The freezing point of water normally occurs at 273.15 K (0◦ C), but water can be \"supercooled\" past this temperature and remain a liquid. In the supercooled state, the presence of a novel liquid-liquid phase transition complicates the thermodynamics and dynamics of the liquid state. In particular, this gives rise to a \"fragile-to-strong\" crossover in the temperature dependence of the dynamics and thermodynamics. We investigate how this fragile-to-strong transition is manifest in the heterogeneous nature of dynamics. We address this challenge using simulations of the ST2 model of water at atmospheric pressure. We see the fragile-to-strong crossover in the diffusion coefficient and the relaxation time. These dynamic quantities enter a new regime from primarily diffusive motion characteristic of fragile liquids to motion dominated by local caging. A temperature specific peak in the fluctuations of the self-intermediate scattering function also indicate a change in the dynamics as a result of the fragile-to-strong crossover. In our thermodynamic quantities, a maximum in specific heat and a minimum in expan- sivity denote the fragile-to-strong crossover. These disparate events all occur within 20 K of one another and are explained by a switch from diffusive motion to networked, collective motion indicating a robust change in the motion and heat exchange of systems of water molecules as a consequence of the fragile-to-strong transition. We also mini- mize the potential energy to investigate the inherent structure of the potential energy landscape. With the absence of vibrational energy, the structure of the liquid becomes more sharply defined and the cooperative motion associated with α-relaxation is laid bare. Finally, we observe a failure in the Stokes-Einstein relation at low temperature as the product of diffusion coefficient and relaxation time diverge from a constant value. Diffusion and relaxation time are affected by different clusters of molecules in liquids exhibiting dynamic heterogeneity causing them to no longer be coupled. We highlight several potential replacements for the Stokes-Einstein relationship. Timescales that are sensitive to specific mobility groups, exhibit coupling even at low temperature.
"},{"label":"Degree","value":["Bachelor of Arts with Departmental Honors"]},{"label":"Author-supplied Keywords","value":["Supercooled water","Heterogeneous dynamics","Fragile-to-strong crossover"]},{"label":"Administrative Note","value":"Drupal Node ID: 24226