Jumping-Drop Vapor Chambers and Related Phase-Change Processes

AbstractHotspot cooling is critical to the performance and reliability of electronic devices, but existing techniques are not very effective in managing mobile hotspots. We report a hotspot cooling technique based on a jumping-drop vapor chamber consisting of parallel plates of a superhydrophilic evaporator and a superhydrophobic condenser, where the working fluid is returned via the spontaneous out-of-plane jumping of condensate drops. While retaining the passive nature of traditional vapor-chamber heat spreaders (flat-plate heat pipes), the jumping-drop technique offers a mechanism to address mobile hotspots with a pathway toward effective thermal transport in the out-of-plane direction. Our jumping-drop chamber has been demonstrated to address multiple hotspots with a performance comparable to copper, despite the limitation posed by the low vapor temperature used in the demonstration.

For optimal performance of the jumping-drop vapor chamber, both its evaporator and condenser need to be optimized. To this end, we present fundamental studies related to these two components: the dryout modes of a liquid film evaporating in a microscale well, and the nucleation and removal of condensate drops on microstructures of hybrid wettability.

When a liquid film evaporates in a micro-well, the film is often pinned on the sidewall of the circular well. Prior studies have shown that the liquid film dries out in the center of the well. In addition to this center breakup mode, we have identified an annular breakup mode, where the evaporating film breaks up in an annular ring between the center and the sidewall. Our experiments suggest that the annular breakup is associated with high-aspect-ratio wells that are shallow and wide. The shallower well has a larger resistance for the liquid flow between the center and the edge, resulting in the annular breakup.

While a nanostructured superhydrophobic condenser enables self-propelled jumping removal of condensate droplets, the nearly non-wetting nanostructures present an additional thermal resistance. We propose a hybrid design integrating superhydrophobic nanostructures with hydrophobic microstructures, so as to retain the jumping removal while enhancing condensation heat transfer. We have demonstrated the proof of principle for a hybrid design, where the condensate droplets first nucleate on the hydrophobic microstructures and then jump away with the aid of the superhydrophobic nanostructures.