Graphene Oxide Colloidal Suspensions as Cutting Fluids for Micromachining: Part 2 — Droplet Dynamics and Film Formation

Part-2 of this paper is focused on studying the droplet spreading and the subsequent evaporation/film-formation characteristics of the graphene oxide colloidal solutions that were benchmarked in Part-1. A high-speed imaging investigation was conducted to study the impingement dynamics of the colloidal solutions on a heated substrate. The spreading and evaporation characteristics of the fluids were then correlated with the corresponding temperature profiles and the subsequent formation of the residual graphene oxide film on the substrate. The findings reveal that the most important criterion dictating the machining performance of these colloidal solutions is the ability to form uniform, sub-micron thick films of graphene oxide upon evaporation of the carrier fluid. Colloidal suspensions of ultrasonically-exfoliated graphene oxide at concentrations < 0.5 wt% are best suited for micromachining applications since they are seen to produce such films. The use of thermally-reduced graphene oxide suspensions at concentrations < 0.5 wt% results in non-uniform films with thickness variations in the 0–5 μm range that are responsible for the fluctuations seen in the cutting force and temperatures. At concentrations ≥ 0.5 wt%, both the thermally-reduced and ultrasonically-exfoliated graphene oxide solutions result in thicker and non-uniform films that are detrimental for machining results. The findings of this study reveal that the characterization of the residual graphene oxide film left behind on a heated substrate may be an efficient technique to evaluate different graphene oxide colloidal solutions for cutting fluids applications in micromachining.