Wetting of nanofluids with nanoparticles of opposite surface potentials on pristine CVD graphene

Comparative wettability studies of graphene are conducted for two different nanofluids with opposite surface potentials of +53 mV (45-nm alumina nanoparticles) and −45 mV (28-nm silica nanoparticles), respectively. Aged graphene surface, which has adsorbed abundant hydrocarbon contaminants, shows weak hydrophobicity of about 90° wetting angles for both nanofluids for the tested volume concentration range from 0 to 10 %. For pristine graphene surfaces, however, the contact angle of alumina nanofluids continually increases from 50° to 70° for the same volume concentration increase, but the contact angle of silica nanofluids shows first increase of up to about 1 % concentration and then remains nearly unchanged with further increasing concentration. Since the nanoparticle–graphene interaction at the solid–liquid (SL) interface is expected to be the most crucial in determining the nanofluid wetting angles, the corresponding surface energy \(\gamma_{\text{SL}}\) is examined from elaboration of \(F_{\text{DLVO}}\), the Derjaguin–Landau–Verwey–Overbeek force. The magnitudes of both the repulsive \(F_{\text{DLVO}}\) on the alumina nanoparticles and the attractive \(F_{\text{DLVO}}\) on the silica nanoparticles show rapid decreases up to 1 % volume concentration and exhibit slower decreases thereafter. The reduced repulsive \(F_{\text{DLVO}}\) of the alumina nanoparticle drives the increasing aggregation of nanoparticles on the SL interface with increasing concentration, thus increasing the SL interfacial energy \(\gamma_{\text{SL}}\). On the contrary, the reduced attractive \(F_{\text{DLVO}}\) on the silica nanoparticle retards their aggregation on the SL interface with increasing concentration and slows the increase in \(\gamma_{\text{SL}}\), eventually settling on the saturated level of \(\gamma_{\text{SL}}\) from a certain concentration onwards. These distinctive behaviors of \(\gamma_{\text{SL}}\) are consistent with the measured contact angles that gradually increase with increasing concentration for the positive surface potential (alumina), but initially increase and then settle for the negative surface potential (silica). This phenomenon strongly supports the critical dependence of nanofluid wetting of pristine graphene on \(F_{\text{DLVO}}\) in the vicinity of the SL interface.