Rheology of graphene oxide suspended in yield stress fluid

Abstract

• Suspensions of graphene oxide (GO) in Carbopol are also elastic yield stress fluids. • Addition of GO nanosheets in Carbopol decreases viscosity and elasticity. • Oxidation level of GO affects the rheological behavior. • Higher level of interaction of the GO/water system oxidized for 96 h. This work investigates the rheology of graphene oxide (GO) suspension on a yield stress model fluid. The effects caused by the variation of the GO concentration in the suspension and by the amount of oxygenated groups pendant present on the nanosheets surface, on the rheology of the suspensions were evaluated. The dispersant used was an aqueous solution of Carbopol® Ultrez 10. This is a transparent, nontoxic elastic yield stress fluid. GO nanosheets were produced from synthesis of graphite oxide (GrO) by modified Hummers method, and the nanosheets were characterized by XRD, Raman, FTIR, TEM and AFM techniques. The rheological behavior of these suspensions was characterized by oscillatory and steady-state flow experiments. The GO structure characterization shows that oxygenated functional groups were incorporated in its graphite surface in different levels after using two distinct times of oxidation. GO oxidized for 96 h (GO 96 h) showed greater interplanar distance and also presented few layers when compared with GO oxidized for 2 h (GO 2 h). The GO exfoliation process directly into the aqueous dispersion of Carbopol® showed to be an effective method. As for the pure Carbopol solutions, the GO suspensions were well modeled by the Hershel-Bulkley equation. The increase of GO concentration in the suspensions impairs the level of fluid structure and leads to a decrease in viscosity, yield stress, and elasticity. When compared, the GO 96 h promoted a lower decrease in viscosity, yield stress and elasticity than the GO 2 h suspension. For the suspension with a higher concentration of GO 96 h, it was observed the appearance of hysteresis at low shear rates. These results show that small changes in the GO nanosheets surface can significantly influence the rheological responses of a non-Newtonian fluid.