Dynamic Measurements of Thin-Films of Boiling Bubbles Using Fluorescence Microscopy

Nucleate boiling is a heat transfer process that efficiently removes energy from a heated surface. This process is important for systems such as nuclear reactors, electronic cooling, etc., where overheating can result in system failure. During boiling, vapor bubbles nucleate from cavities on the heated surface, grow, and then depart from the surface. There is a thin (nanometers to microns) wedge-shaped film underneath the bubble, between the bubble and the heated surface; evaporation from this thin-film contributes to approximately 20% of the total heat transfer to the bubble. Understanding the heat and mass transfer process in this thin-film is critical if one needs to incorporate models for thin-film evaporation into numerical simulations. Characterization of the film thickness during the bubble growth cycle is a crucial step in this process. The thickness was quantified using fluorescence microscopy, an optical technique that measures the intensity of the emitted electromagnetic radiation, stimulated by the absorption on white light. A fluorescent dye, that emits light at a different wavelength than the incident light, was added to the test liquid. The incident light illuminated the solution from below the device through an objective lens in an inverted microscope. This excited the fluorescent molecules and produced an emission band that passed through a filter in the objective lens; this filter narrowed the receptivity of the camera. The captured images indicated the intensity of the emitted light, which is linearly proportional to the thickness of the thin-films.