From Dynamical Superhydrophobicity to Thermal Diodes
The interaction between liquid drops and textured surfaces not only offers fundamental challenges in capillarity and wetting, but also enables new applications ranging from self-cleaning materials to self-sustaining condensers. The first part of this dissertation deals with the fundamental wetting and dewetting dynamics of drops on textured surfaces, and the self-propelled jumping of dropwise condensate on superhydrophobic surfaces. The second part builds upon these findings in dynamical superhydrophobicity to develop a jumping-drop thermal diode that rectifies heat flow between textured superhydrophilic and superhydrophobic surfaces.
On the fundamental side, anti-dew is an essential property of robust superhydrophobic surfaces, particularly those deployed in ambient environments or phase-change systems. A superhydrophobic lotus leaf retains water repellency after repeated condensation in nature but becomes sticky to water drops after condensation on a fixed cold plate. To solve this mystery, we first study the possible wetting states of superhydrophobic surfaces possessing two-tier surface roughness mimicking that on the lotus leaf. By incrementally increasing the ethanol concentration of water/ethanol drops, two distinct wetting transitions are observed on two-tier surfaces. Drops in the intermediate wetting state uniformly wet the microscale roughness but not the nanoscale roughness. Dew drops exhibited a similar intermediate wetting state. Our experiments show that mechanical vibration can be used to overcome the energy barrier for transition from the intermediate wetting (Partial Wenzel) state to the fully dewetted (Cassie) state, and the threshold for the dewetting transition follows a scaling law comparing the kinetic energy imparted to the drop with the work of adhesion.
Although vibration-induced dewetting is effective for removing millimetric condensate from the surface, micrometric condensate cannot be removed as surface energy dominates at small scales. We report a new discovery in which the micrometric condensate can spontaneously dewet and jump off the superhydrophobic surface. The spontaneous jumping results from the surface energy released upon drop coalescence, which leads to the rapid out-of-plane jumping motion of the coalesced drops. The jumping drops follow an inertial-capillary scaling and give rise to self-sustained dropwise condensation with a micrometric average diameter. Using two approaching Leidenfrost drops suspended on a vapor layer to simulate superhydrophobicity, we show that the out-of-plane directionality results from the impingement of the expanding liquid bridge against the heated Leidenfrost surface, which is initially formed between coalescing drops above the substrate.
On the practical side, textured surfaces offer new possibilities for phase-change heat transfer. Taking advantage of the self-propelled jumping condensate, we developed a planar phase-change thermal diode that transports heat in a preferential direction. The jumping-drop diode is composed of parallel superhydrophobic and superhydrophilic plates, and the thermal rectification is enabled by spontaneously jumping dropwise condensate which only occurs when the superhydrophobic surface is colder. The superhydrophobic surface has nanoscale surface roughness that is anti-dew, while the superhydrophilic surface consists of porous copper wick borrowed from heat pipes. Our planar thermal diode with asymmetric wettability is scalable to large areas with an orientation-independent diodicity of over a hundred.
More broadly speaking, the self-propelled jumping offers an alternative means to return liquid condensate in phase-change systems. We systematically investigate the heat transfer performance of a vapor chamber enabled by the jumping condensate. When the non-condensable gases are removed, the effective heat transfer coefficient is mainly governed by the interfacial resistance of the phase-change processes and the conduction resistance across the superhydrophilic wick. Potential routes for improving the heat transfer performance are discussed, including the optimization of the superhydrophilic wick and its separation with the opposing superhydrophobic surface. The new jumping return mechanism is unique in that it neither relies on external forces nor requires wick structures along the return path, and is expected to be applicable to a variety of phase-change heat transfer systems.
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