New technique for visualizing microboiling phenomena and its application to water pulse heated by a thin metal film

This article presents a laser strobe microscopy method for photographing fast transient microscale processes. The technique is illustrated by capturing time-lapsed images of bubble nucleation from pulse-heated thin films immersed in a pool of water. The method combines a pulsed laser for the light source and a microscope with 100x magnification of a heater element (platinum film, 30 mu m long, 15 mu m wide, and 0.2 mu m thick), and associated instrumentation to coordinate the laser pulse with the electrical pulse at various delay times to step through the evolutionary process of the phase change. The emphasis of this article is on describing the experimental method and illustrating its capabilities to record the nucleation and growth of microbubbles. Results are presented for bubble nucleation and morphology in subcooled water for electrical (heating) pulses with duration ranging from 0.50 to 1 mu s. The repeatability of the process, which is an essential requirement for obtaining meaningful information on the evolution of the phase change process, is demonstrated which allows high effective framing rates to be achieved (> 10(8) s(-1)) using a pulsed light source with a controlled delay. New bubble morphologies are shown. For 0.5 mu s pulses, bubbles appear to nucleate first at the corners of the heater followed by lateral coalescence and spreading to form a thin vapor film which later thickens on the heater footprint and evolves into an oblate spheroidal shape. For 1 mu s pulses, individual bubbles are observed which appear at specific nucleation "sites" that may indicate surface imperfections typical of nucleate boiling processes even though the superheat is close to the superheat limit of water. (c) 2006 American Institute of Physics.