium of intermediate permittivity—it could be repulsive. In fact, the repulsive Casimir-Lifshitz force is responsible for liquid helium's tendency to climb the walls of its container: The container repels the ambient vapor, and the liquid rises to fill the gap. Now, a group of researchers led by Harvard University's Federico Capasso have observed a repulsive Casimir-Lifshitz interaction between two solid objects, a silica surface and a 40-μm-diameter gold-coated sphere, immersed in bromobenzene. To monitor the force, they attached the sphere to an atomic force microscope cantilever and measured the cantilever's deflection using a light beam and a split-quadrant photodetector, as shown in the figure. A repulsive force of a few tens of piconewtons was measurable when the objects were brought within 40 nm of each other, and it increased as their separation decreased. The researchers suggest that the force they observed could levitate a solid within a liquid, which may lead to very low-friction sensors of force and torque. (J. N. Munday, F. Capasso, V. A. Parsegian, Nature 457, 170, 2009.) —Johanna L. Miller
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The loops and folds that result when a sheet, tape, or wire crumples are of practical and theoretical interest. Engineers want to predict how structures deform under stress; physicists want to reduce diverse crumpling behavior to a few simple principles. Toward that second aim, Norbert Stoop, Falk Wittel, and Hans Herrmann of ETH Zürich have conducted an experimental study of one elementary system: a length of metal wire stuffed from two opposing directions into a cylindrical container so shallow that the crumpling is two-dimensional. At the start of each run, the wire spanned the container in a straight line. Two counterrotating drums then pushed more and more of the wire into the container until, having bent to form a loop, the wire touched the side. What happened next, the researchers found, depended on the wire's elasticity and on the friction between the wire and the container. When friction is high, the wire adopted near-symmetric looping patterns, which the researchers termed classical. When friction is low and the wires are stiff and springy (the researchers used steel), the wire adopted spiral patterns. Floppy, soft wires (solder) adopted messy, asymmetric patterns, which the researchers termed plastic. By adjusting the elasticity and friction in their experiment, the researchers could delineate the three regimes in a morphological phase diagram. And, as the figure shows, they could reproduce the three phases with a simple continuum model. The ETH team anticipates their phase diagram could prove useful in characterizing the packing of DNA inside viral capsids and other crumpling systems. (N. Stoop, F. K. Wittel, H. J. Herrmann, Phys. Rev. Lett., in press.) ― Charles Day
