Frequency dependence of specific heat in supercooled liquid water and emergence of correlated dynamics

TL;DR

This study investigates the microscopic origins of the specific heat anomaly in supercooled water, revealing emergent slow, long-range temperature fluctuations and a fragile-to-weak transition in dynamics below 220 K.

Contribution

The paper combines simulations and theoretical analysis to connect specific heat anomalies with the emergence of correlated, slow dynamics and long-range fluctuations in supercooled water.

Findings

Specific heat increases sharply below 270 K in simulations.

Emergence of slow, long-range temperature fluctuations at low temperatures.

A fragile-to-weak transition occurs around 220 K.

Abstract

Molecular origin of the well-known specific heat anomaly in supercooled liquid water is investigated here by using extensive computer simulations and theoretical analyses. A rather sharp increase in the values of isobaric specific heat with lowering temperature and the weak temperature dependence of isochoric specific heat in the same range are reproduced in simulations. We calculated the spatiotemporal correlation among temperature fluctuations and examined the frequency dependent specific heat. The latter shows a rapid growth in the low frequency regime as temperature is cooled below 270 K. In order to understand the microscopic basis of this increase, we have performed a shell wise decomposition of contributions of distant molecules to the temperature fluctuations in a central molecule. This decomposition reveals the emergence, at low temperatures, of temporally slow, spatially long ranged large temperature fluctuations. The temperature fluctuation time correlation function (TFCF) can be fitted to a William-Watts stretched exponential form with the stretching parameter close to 0.6 at low temperatures, indicating highly non-exponential relaxation. Temperature dependence of the relaxation time of the correlation function can be fitted to Vogel-Fulcher-Tamermann expression which provides a quantitative measure of the fragility of the liquid. Interestingly, we find that the rapid growth in the relaxation time of TFCF with lowering temperature undergoes a sharp crossover from a markedly fragile state to a weakly fragile state around 220 K.