High-school physics books tell us that every substance can occur in three states: solid, liquid, gas. Can a substance have two liquid states? The obvious answer seems “no”: you would expect that at any given temperature a substance has only one disordered thermodynamic state (as opposed to the well-known possibility of having more than one ordered state: think of the diamond and graphite phases of carbon). However, recent theory [1-3] and experiments  indicate that some substances may in fact have two liquid states. As a consequence, phase transitions between these two liquid states may occur (similar to the solid/liquid, liquid/gas phase transitions that we know from daily life), leading to strange thermodynamic properties such as liquid-liquid critical points, and the corresponding divergences of thermodynamic properties. It is suspected that water might be one of the substances having two liquid phases.
At present, the existence of liquid-liquid phase transitions is still very controversial , one of the main reasons being that the liquid-liquid phase transitions reported so far were observed mostly in mixtures (solutions) rather than in pure substances. In this project, you will investigate a pure substance for which we have strong evidence that it has two liquid phases. You will use a range of different experimental techniques to discover how the two liquid phases differ in structure and dynamics, both at the macroscopic and molecular level. Optionally, you can complement your experiments with computer simulations of the liquid-liquid transition.
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