Flow Boiling Heat Transfer Enhancement Using Tuned Geometrical Contact-Line Pinning.

Wettability patterning is a promising method to manipulate bubble dynamics in microscale boiling systems, allowing the transfer of large heat fluxes at low wall temperatures. Herein, we experimentally investigate the enhancement of flow boiling through exploitation of contact-line pinning using superbiphilic wettability patterns with a range of geometries and orientations. We compare the boiling performance on symmetrical (i.e., circular, square, and diamond-shaped) and asymmetrical (i.e., triangular) superhydrophobic patches and also create rings and chevrons through insertion of self-similar, recursive superhydrophilic cut-outs. Two main principles for boiling heat transfer enhancement are demonstrated: first, the ease of bubble departure from the superhydrophobic patches is shown to depend upon the interaction between the local contact angle and the bubble's tilt due to hydrodynamic drag; second, we find that ring-shaped superhydrophobic patches may trap droplets inside the forming bubbles, thus supplementing the heat transfer coefficient and critical heat flux through latent heat. Through application of these principles, the heat transfer coefficient and critical heat flux of heterogeneous surfaces was enhanced over the homogeneous analogues by 62% and 24%, respectively. Finally, we establish and validate a general model to estimate the ease of bubble departure through the use of geometric arguments.