Using a static contact angle as a main experimental parameter, photographic study was carried out to investigate the dynamics of vapor bubbles during subcooled pool boiling on a vertical heated surface. Test fluid was distilled water and the experiments were performed under the atmospheric pressure; liquid subcooling was set to around 5 K. To enable clear observation of bubble behavior with a high speed camera, the experiments were conducted in an isolated bubble regime near the onset of nucleate boiling. Distinctly different bubble behaviors were observed on hydrophobic and hydrophilic surfaces: the bubbles were adhered to the surface for a long period of time when the contact angle was large while lifted off the surface within a short period of time after the nucleation when the contact angle was small. Since buoyancy does not remove the bubble from a vertical surface, mechanisms of lift-off were investigated. It was shown that the change in bubble shape induced by the surface tension force, unsteady growth force and local liquid flow induced by heterogeneous condensation around the bubble are considered to promote the bubble lift-off and the surface tension force acting on the three-phase common line prevents the lift-off. Dependences of bubble size and time at lift-off and the condensation rate on the surface wettability were also investigated.

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