Nucleate boiling in graphene oxide colloids: Morphological change and critical heat flux enhancement

We report an experimental investigation into the boiling heat transfer characteristics of water based graphene oxide (GO) colloidal suspensions on a silicon dioxide plate surface. Pool boiling experiments were conducted with GO colloidal suspensions at three concentrations: 1, 5, and 10 mg/L. The characteristics of the GO colloidal suspensions were compared to those of reduced GO (RGO) colloidal suspensions examined in our previous studies, in which we investigated the critical heat flux (CHF) enhancement mechanism in graphene colloids. Even though GO and RGO share almost identical geometrical and chemical characteristics, the GO and RGO flakes formed a two-dimensional (2D) laminate film and a three-dimensional (3D) porous network on a 2D film by nucleate boiling, respectively. Both species showed dramatic CHF enhancement, but not surface wettability enhancement. The 2D structure formed by nucleate boiling exhibited a delayed CHF phenomenon. We found that the GO flakes were well-aligned in the 2D film, such that the film acted as a heat spreader, with superior thermal conductivity. The CHF enhancement data were compared with thermal activity analysis results. Boiling hydrodynamics was examined under high-speed camera visualization.