Flow boiling is a promising method for the cooling of sensitive computational and industrial components, facilitating the transportation of large quantities of heat at near-constant temperature and in a small form factor. The prevention of vapour film formation is a fundamental challenge for the enhancement of boiling systems, and an impetus therefore exists for the discovery of new techniques to segregate nucleating bubbles during their formation. Herein, we demonstrate three methods to manipulate variables in the force balance on a vapour bubble in order to expedite its departure from the nucleation site. In the first enclosed study, we experimentally investigate the application of superbiphilic wettability patterns with a range of geometries and orientations in order to influence the...[ Read more ]

Flow boiling is a promising method for the cooling of sensitive computational and industrial components, facilitating the transportation of large quantities of heat at near-constant temperature and in a small form factor. The prevention of vapour film formation is a fundamental challenge for the enhancement of boiling systems, and an impetus therefore exists for the discovery of new techniques to segregate nucleating bubbles during their formation. Herein, we demonstrate three methods to manipulate variables in the force balance on a vapour bubble in order to expedite its departure from the nucleation site. In the first enclosed study, we experimentally investigate the application of superbiphilic wettability patterns with a range of geometries and orientations in order to influence the surface-tension forces which resist the bubble’s departure. 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 thereby: firstly, the ease of bubble departure from the superhydrophobic patches is shown to depend upon the interaction of the region with weakest contact-line pinning and the bubble’s tilt due to hydrodynamic drag, and we secondly propose 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. We establish and validate a general model to estimate the ease of bubble departure through use of geometric arguments, demonstrating that the surface-tension component of the force balance may be locally weakened through careful design of the hydrophobic patch. In the second enclosed study, we utilise the strong capillary forces generated by nanostructures to pin the liquid/vapour interface on 3-dimensional hierarchical micro/nanostructures, and thereby control the coalescence and flow interactions of developing bubbles. The drag component of the bubble’s force balance may be enhanced by this means, resulting in improved boiling performance. We demonstrate this principle on both symmetrical and asymmetrical superbiphilic microstructures, showing enhancement of peak heat transfer coefficient by 81% and 113%, respectively, when compared to the best superhydrophilic and superhydrophobic analogues. In the third enclosed study, we apply greyscale lithography to produce novel gradient microstructures, demonstrating the enhancement of flow boiling through the manipulation of drag forces upon the forming bubbles. The use of engineered 3D anisotropy allowed directional control of boiling enhancement, with respective CHF and peak HTC enhancements of up to 20 % and 93 % observed over the unstructured substrate. It is our suspicion that the sheltering of bubbles inside of cavity microstructures deteriorates their departure characteristics, while boiling is enhanced on protruding microstructures due to the combination of induced turbulence and available surfaces to capture passing liquid. Nonetheless, we demonstrate the promising implementation of novel gradient microstructures in the boiling discipline, which may serve as a useful foundation for future work. Overall, the enclosed approaches to boiling enhancement show a potential future direction for engineered boiling micro/nanostructures, wherein bubble dynamics are directly manipulated on bespoke, 3-dimensional substrates to manipulate bubble dynamics in micro-scale boiling systems, thus allowing the transfer of large heat fluxes at low wall temperatures

Keywords: Flow boiling; Contact-line pinning; Biphilic microstructure; Bubble dynamics; Silicon nanograss; Greyscale lithography; Flow-structure interactions.