A facilitation-induced fluidization transition in supercooled water triggered by a few active molecules

TL;DR

The paper addresses whether a facilitation-induced fluidization transition, previously observed in a model liquid, also arises in real water. It uses TIP5P-based simulations with reaction-field electrostatics and an activation protocol that periodically energizes a small fraction of molecules to mimic facilitation, assessed through diffusion, relaxation times, and dynamic heterogeneity metrics. The authors report a dynamic transition at 220 K triggered when the activated fraction reaches about 10% (effectively ~2.5%), marked by a sharp increase in diffusivity and a rapid decrease in relaxation time, along with the loss of the cage plateau and a surge in cooperative motion that extends to non-active molecules. The transition shows no strong active aggregation and is potentially influenced by an accompanying LDL-HDL structural competition, suggesting a universal mechanism linking kinetic energy to fluidization and providing insight into glassy dynamics of water.

Abstract

Using an activation mechanism reproducing facilitation, a dynamic phase transition triggered by a few active molecules was recently found in a supercooled model liquid. Prompted by this finding we investigate the presence of a similar transition in supercooled water. We find the presence of the phase transition in water despite the numerous anomalies of water, suggesting universality of the transition. The transition appears at constant temperature, being only induced by the motion of a small percentage of active molecules and the system cooperativity. We observe that cooperative motions strongly increase at the transition and do not disappear when the medium viscosity drops. An increase of temperature is needed to make the cooperative motions disappear, suggesting a connection to the kinetic energy to potential energy ratio instead of the medium viscosity.

Figures (14)

• Figure 1: (color online) Diffusion coefficient $D$ versus the concentration $C$ of active molecules at constant temperature $T=220K$. We observe a decrease of $D$ just before the transition, followed by a large increase at the transition. Active molecules are more diffusive than non actives.
• Figure 2: (color online) Inverse of the alpha relaxation time $\tau_{\alpha}$ versus the concentration $C$ of active molecules at constant temperature $T=220K$. The Figure show a decrease of $1/\tau_{\alpha}$ just before the transition, then a large increase at the transition. Active and inactive molecules display a very similar behavior.
• Figure 3: (color online) Mean square displacement of (a) active and (b) non active water molecules versus the concentration $C$ of active molecules.
• Figure 4: (color online) Stokes-Einstein breaking ($D.\tau_{\alpha}$ variation) versus the concentration $C$ of active molecules at constant temperature $T=220K$.
• Figure 5: (color online) Maximum value of the dynamic susceptibility of the whole medium versus the concentration $C$ of active molecules.