Excitations Provide a Thermodynamic Order Parameter for Liquid Dynamics

Recent evidence from simulation and model systems suggests that locally excited structures (excitations) play an important role in the dynamics of supercooled liquids and glasses. Here, for the first time, we measure excitations in real liquids and show that they obey Bose-Einstein statistics with enthalpy and entropy of creation closely related to that of melting. We also show that excitations - particularly their population in the first solvent shell - serve as an order parameter for driving transformations between distinct mechanistic regimes of liquid relaxation, and specifically for the appearance of dynamic heterogeneity. The connection between excitations and mechanistic changes in relaxation suggests a simple and compelling explanation for previously cryptic aspects of these changes and provides a new connection between the dynamics and thermodynamics of liquids. The thermodynamic underpinning of the excitations suggests a path for connecting the full range of complex liquid dynamics with easily measurable quantities.