Wetting and Evaporative Aggregation of Nanofluid Droplets on CVD-Synthesized Hydrophobic Graphene Surfaces

The wetting and evaporative aggregation of alumina nanofluids (Al2O3) are examined for CVD-synthesized graphene-coated (GC) surfaces that are known as strongly hydrophobic (theta(contact) approximate to 90 degrees). Our findings are compared to those associated with a hydrophilic cover glass (CG) substrate (theta(contact) approximate to 45 degrees). The nanofluidic self-assemblies on the GC substrate are elaborately characterized in terms of the droplet wetting/crack formation, the particle migration time over the evaporative time (C-R), the Derjaguin-Landau-Verwey-Overbeek forces (F-DLVO), and the relative thermal conductivity (K-R). The GC substrate forms relatively thicker and larger cracks and requires a longer evaporation time. Both the GC and CG substrates share approximately the same time constant C-R, which suggests the formation of coffee-ring patterns for both substrates. The GC shows negative F-DLVO, which implies a repulsive force between the nanoparticles and the substrate, and the CG shows a positive F-DLVO of attraction. Furthermore, a more than 3 order of magnitude larger thermal conductivity of GC compared to that of CG drives significantly different particle/fluid motions near the drop edge areas between the two substrates.