Origin, Evolution, and Movement of Microlayer in Pool Boiling.

The microlayer thin film is visualized in situ in a vapor bubble during pool boiling. Contrary to current understanding, bubbles originate on hydrophilic and silane-coated hydrophobic surfaces without a three-phase contact line, i.e., the microlayer completely covers the bubble base. The occurrence of such a wetted bubble base is found to be dependent on the liquid-solid interaction. As the bubble grows in time, the film decreases in thickness, eventually forming the contact line and dry region. During this drying out process, curvature at the center of the microlayer shows a cyclical behavior due to competing Marangoni and capillary flows, and is characterized as a "dryout viscosity". After the dry region forms, the mechanism of contact line/microlayer movement of a single bubble on the hydrophilic surface is experimentally determined, and a generalized expression of energy required for its unpinning and movement is defined.