Communication: Existence and control of liquid polymorphism in methanol under shear

Abstract

The liquid-liquid hypothesis, which states that a pure substance can exhibit two liquid forms (or polymorphs), has drawn considerable interest in recent years. The appeal of this theory is that it provides the basis for a deeper understanding of the properties of supercooled liquids. However, the study of this phenomenon is extremely challenging and a complete understanding of its impact on fluid properties has remained elusive so far, since the low-temperature liquid form is generally not stable and undergoes rapid crystallization. Using a coarse-grained model for methanol, we show that methanol under shear can exhibit, in the steady state, two liquid forms that respond differently to the applied shear. Using molecular simulations, we show that the difference in dynamical response is correlated with structural differences between the two liquid forms. This establishes the existence of liquid polymorphism for systems driven out-of-equilibrium. Our findings also show how, by varying the pressure or the shear stress applied to the system, liquid-liquid transitions can be triggered and how a control of liquid polymorphism can be achieved. The resulting solid-liquid-liquid nonequilibrium phase diagram leads us to identify new ways for the stabilization and study of liquid polymorphism.