PMC

(a) Water boiling on a clean copper heater. Before CHF, a large mushroom of vapor is formed on an entire heater. At CHF, the large dry patch (spot) is observed in the large mushroom. At the edge of heater, the vapor film goes into the large dry path and meets with the large dry patch. Finally, the vapor film covers the entire heater. A film boiling is developed with the rapid wall temperature increase. (b) Water boiling on RGO flakes pre-coated copper heater. At the CHF, the wall temperature starts to increase slowly and the large dry patch can be observed in the large mushroom. Even though the wall temperature reaches to 209°C, the nucleate boiling is still maintained with the large dry patch in the large mushroom, contrary to the film boiling development in water boiling on a clean copper heater.

Evaporation/boiling and their heat transfer performance
(A) Evaporation on capillary wicking structures. Hybrid wicking structures are used to enhance evaporation by reducing the thickness of capillary liquid film with reduced flow resistance.
(B) Nucleate boiling heat transfer in a liquid pool. Micro/nano-structured surfaces can enhance nucleate boiling by both increasing nucleation sites and increasing the liquid wicking capability.
(C) Strategy in enhancing evaporation through liquid film boiling: By introducing dynamic bubbles into a liquid film in the capillary wicking structure, heat transfer efficiency is increased due to both the heat carried by bubble motion and the thermal convection of the liquid film.
(D) Demonstrated liquid film boiling heat transfer on a hierarchical mesh-covered (hi-mesh) wicking structure. Nucleated bubbles can rapidly grow and escape from the hi-mesh wicking structure by bursting on the capillary liquid film, frequently fluctuating the liquid film to increase convection. Reprinted with permission from (). Copyright (2018) Elsevier.
(E) Significantly enhanced heat transfer performance was demonstrated on the hi-mesh wicking structure: enhanced CHF and HTC compared with capillary evaporation on mesh wicking structure () and enhanced HTC compared with nucleate boiling on nanowired surfaces (). The heat transfer enhancement of liquid film boiling strongly indicates the success of the liquid film boiling strategy for enhancing evaporation. Published by Elsevier.
(F) Comparison of HTC and CHF of various evaporation/boiling heat transfer processes.

(a), SEM images of the graphene film. (b), Boiling curves. (c), Static contact angle measurements.

Schematic diagram of (a) Film boiling, (b) Transition boiling (c) Nucleate boiling. The dark blue circles represent the nucleation process.