COOPERATIVE EVAPORATION, a process whereby droplets on a substrate do not evaporate independently but in a coordinated fashion, has been observed for the first time by physicists at the University of Konstanz (Claudia Schafle, claudia.schaefle@uni-konstanz.de). The researchers begin by laying down a periodic array of diethylene glycol drops 0.75 microns in radius and spaced by 2.5 microns (see figure at Physics News Graphics). (Condensing the droplets out of a supersaturated vapor onto a patterned grid of adsorption sites imposed on the surface with microcontact-printing was itself something of a feat). The Konstanz scientists found that some rows of droplets evaporated faster than other rows, leading to a sort of "superstructure." In other words, some drops would survive at the expense of the preferential evaporation of other drops in a methodical way. Previously scientists have considered how gas sensors comprised of liquid droplet arrays could be designed. The droplet size in such sensors can be made sensitive to environmental conditions by selective uptake of certain molecules. When monitoring the average droplet size by light scattering techniques, the concentration of the molecules can be determined. But for this to work the cooperative evaporation effect will have to be taken into effect. (Schafle et al., Physical Review Letters, 20 December 1999; Select Article.)