Self-assembled foam-like graphene (SFG) structures were formed using a simple nucleate boiling method, which is governed by the dynamics of bubble generation and departure in the graphene colloid solution. The conductivity and sheet resistance of the calcined (400°C) SFG film were 11.8 S·cm–1 and 91.2 Ω□−1, respectively, and were comparable to those of graphene obtained by chemical vapor deposition (CVD) (~10 S·cm–1). The SFG structures can be directly formed on any substrate, including transparent conductive oxide (TCO) glasses, metals, bare glasses, and flexible polymers. As a potential application, SFG formed on fluorine-doped tin oxide (FTO) exhibited a slightly better overall efficiency (3.6%) than a conventional gold electrode (3.4%) as a cathode of quantum dot sensitized solar cells (QDSSCs).

Based on the unique design of the surface morphology, we investigated the effects of gravity and capillary pressure on Critical heat flux (CHF). The micro-structured surfaces for pool boiling tests were comprised with both the rectangular cavity and microchannel structures. The microcavity structures could intrinsically block the liquid flow by capillary pressure effect, and the capillary flow into the boiling surface was one-dimensionally induced only through the microchannel region. Thus, we could clearly establish the relationship between the CHF and capillary wicking flow. The driving potentials for the liquid inflow can be classified into the hydrostatic head by gravitational force, and the capillary pressure induced by the interactions of vapor bubbles, liquid film, and surface solid structures. Through the analysis of the experimental data and visualization of vapor bubble behaviors, we present that the liquid supplement to maintain the nucleate boiling regime in pool boiling condition is governed by the gravitational pressure head and capillary pressure effect.

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Boiling heat transfer (BHT) is a particularly efficient heat transport method because of the latent heat associated with the process. However, the efficiency of BHT decreases significantly with increasing wall temperature when the critical heat flux (CHF) is reached. Graphene has received much recent research attention for applications in thermal engineering due to its large thermal conductivity. In this study, graphene films of various thicknesses were deposited on a heated surface, and enhancements of BHT and CHF were investigated via pool-boiling experiments. In contrast to the well-known surface effects, including improved wettability and liquid spreading due to micron- and nanometer-scale structures, nanometer-scale folded edges of graphene films provided a clue of BHT improvement and only the thermal conductivity of the graphene layer could explain the dependence of the CHF on the thickness. The large thermal conductivity of the graphene films inhibited the formation of hot spots, thereby increasing the CHF. Finally, the provided empirical model could be suitable for prediction of CHF.

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