Ice-Amorphization of Supercooled Water Nanodroplets in No Man’s Land

Abstract

Elucidating freezing mechanisms of liquid water into ice, especially in "No Man's Land" (150 K< T < 235 K), carries scientific and technological importance. Indeed, superior predictions of upper-troposphere cirrus-cloud formation and surface-bound ice-fog formation constitute powerful motivations in addition to unravelling long-standing puzzles such as persistent liquid fogs well below frost point and understanding interstellar-space water states, together with advancing cryopreservation technology. Unlocking the secrets of water's anomalous deep-cooling complexities, such as structural ordering and microscopic nucleation mechanisms, are the subject of lively debate. Exploring nucleation mechanism in No Man's Land (NML) is technically demanding, owing to rapid nucleation rates with, unsurprisingly, very few reported experimental studies. However, amorphization is a key intermediate stage in NML-based nucleation, and it is also not particularly well understood. In this microsecond long molecular dynamics study, we have explored microstructural processes involved in the amorphization of aggressively quenched supercooled water nanodroplets in the gas phase where surface effects are non-negligible. A dynamically arrested state is observed in these droplets that resembles structurally low-density amorphous polymorphs of ice. Importantly, the curved geometry of the nanodroplets themselves is seen to inhibit amorphization relative to bulk systems under identical thermodynamic conditions.

Thermodynamic and structural anomalies of water nanodroplets

“…Despite the disruption of bulk-like structure occurring at the nanodroplet surface, the evolution of our density profiles as T decreases is thus driven by the formation of a low-density RTN in the droplet core. A similar low density core was observed in recent simulations of glassy water nanoparticles 29 . Notably, the onset of ice crystallisation in nanodroplets is observed experimentally also when N reaches 250–300 24 , consistent with our finding that this is the range of N in which a density maximum and a RTN structure emerge with decreasing T .…”

“…4a . As noted in previous simulations of water nanodroplets 19 , 27 , 29 , we observe oscillations in ρ o ( r ) that are especially prominent near the surface, indicating that the interface with the vacuum is a well-defined molecular layer, the influence of which propagates inward as a succession of concentric shells. The amplitude of these oscillations is larger at lower T and for smaller N .…”

“…This is due to the significant experimental challenges associated with studying liquid nanodroplets that are not in contact with a supporting or confining surface. To date, experimental and simulation studies of pure liquid water nanodroplets have focussed largely on freezing and melting behaviour 7 , 8 , 19 , 22 – 28 , as well as the formation of amorphous solid nanoparticles 29 . However, a systematic description is lacking for how basic nanodroplet properties, such as R , P L , or the droplet density profile, vary with both N and T .…”

Low Density Interior in Supercooled Aqueous Nanodroplets Expels Ions to the Subsurface

“…These observations have some commonalities with studies of nucleation within droplets and thin films. [32][33][34][35][36] We hypothesize that the low density water core in supercooled droplets will differentiate the ion distribution from that at room temperature. The hypothesis could not be explored in earlier research 14 because the small cluster sizes (number of water molecules ≤ 100) precluded the formation of the low density core.…”

Low Density Interior in Supercooled Aqueous Nanodroplets Expels Ions to the Subsurface

“…These observations have some commonalities with studies of nucleation within droplets and thin films. [32][33][34][35][36] We hypothesize that the low density water core in supercooled droplets will differentiate the ion distribution from that at room temperature. This hypothesis has not been explored earlier.…”