The spontaneous motion of particles, such as Robert Brown's observation of dancing pollen particles, has long been a topic of fascination and interest. In the evaporation of arsenic from a gallium arsenide surface, gallium droplets can form and migrate. Tersoff et al. (p. 236) monitored the motion of the Ga droplets as they moved back and forth during the evaporation of hundreds of surface layers. While one might expect the droplet velocity to increase with increasing temperature, instead the authors observed a temperature atrnwhich the motion of the droplets would cease, with an increase in velocity both above and below this temperature. The motion results from the interplay between surface evaporation effects and the motion of the droplets, which prevent the evaporation of the regions they are covering.
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