No abstract

Hydrodynamic flow can have complex and far-reaching consequences on the rate of homogeneous nucleation. We present a general formalism for calculating the nucleation rates of simply sheared systems. We have derived an extension to the conventional Classical Nucleation Theory, explicitly embodying the shear rate. Seeded molecular dynamics simulations form the backbone of our approach. The framework can be used for moderate supercooling, at which temperatures brute-force methods are practically infeasible. The competing energetic and kinetic effects of shear arise naturally from the equations. We show how the theory can be used to identify shear regimes of ice nucleation behavior for the mW water model, unifying disparate trends reported in the literature. At each temperature, we define a crossover shear rate in the limit of 1000 s−1–10 000 s−1, beyond which the nucleation rate increases steadily up to a maximum, at the optimal shear rate. For 235 K, 240 K, 255 K, and 260 K, the optimal shear rates are in the range of ≈106 s−1–107 s−1. For very high shear rates beyond 108 s−1, nucleation is strongly inhibited. Our results indicate that the optimal shear rates have a non-monotonic dependence on temperature.

Inspired by the discovery of high-pressure ice inclusions in diamond, we investigate the rich structural diversity of high-pressure phases of quasi-two-dimensional water constrained by diamond matrices, using molecular dynamics simulations. Monolayer ices formed are structurally similar to monolayer ices constrained by graphene. We report new bilayer and trilayer ice phases, specifically, the AB II, ABC I, ABC II, and AAB/ABB ordered phases. The relative stability of the interlayer hydrogen bonds between adjacent layers reveals differences in structural properties. We observe grain-boundary migration between pockets of AAB and ABB ices, during the pressurization process. The compressionlimit phase diagram in the slit width−lateral pressure plane has been constructed and analyzed. Our results indicate that the phase behavior of confined ice is qualitatively independent of the diamond−water force-field parameters and is significantly influenced by the lattice structure and geometry of the confining surfaces.

We develop intuitive metrics for quantifying complex nucleating systems under confinement. These are shown to arise naturally from the analysis of the topological ring network, and are amenable for use as order parameters for such systems. Drawing inspiration from qualitative visual inspection, we introduce a general topological criterion for elucidating the ordered structures of confined water, using a graph theoretic approach. Our criterion is based on primitive rings, and reinterprets the hydrogen-bond-network in terms of these primitives. This approach has no a priori assumptions, except the hydrogen bond definition, and may be used as an exploratory tool for the automated discovery of new ordered phases. We demonstrate the versatility of our criterion by applying it to analyse well-known monolayer ices. Our methodology is then extended to identify the building blocks of one-dimensional n-sided prismatic nanoribbon ices.Confinement is known to cause deviations from bulk system properties [1][2][3][4], imparting unique complexities to the structure determination of icy confined water. The structure and phase behaviour of quintessential confined systems, like that of two-dimensional monolayer and one-dimensional nanoribbons are of enormous theoretical[4-6] and experimental interest [7,8]. However, well-defined simulation methodologies and methods of analysis for bulk systems like the mean squared displacement (MSD), often used as a broad indicator of phase transitions, may be unreliable for constrained ice systems. The reasons are two-fold: confined water often exhibits continuous freezing transitions [9][10][11], and the mobility of water layers close to the confining sheet is lower than that of intermediate layers [12,13]. Parameters engineered to characterize phase transitions in simpler Lennard-Jones systems, inspired by the MSD, are not universally valid. For example, the Lindemann parameter, which has been used for melting studies [14,15], fails for confined systems with attractive pores due to instabilities in the MSD [16]. Structure determination techniques, which are accurate for bulk water, are often intractable for nucleating water systems under confinement, as the hydrogen-bonding network (HBN) is significantly influenced by the confining wall. Compared to bulk water, water under nanoscale confinement shows considerable polymorphic diversity [17].Visual inspection of the HBN, paired with the angle distribution, has been used in the literature for the analysis of confined water systems. Such techniques are often used for qualitative analyses [5,10,11,18,19] but are unable to describe the structures quantitively. Voronoi tessellation [20] has been previously used to quantify the structures of two-dimensional confined ice systems. The bond orientational parameter has also been used for distinguishing the degree of square and hexagonal order in ice [21]. However, the bond orientational parameter has implicit a priori * Corresponding Author arXiv:1909.09827v1 [physics.chem-ph] 21 Sep 2019 PREPRINT -SEPTE...

One of the most ubiquitous and technologically important phenomena in nature is the nucleation of homogeneous flowing systems. The microscopic effects of shear on a nucleating system are still imperfectly...