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Number 418, March 15, 1999 by Phillip F. Schewe and Ben Stein
CHAOTIC GRANULAR MIXING has been observed for the first time. Studies of chaos in the mixing of fluids is common, but it was thought that grains mixed by a combination of steady motion and diffusion. Now an experiment by Troy Shinbrot, Fernando Muzzio, and Albert Alexander at Rutgers (shinbrot@sol.rutgers.edu, 732-445-6710), using identical (initially segregated) red and green particles in a cylindrical drum being gently tumbled, shows that grains can spontaneously interpenetrate chaotically, and the green-red interface was fractal in nature. Even more unexpected was the speed at which the interface grew in complexity---many orders of magnitude greater than expected. These results should have an impact on the mixing industry, which worries about how long and how hard to mix commodities such as pharmaceuticals, explosives, makeup, and powdered foods. (Troy Shinbrot et al., Nature, 25 February 1999; see figure at Physics News Graphics.)
PINPOINT POLYMERIZATION, in which laser light is absorbed two photons at a time within tiny volumes, can be used to turn chemical reactions on and off and to fabricate microstructures from the inside out. An Arizona-Caltech collaboration (Joseph Perry, jwperry@u.arizona.edu, 520-626-9331) has developed a new highly sensitive resin which when bombarded by intense laser light is converted into polymer, but only within the tiny micron-sized laser focus. By scanning the laser, a pattern of chemical changes is imposed on the sample. Although not exactly a holographic process since interference effects are not at work, the photo-polymerization does result in permanent changes in the local environment, such as index of refraction. The implications of this are chemical (reactions can be activated in tiny zones and not in neighboring zones), optical (rows of fluorescent binary bits can be encoded), and mechanical (microstructures can be built, including waveguides, photonic crystals, and arrays of cantilevers---see the figure at Physics News Graphics). For example, the photonic crystal (honeycomb structures which exclude or trap light at select wavelengths) was built by polymerizing some sections of the solid and then washing away the unwanted parts with solvents, a process not unlike photo-lithography except that it's done in three dimensions and with two-photon excitation. (Cumpston et al., Nature, 4 March 1999.)
WHERE DOES FRICTIONAL HEAT GO? Rub your hands together and they get warm. How does this come about, at the atomic level? Miquel Salmeron and his colleagues at LBL (510-486-6230, salmeron2stm.lbl.gov) addressed this problem by running a nanoscopic hoe (an atomic force microscope probe) through a field of pliant stalks (a monolayer of closely packed, upright alkylsilane molecules, strandlike molecules used in lubrication) self- assembled on a mica prairie. When the probe pushed harder, the contact area increased and so did the friction. The surface molecules had to tilt and to do that they had to unlock from each other, and this consumes energy, energy which is not recovered when the probe passes, allowing the molecules to untilt. This is the energy of friction. (The same AFM tip bends and images the sample; see figures at Physics News Graphics.) By adjusting the loading force of the probe, the researchers could get the molecules to tilt in discrete steps (cramped atoms displacing into new notches along the molecule, one at a time) resulting in a quantized form of friction. The exertion felt by the probe provides a measure of this energy dissipation, thus quantifying, for the first time, the direct relation between physics at the molecular level and macroscopic friction. (Barrena et al., Physical Review Letters, 5 April 1999)
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