In situ generated hydrophobic micro ripples via π–π stacked pop-up reduced graphene oxide nanoflakes for extended critical heat flux and thermal conductivities

We report the synthesis of thermally heated pop-up reduced graphene oxide (Pop-rGO) and its nanofluid (Pop-rGO-Nf) in DI water for extended critical heat flux (CHF) in a nucleate pool boiling experiment. When Pop-rGO-Nf is boiled over a nichrome (NiCr) wire heater the CHF values were increased up to 132%, 156%, and 175% with increasing concentrations of 0.0005 vol%, 0.001 vol%, and 0.005 vol% at heat fluxes of q′′ = 264 333 kW m(−2), 339 202 kW m(−2), and 327 895 kW m(−2), respectively, because of the higher surface area of 430 m(2) g(−1). We also found a decrease in the CHF value from 0.05 vol% (175%) to 0.01 vol% (153%) for Pop-rGO-Nf due to the nanofluid concentration reaching the saturation point. After nucleate pool boiling, the developed Pop-rGO-Nf built-up layer on the NiCr wire surface showed regular π–π stacking with novel micro-rippled structures having uniform nanocavities and nanochannels. The nanocavities strongly helped vapor bubbles to escape from the NiCr wire surface. In addition, the nanochannels were formed by hydrogen bonding of adjacent carboxyl groups of each Pop-rGO nanosheet. The surface hydrophobicity of the built-up layers increased with the increase of the concentration of the Pop-rGO-Nfs, and the surface morphology, roughness average (R(a)) and hydrophobicity were determined using FE-SEM, AFM and contact angle (CA) analysis. In our present investigation, during and after the nucleate CHF experiments with Pop-rGO-Nfs, for the first time, we obtained a higher CHF value of 175% at 0.01 vol% and a higher CA of 118° obtained at 0.05 vol%, due to the increase in surface hydrophobicity and the novel micro-rippled structures. We anticipate that the present results suggest that pool boiling employing Pop-rGO-Nf can dissipate the critical heat flux of electronic chips to a greater extent, allowing the enhancement of the cooling performance in existing two-phase heat transfer devices.