Experimental Study of Aqueous Binary Mixture Droplet Vaporization on Nanostructured Surfaces

In this study heat transfer due to vaporization is investigated for low concentration binary mixtures of 2-propanol/water on nanostructured surfaces. The surfaces are comprised of zinc oxide (ZnO) nanocrystals grown by hydrothermal synthesis on a smooth copper substrate having an average roughness of 0.06 μm. Three nanostructured surfaces used in this study differ only in the duration of the hydrothermal synthesis consisting of 4, 10, and 24 hours of surface growth. Surface geometries were observed to be a function of hydrothermal synthesis time with an increase in area coverage, length, and diameter of nanocrystals with increase synthesis time. ZnO nanocrystals exhibit mean diameter of 500–700 nm, mean length of 1.7–3.3 μm and porosities of 0.04–0.58. Individual droplets between 2.5–3.9 mm in diameter consisting of a binary mixture of 2-propanol/water with concentration of either 0.01 M or 0.03 M were deposited at a minimum distance above the surface that would be sufficient for droplets to detach on their own due to gravity onto a nanostructured surface at temperatures between 110–140 °C. High speed video was used to record the deposition and vaporization process and through image analysis it was possible to measure heat transfer coefficients based on the wetted area, as well as other parameters. Through the video analysis it was observed that droplets which are approximately spherical, impact the surface and spread into a thin film with mean film thickness between 65–400 μm which then evaporated by film evaporation without nucleate boiling. Wettability of each of the surfaces was characterized through contact angle measurements from photographs of the droplet profile when the droplet profile was discernible. When profiles were not discernible due to hydrophilicity of some surfaces, contact angles were calculated by utilizing droplet volume and spread area. Contact angle measurements were performed on the surfaces before and after each experiment in order to document changes in wettability as a result of experimentation. Results from this experiment are compared to water droplet vaporization results from a previous experiment in order to determine whether 2-propanol enhances the heat transfer, and found that the heat transfer coefficient was increased by up to 128% in some cases. Heat transfer enhancement was found to be a function of droplet diameter as well as mixture concentration with 3.9 mm 0.01 M 2-propanol/water droplets showing larger enhancement. Potential uses of heat transfer in this application are also discussed.