Anomalously enhanced subcooled flow boiling in superhydrophobic graphene-nanoplatelets-coated microchannels

In boiling heat transfer, superhydrophobic surface is deemed to reduce the critical heat flux (CHF) due to the immediate formation of an insulating vapor film that inhibits effective heat transfer despite its smaller energy barrier for bubble nucleation than that of a superhydrophilic surface. Here, a subcooled flow boiling investigation is carried out in a microchannel heat sink at a low mass flux condition using the superhydrophilic and superhydrophobic graphene nanoplatelets (GNPs) composite coatings. Interestingly, the flow boiling performance of the superhydrophobic GNPs-coated microchannel significantly transcends that of its superhydrophilic counterpart. By benchmarking with the performance of an uncoated microchannel, the CHF and heat transfer coefficient of the superhydrophobic GNPs-coated microchannel are enhanced up to 134% and 135%, respectively. This anomaly is ascribed to the Cassie Baxter-to-Wenzel transition on the superhydrophobic GNPs-coated surface, which is infiltrated with liquid, restricting the nucleating bubbles from expanding into a vapor film and sustaining an efficient boiling. The initial Wenzel state of the superhydrophobic GNPs surface is primarily ascribed to the rapid intercalation of water molecules via the unique graphene nanostructure. This study sheds new light on the unique wetting behavior of graphene-nanostructured surfaces in the context of microscale flow boiling heat transfer.