*3.2. CO2 Mechanism of Action*

In a conventional LN2 cryogenic machining strategy, the tool is cooled by proximity to the cryogenic medium. In such a scenario, the extremely cold fluid serves to function as a heat sink, cooling the tool primarily by convection, driven by temperature gradient (between the tool and cryogen). CO2 coolants on the other hand are delivered at ambient temperature (Figure 1), and as such, their function as a coolant is not a consequence of their cryogenic temperature at delivery, but rather, the Joule–Thomson effect. In a Joule–Thomson expansion, the fluid is adiabatically throttled through the exit valve of the coolant system. As the gas is throttled, it undergoes a sudden drop in pressure. This reduction in pressure is accompanied by an increase in the potential energy of the fluid owing to the increasing Van der Waals attraction at a larger atomic separation. As such, in order for enthalpy to remain constant across the throttle (which is observed in a Joule– Thomson expansion), the thermal kinetic energy of the fluid post throttle must reduce to accommodate this increase in potential energy, ultimately generating a cooling effect in the system. The throttling process also corresponds a phase-transformation where, in the case of CO2, LCO2 becomes a combination of 60% solid (generally in the form of CO2 "snow") and 40% gas [7], which equally has a novel impact upon the mode of cooling, and the machining process in general.
