Dropwise condensation on superhydrophobic surfaces could potentially enhance heat transfer by droplet spontaneous departure via coalescence-induced jumping. However, an uncontrolled droplet size could lead to a significant reduction of heat transfer by condensation, due to large droplets that resulted in a flooding phenomenon on the surface. Here, we introduced a dropwise condensate comb, which consisted of U-shaped protruding hydrophilic stripes and hierarchical micro-nanostructured superhydrophobic background, for a better control of condensation droplet size and departure processes. The dropwise condensate comb with a wettability-contrast surface structure induced droplet removal by flank contact rather than three-phase line contact. We showed that dropwise condensation in this structure could be controlled by designing the width of the superhydrophobic region and height of the protruding hydrophilic stripes. In comparison with a superhydrophobic surface, the average droplet radius was decreased to 12 μm, and the maximum droplet departure radius was decreased to 189 μm by a dropwise condensate comb with 500 μm width of a superhydrophobic region and 258 μm height of a protruding hydrophilic stripe. By controlling the droplet size and departure on hierarchical micro-nanostructured superhydrophobic surfaces, it was experimentally demonstrated that both the heat transfer coefficient and heat flux could be enhanced significantly. Moreover, the dropwise condensate comb showed a maximum heat transfer coefficient of 379 kW m–2 K–1 at a low subcooling temperature, which was 85% higher than that of a superhydrophobic surface, and it showed 113% improvement of high heat flux or heat transfer coefficient when it was compared with that of the hierarchical micro-nanostructured superhydrophobic surface at a high subcooling temperature of ∼10.6 K. This work could potentially transform the design and fabrication space for high-performance heat transfer devices by spatial control of condensation droplet size and departure processes.

Boiling can transfer a vast amount of heat and thereby is widely used for cooling advanced systems with high power density. However, the capillary force of most existing wicks is insufficient to surpass the liquid replenishing resistance for high-efficient boiling. Herein, we report a new microgroove wick on high-conductive copper substrates that was constructed via ultraviolet nanosecond pulsed laser milling. The phase explosion, combined with melting and resolidification effects of laser milling induces dense microcavities with sizes around several micrometers on the microgroove surface. The hierarchical microstructures significantly improve the wettability of the microgroove wicks to obtain strong capillary and meanwhile provide abundant effective nucleation sites. The boiling heat transfer in a visualized flat heat pipe shows that the new wicks enable sustainable liquid replenishing even under antigravity conditions, thus resulting in maximum 33-fold improvement of equivalent thermal conductivity when compared with the copper base. This research provides both scientific and technical bases for the design and manufacture of high-performance phase change cooling devices.