A Bar of Cutting Fluid: Deep Eutectic Fluids with a Novel Flavour

Deep-eutectic fluids (DEFs) are a novel class of materials that, to date, have been generally used in a single-phase (liquid) for a growing number of applications (e.g., extraction, catalysis, synthesis, etc). In contrast, we present a novel approach to utilise the advantages associated with dual-phase control of DEFs. In this study, we exploit the compositional tunability of DEFs to adapt their functional response. Specifically, we demonstrate the synthesis and application of a crystalline DEF (SolBar) that melts in a controlled manner and under desired conditions. We show how the concept of dual-phase control of a DEF can reduce thermo-mechanical loads developed during shear-based machining operations and offer a substitute for conventional cooling/lubrication means. To this end, we formulated the chemistry of SolBar (a citric acid and choline chloride DEF) so that, as it transforms into liquid, it directly affects lubrication and cooling at the tool-workpiece interface. This was demonstrated by temperature and force measurements. The generation of a tribo-active layer was evidenced through X-ray photoelectron spectroscopy. This layer enhances the lubrication capabilities of the DEF. The concept is not limited to machining, which serves to demonstrate a useful application in this study but can be extended to multiple fields where surfaces are in contact and control of the physico-chemical response of the system is required. Here, we pave the way to new, environmentally friendly, and cost-effective alternatives to conventional tribological media.