Understanding the growth mechanism of single vapor bubble on a hydrophobic surface: Experiments under nucleate pool boiling regime

Growth and departure characteristics of single vapor bubble on hydrophobic surfaces have been investigated under saturated bulk conditions. Nucleate pool boiling experiments, with water as the working fluid, have been carried out on two sets of hydrophobic surfaces; (1) black painted ITO-coated glass substrate, and (2) ITO-coated sapphire substrate subjected to two different levels of constant heat flux conditions. High speed camera was employed to characterize the bubble dynamics. In conjunction with the high speed camera, an IR thermal imaging camera was used to simultaneously map the spatio-temporal temperature distribution of the substrate underneath the growing vapor bubble. Compared to the black paint coated glass substrate, significantly higher transmissivity of sapphire substrate in the mid-IR spectral range ensured the authenticity of the spatio-temporal accuracy of temperature measurements of the bubble nucleation site using the employed IR camera. Results on the bubble growth clearly showed that, irrespective of the substrate employed, the bubble exhibits almost zero waiting time, leaving a small residue of vapor on the substrate after the bubble departure, which further acts as a nucleus for the next bubble. Moreover, the bubble growth process was observed to be extremely slow. The IR camera-based temperature field of the hydrophobic substrates revealed that the presence of vapor bubble does change the local temperature of the substrate underneath the vapor bubble. However, it was observed that the temperature field of the nucleation site does not change with bubble growth and its departure, which, in turn, indicated towards the possibility that the microlayer ceases to exist underneath the vapor bubble. This is in contrast to the microlayer evaporation-driven growth of single vapor bubble on a hydrophilic substrate. Based on these direct experimental observations, plausible bubble growth mechanism(s) on the given hydrophobic surfaces have been predicted and discussed. (C) 2020 Elsevier Ltd. All rights reserved.