Secondary nucleation in superheated water drops on a heated quartz surface

An examination of the behaviour of an initially subcooled water drop placed on a smooth quartz surface at 700 0C led to the following observation. The water boiled for about 100 ms, then nucleation ceased and the drop became static. The rate of heat transfer from the water surface therefore decreased and the water consequently acquired a considerable superheat, temperatures of up to 200 0C being measured at the wetted surface. When a further 100 ms nucleation recommenced the drop departed from the surface in an explosive manner. Evidence is provided that propagation of this explosive nucleation involves the impact on the water surface of droplets created during the rupture of an initiating vapour bubble. It is argued that this observation lends support to the hypothesis of secondary nucleation for nucleate boiling of highly superheated liquids. (author)