Orientation effect on the rheology of graphene oxide dispersions in isotropic phase, ordered isotropic biphase, and discotic phase

Abstract

We present a combined simulation and experimental study of the structure and dynamics of dilute, semidilute, and concentrated graphene oxide aqueous alkaline dispersions. These materials behave as lyotropic systems, with phase ordering as the concentration increases. The sheet spacing in the ordered phases is much broader than that expected by the classical Derjaguin–Landau–Verwey–Overbeek theory. Rheological responses in the isotropic phase are similar to rod-like liquid crystals (LCs), which follow the simplified Leslie–Ericksen (LE) model. The dispersions in the biphase and the discotic phase behave similarly to polydomain LC polymers, following the mesoscopic LE model [Larson–Doi (LD) model]. The LD model fits the time evolution of shear stress at startup flow, re-startup flow after the cessation of flow, and reversal flow in the discotic phase. Further, the Folgar–Tucker–Lipscomb model fits the stress overshoot in a startup flow, but not the reversal flow.

Simulations of Graphene Oxide Dispersions as Discotic Nematic Liquid Crystals in Couette Flow Using Ericksen-Leslie (EL) Theory

“…The shear-thinning behavior has previously been reported for flow-aligning nematic LCs and GO suspensions [38,47,48]. Choi et al [38] and Adnan et al [49] investigated the viscosity of GO suspensions for shear rates up to 10 s −1 and showed that it increased for different concentrations of GO from the isotropic to discotic phase.…”

“…The shear-thinning behavior has previously been reported for flow-aligning nematic LCs and GO suspensions [38,47,48]. Choi et al [38] and Adnan et al [49] investigated the viscosity of GO suspensions for shear rates up to 10 s −1 and showed that it increased for different concentrations of GO from the isotropic to discotic phase. Giudice and Shen [48] explored GO aqueous suspensions viscosity further, considering the higher shear rate of 10 2 s −1 , and showed that the shear-thinning behavior becomes more noticeable at higher concentrations.…”

“…The LCs are prone to having multiple orientation solutions and the solution branch selected depends on the initial guess given to the solver [38,42]. In this section, studies have been performed to show the stable and unstable branches of the solution.…”

“…Furthermore, shear and pressure-driven flows of nematic LCs were studied by [37] under the thin-film approximation. Recently, GO aqueous alkaline dispersion with and without shear flow was studied by [38], which showed that the orientational dynamics play a critical role in the rheology of GO dispersions.…”

Crystallographically Textured Electrodes for Rechargeable Batteries: Symmetry, Fabrication, and Characterization

“…In the former category, anisotropic materials suspended in the solvent are known to tend to develop preferential orientation under mechanical stimulus. For example, the rheological responses of graphene-based suspensions have been extensively studied; pronounced shear alignment phenomena are observed as the shear rate exceeds critical values. − Because of the suspension’s fluidity, upon removal of the mechanical stimulus, this preferential orientation could decay, and the original order is therefore restored after resting/annealing; this transient process is associated with a characteristic relaxation time τ. If the solvent of the suspension is evaporated within a period of time t sufficiently shorter than the relaxation time (i.e., t ≪ τ), the preferential orientation is then preserved and built in permanently to the solid phase formed by the suspension after evaporation of the solvent, for example, freeze-dry. , τ is on the order of seconds/mins and more in graphene-based suspensions. , As such, one can synthesize macroscopic assemblies with preferred out-of-plane orientation.…”