Causality in Liquid Water as a Hallmark of Emergent Glassy Dynamics

TL;DR

This study uses causal inference to reveal asymmetric dynamical couplings in water, showing a transition from decoupled to facilitation-driven dynamics in supercooled regimes, indicating emergent glassy behavior.

Contribution

It introduces a causal inference approach to analyze molecular water dynamics, uncovering asymmetries and a reorganization of couplings across different thermodynamic states.

Findings

At room temperature, rotational and translational modes are largely decoupled.

In supercooled water, translational motions become the main drivers of dynamics.

Emergent directionality in fluctuation couplings suggests glassy relaxation mechanisms.

Abstract

In molecular liquids such as water, time-delayed influences between microscopic or mesoscopic variables are typically probed using time-correlation functions, which are symmetric under detailed balance and therefore blind to dynamical asymmetries. Here, we characterize waters dynamics using a causal inference metric that captures asymmetric couplings between collective variables. Analyzing equilibrium molecular dynamics simulations at ambient conditions and in the high-density liquid (HDL) regime of supercooled water, we uncover pronounced asymmetries in the couplings between orientational and translational degrees of freedom across multiple time and length scales. At room temperature, rotational modes remain largely decoupled from translations. In contrast, in the supercooled HDL regime, translational motions emerge as the primary drivers of the dynamics, suggesting facilitation-like relaxation mechanisms characteristic of glassy systems. These results reveal a qualitative reorganization of dynamical couplings across thermodynamic conditions, implying that molecular liquids at thermal equilibrium can exhibit an emergent directionality in their fluctuation couplings. As a consequence, our analysis reveals that external perturbations acting on specific degrees of freedom can induce a stronger arrow of time in the causal relations between translational and orientational modes.