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PHYSICS
Colloid science is important for applications ranging from drugs to dairy products. Less well known is that it can also illuminate basic physics questions, because in certain crucial respects, colloids behave as "big atoms." The report on page 847 of this issue by Aarts et al. (1) beautifully illustrates this approach, which was pioneered by Einstein. In particular, the results show that phenomena at the interface between a liquid and a vapor can be studied with a colloidal model.
Beginning with his doctoral thesis, Einstein showed that the incessant, random jiggling of colloidal particles known as Brownian movement was the visible manifestation of the "graininess"-the molecular nature-of the surrounding liquid. One consequence is that the density of particles as a function of height in a dilute suspension in sedimentation equilibrium is given by an equation that depends on the particle's buoyant mass, the gravitational acceleration, Boltzmann's constant, and the absolute temperature. It turns out that this equation also expresses the distribution of gas molecules in a constant-temperature atmosphere in gravity, where it is known as the barometric distribution. In other words, colloids made up of relatively large particles can behave in the same way as much smaller counterparts in the molecular world; for some purposes, colloids behave as "big atoms." Jean-Baptiste Perrin's experimental verification of the "colloidal barometric distribution" contributed toward his 1926 physics Nobel Prize and the widespread acceptance of the reality of molecules.
Today, the study of colloids is throwing new light on fundamental problems of condensed matter physics, from the kinetics of crystallization (2) to the nature of glassy states [(3, 4); see (5) for a review]. In their work, Aarts et al. (1) use colloids to study the vapor-liquid interface. At conditions far from the critical point (the temperature and pressure beyond which vapor and liquid do not exist separately), such interfaces are macroscopically flat. Microscopically, however, thermal energy...