Physics News Update, Number 409

Number 409, January 7, 1999 by Phillip F. Schewe and Ben Stein

WHAT MAKES OBJECTS SO STICKY? Removing one's finger from a sticky surface often requires an unexpectedly large amount of work, sometimes up to 10,000 times more than simple theoretical considerations would suggest. The forces primarily involved in making objects sticky are the weak attractions between molecules known as van der Waals forces, but their effect is enhanced by mechanisms whose exact nature and role have remained a mystery. Moreover, in controlled experiments where a metallic probe is removed from a sticky polymer at a constant rate, no one has explained the observed sequence of forces, which quickly reach a peak value, then remain roughly constant before dropping to zero. Now, researchers in France (Cyprien Gay and Ludwik Leibler, CRNS-Elf Atochem, cgay@pobox.com, 011-33-147-59-1494) suggest that a combination of surface roughness and air suction effects is what makes things sticky. In their theory, air bubbles are trapped as the rough, wavy surfaces of the metallic probe and of the deformable polymer touch each other. Pulling apart the surfaces causes the bubbles to change shape. At first this creates a suction-cup effect which makes it harder to separate the surfaces (corresponding to the force peak), until air rushes in. Then, isolated bubbles connect and evolve into a network of contact points between the probe and polymer. Fractures that propagate through this network reduce the force required to separate the surfaces, and keep it at a plateau before the probe is finally removed, dropping the force to zero. Developing a more sophisticated understanding of stickiness will help researchers better design adhesives, coatings and paints. (Gay and Leibler, Physical Review Letters, 1 February 1999.)

THE TOP PHYSICS STORIES FOR 1998 were, according to us, the realization (based on observations of distant supernovas) that the cosmological expansion of the universe is not only not slowing but actually accelerating (Updates 355, 361) and the observation of neutrino oscillation (Update 375). Other highlights from last year included the mapping of the cosmic infrared background (Update 354), the localization of near-visible light (Update 356), Bose-Einstein research (Updates 362, 382, 402), progress in quantum teleportation (Update 356), the complementarity principle demonstrated for electrons (Update 362), quantum computing used to perform simple searches (Update 367), the detection of gamma rays from a high-magnetic-field pulsar (or "magnetar," Updates 374, 394), the idea of chaos- based computing (Update 389), Physics Nobel Prize (Update 396), low-field MRI (Update 398), direct observation of time-reversal asymmetry (Update 402), no end in sight for cosmic-ray energies (385), and some indication of CP violation in B meson decays (405).

FURLONGS PER FORTNIGHT is not an acceptable unit for velocity in the study of physics. This is because such mongrel units do not abet the sort of consistency needed for carrying out scientific research, which is already complicated enough. Instead the Systeme International (SI) dictates a strict code of kilograms, meters, and seconds. Nevertheless, physicists are human and (especially in the largely non-metric US) surround themselves with many non-SI units like miles-per-hour and atmospheres. Robert Romer, editor of the American Journal of Physics, argues that this is inevitable and partly desirable since we all, even scientists, continue to live in a world where BTU's, horsepower, and barrels of oil are still common parlance. (Editorial, AJP, Jan 1999.)