Relaxing in No Man’s Land: Structural Relaxation of Water from 170 – 260 K*

The origin of water’s anomalies is still debated, in part, because of the experimental difficulty in measuring the kinetics and thermodynamics at temperatures where key phenomenon are predicted. Recently, we have shown that by transiently heating nanoscale water films at rates on the order of 10¹⁰ K/s, the structural transformation and relaxation of liquid water can be examined in the elusive temperature region known as “no man’s land” (Kringle, et al., Science 369, 1490 (2020)). We observed reversible transformations of water, which are described by a linear combination of high- and low-temperature structural motifs. The temperature-dependent transformation follows a sigmoid function, centered at 210 K. For all temperatures examined, the relaxation to a metastable state occurs prior to crystallization. The relaxation rate is dependent on the initial structural motif, with the higher temperature motif typically relaxing faster, with greater dynamical heterogeneity. The relaxation dynamics can be described by a random two-well model; accounting for the heterogeneity in the context of two-component models.