Experimental study of the effect of a reduced graphene oxide coating on critical heat flux enhancement

Critical heat flux (CHF) nucleate pool boiling was investigated in a reduced graphene oxide (RGO) colloid consisting of 0.0005 wt.% flake-based RGO in distilled water using Ni–Cr thin-wire heaters 0.1 and 0.2 mm in diameter and 105 mm in length. One- and two-sided coatings were tested to study the RGO flake aggregation as a function of coating time using the Joule heating method. In the one-sided coating tests, RGO flake aggregation was biased on the anode so that the CHF occurred on the cathode side with slight enhancement. The RGO coating on the cathode increased with coating time; however, the coating on the anode did not appear to be affected by coating time. The biased deposition of the RGO coating was attributed to the negatively charged carboxyl group (–COOH) in the RGO flakes. In the two-sided coating tests, the RGO flake aggregation showed dramatic CHF enhancement with increasing coating time. The enhancement mechanisms were examined based on a surface analysis considering the deposition mechanism, thermal activity, water absorption, and the modulated Rayleigh–Taylor wavelength of the RGO coating layer on the wire heaters.