Evaluation of heat flux around water vapor-rich microbubbles using quantitative phase imaging

Water vapor-rich micro bubbles generated by local heating of degassed water induce strong convection [1], and the maximum flow speed around the bubble can reach 1 m/s. In this study, we measure flow velocity and temperature distributions around the bubble and evaluate the amount of heat transferred by the convection. By focusing a laser onto a FeSi₂ thin films, we generated the bubble in degassed water. The flow velocity distributions around the bubble were obtained by observing the movement of polystyrene spheres suspended in the degassed water with a bright-field microscopy. Furthermore, the phase distribution of light transmitted through the water was measured using quantitative phase microscopy and converted into a temperature distribution, considering temperature-dependent changes in the refractive index of water. Comparison of flow velocity and temperature distributions revealed that hot water was jetting in a direction perpendicular to the substrate surface. Therefore, from those two distributions, we estimated the amount of the heat transferred by the convection. The results showed that about 3 mW was carried away by the convection, when the heat generation at the laser spot is 16.9 mW. Detailed evaluation such as a cooling efficiency per unit area will be explained in the presentation.

[1] K. Namura et al., Sci. Rep., 7, 45776 (2017).