Yuejun Zhao

Planar jumping-drop thermal diodes

Phase-change thermal diodes rectify heat transport much more effectively than solid-state ones, but are limited by either the gravitational orientation or one-dimensional configuration. Here, we report a planar phase-change diode scalable to large areas with an orientation-independent diodicity of over 100, in which water/vapor is enclosed by parallel superhydrophobic and superhydrophilic plates. The thermal rectification is enabled by spontaneously jumping dropwise condensate which only occurs when the superhydrophobic surface is colder than the superhydrophilic surface.

Thermocapillary actuation of binary drops on solid surfaces

On hydrophobic solid surfaces, aqueous drops are typically not conducive to thermocapillary actuation. This letter reports thermal mobilization of water drops by encapsulating them with a long-chain alcohol. On a parylene-coated silicon substrate, a water-heptanol binary drop can assume a unique shape: the dome-shaped water drop is capped by a layer of heptanol, and the heptanol cap protrudes through a “foot” to a precursor film. For intermediate drop diameters, the speed of the binary drop is linearly proportional to its diameter and the imposed temperature gradient, with an offset accounting for the hysteresis force.

Beetle Inspired Electrospray Vapor Chamber

We present the proof-of-concept for a biomimetic electrospray vapor chamber (BEVAC) which can potentially eliminate the wick structures and thermal interface materials used in conventional vapor chambers, and enable direct cooling of the backside of a microprocessor. This vapor chamber has a beetle-inspired superhydrophobic condenser with hydrophilic bumps on which condensate of the working fluid accumulates. The condensate is returned to the evaporator by electrostatic forces (electrospray atomization). We have demonstrated this novel liquid return mechanism with an open-system BEVAC prototype in which an external voltage is applied between a wickless evaporator and a microfabricated condenser with hybrid wettability.Copyright

Development of an Adaptive Vapor Chamber With Thermoresponsive Polymer Coating

We propose a novel concept for an adaptive vapor chamber using a thermoresponsive polymer coating to enhance heat transfer and reduce local thermal gradients. By coating the wick structures with stimulus-responsive polymer brushes with an upper critical solution temperature (UCST), the hotter surface becomes more wettable than the colder surface. The smaller contact angle at higher temperature generates larger capillary forces and promotes stronger return flow toward the hotspots. In this paper, we present our progress toward developing the adaptive vapor chamber. We have grafted poly(2-(meth-acryloyloxy)ethyl(dimethyl(3-sulfopropyl) ammonium hydroxide) (PMEDSAH) brushes on silica wafers, and the PMEDSAH polymer coating exhibits UCST properties with stable and tunable wettability. We have captured infrared images of the evaporator with steady and transient heating, and developed a thermographic technique that can be used to test the adaptive wick functionality in a vapor chamber.Copyright