Foam Flows in Turbulent Liquid Exfoliation of Layered Materials and Implications for Graphene Production and Inline Characterisation
Surfactants are often used to stabilise two-dimensional (2D) materials in environmentally friendly solvents such as water. Aqueous-surfactant solutions prevent agglomeration of nanosheets through steric and electrostatic repulsion, facilitating the production of high concentration nanomaterial dispersions. Turbulent, shear-assisted liquid exfoliation of layered precursor materials produces defect-free nanosheets by promoting mixing and generating sufficiently high shear rates to overcome out-of-plane van der Waals bonds. In the presence of a liquid-gas interface, a consequence of using surfactants in turbulent flows is the formation of foam. In this experimental study, batch exfoliation of graphite particles into few-layer graphene was performed using a kitchen blender modified to operate across Reynolds numbers, $Re \sim 10^{5} - 10^{6}$. Foam formation during turbulent operation was found to influence the hydrodynamics of the liquid exfoliation process. Measurements on the motion of graphite particles indicate that surfactant concentration alters the rheology of the mixture under dynamic conditions and changes the material flow patterns within the device. As a result, the surfactant concentration that maximised graphene concentration was non-unique. This highlights that the design and selection of surfactants should consider both molecular scale repulsion effectiveness and macroscale hydrodynamics of the liquid exfoliation process. Furthermore, the multi-phase turbulent flows and complex fluids that exist during batch exfoliation in aqueous-surfactants create major challenges for realising $\textit{in situ}$ 2D material characterisation and quality control. Here, we have developed a protocol to enable inline uv-vis-nIR spectroscopy to determine graphene production and atomic layer number changes in-process.