Thermodynamic and structural anomalies of water nanodroplets from computer simulations

Malek, Shahrazad (2018) Thermodynamic and structural anomalies of water nanodroplets from computer simulations. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Liquid water nanodroplets are valuable for studying supercooled water because they resist nucleation well below the bulk freezing temperature and conveniently selfpressurize in the interior. These features make nanodroplets good candidates for studying the properties of liquid water and for probing the liquid-liquid critical point (LLCP) in water hypothesized to exist in the deeply supercooled state at high pressure, at which a distinct low density liquid (LDL) phase becomes distinct from a high density liquid (HDL) phase. We conduct extensive molecular dynamics computer simulations to study the properties of water nanodroplets using the TIP4P/2005 potential over a wide range of size and temperature. In order to improve the sampling of independent microstates, we conduct “swarms” of independent simulations, in which we monitor the approach to equilibrium from the potential energy autocorrelation function. After a swarm of this size attains equilibrium, the ensemble of final microstates from each run is sufficient to evaluate equilibrium properties and their uncertainties in the shortest real time. In order to study the possibility of recovering bulk properties using nanodroplets, we evaluate the Laplace pressure inside the nanodroplets from direct evaluation of the local pressure tensor. We use a modification of a coarse-graining pressure tensor method that calculates the components of the microscopic pressure tensor as a function of radial distance r from the centre of a spherical water droplet. The pressure tensor beneath the surface region becomes approximately isotropic and constant with r. From this region where the components of the pressure tensor are equal, we determine the Laplace pressure of the droplets. Defining the pressure and the density inside the nanodroplets enables us to probe the properties of liquid water nanodroplet cores. We find that the bulk properties and related anomalies are present in the nanodroplets, such as the appearance of a density maximum. We simulate water nanodroplets under extremly low temperature conditions that have not been investigated thoroughly before. At such low temperatures, the nanodroplets show interesting emergence of structural complexity in the interior which may be linked to the LLCP and may indicate a HDL-like to LDL-like transformation in the nanodroplets. We also study the surface tension of water nanodroplets using different approaches. When employing the thermodynamic route to calculating surface tension, we find that the Tolman correction is small and can be neglected. Therefore, the surface tension of nanodroplets can be approximated by the planar surface tension. We also observe a sudden increase in the planar surface tension at low temperature on crossing the Widom line, which may signal the emergence of a LDL-like network in the interior of water nanodroplets.

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