59 research outputs found
Large-scale structure and hyperuniformity of amorphous ices
We investigate the large-scale structure of amorphous ices and transitions
between their different forms by quantifying their large-scale density
fluctuations. Specifically, we simulate the isothermal compression of
low-density amorphous ice (LDA) and hexagonal ice (Ih) to produce high-density
amorphous ice (HDA). Remarkably, both HDA and LDA are nearly hyperuniform,
meaning that they are characterized by an anomalous suppression of large-scale
density fluctuations. By contrast, in correspondence with both non-equilibrium
phase transitions to HDA, the presence of structural heterogeneities strongly
suppresses the hyperuniformity and, remarkably, the system becomes
hyposurficial (devoid of "surface-area" fluctuations). Our investigation
challenges the largely accepted "frozen-liquid" picture, which views glasses as
structurally arrested liquids. Beyond implications for water, our findings
enrich our understanding of the structural transformations that occur in
glasses with varying pressures
Structural Order for One-Scale and Two-Scale Potentials
We perform molecular dynamics simulations to investigate the relationship
between structural order and water-like dynamic and thermodynamic anomalies in
spherically-symmetric potentials having either one or two characteristic length
scales. %The first potential has only one length scale which is the diameter of
the ramp %without the hard core, and the second potential has two length
scales: one is the %diameter of a ramp(softcore) and another one is the
diameter of a %hard core with a ratio of 1.76. Structural order is
characterized by translational and orientational order parameters. %analogous
to those used in previous cases for water and %silica.Only the two-scale ramp
potential exhibits properties %remarkably similar to those found for water and
silica regarding the %relationship between structural order, dynamic anomalies,
and thermodynamic %anomalies. We find that (i) dynamic and thermodynamic
anomalies exist for both one-scale and two-scale ramp potentials, and (ii)
water-like structural order anomalies exist only for the two-scale ramp
potential. Our findings suggest that the water-like relationship between
structural order and anomalies is related to the presence of two different
length scales in the potential.Comment: 12 pages, 5 figure
Phase Diagram of Water Confined by Graphene
The behavior of water confined at the nanoscale plays a fundamental role in biological processes and technological applications, including protein folding, translocation of water across membranes, and filtration and desalination. Remarkably, nanoscale confinement drastically alters the properties of water. Using molecular dynamics simulations, we determine the phase diagram of water confined by graphene sheets in slab geometry, at T = 300 K and for a wide range of pressures. We find that, depending on the confining dimension D and density σ, water can exist in liquid and vapor phases, or crystallize into monolayer and bilayer square ices, as observed in experiments. Interestingly, depending on D and σ, the crystal-liquid transformation can be a first-order phase transition, or smooth, reminiscent of a supercritical liquid-gas transformation. We also focus on the limit of stability of the liquid relative to the vapor and obtain the cavitation pressure perpendicular to the graphene sheets. Perpendicular cavitation pressure varies non-monotonically with increasing D and exhibits a maximum at D ≈ 0.90 nm (equivalent to three water layers). The effect of nanoconfinement on the cavitation pressure can have an impact on water transport in technological and biological systems. Our study emphasizes the rich and apparently unpredictable behavior of nanoconfined water, which is complex even for graphene
Relation between the High Density Phase and the Very-High Density Phase of Amorphous Solid Water
It has been suggested that high-density amorphous (HDA) ice is a structurally
arrested form of high-density liquid (HDL) water, while low-density amorphous
(LDA) ice is a structurally arrested form of low-density liquid (LDL) water.
Recent experiments and simulations have been interpreted to support the
possibility of a second "distinct" high-density structural state, named very
high-density amorphous (VHDA) ice, questioning the LDL-HDL hypothesis. We test
this interpretation using extensive computer simulations, and find that VHDA is
a more stable form of HDA and that in fact VHDA should be considered as the
amorphous ice of the quenched HDL.Comment: 5 pages, 4 fig
Structure of the First and Second Neighbor Shells of Water: Quantitative Relation with Translational and Orientational Order
We perform molecular dynamics simulation of water using the TIP5P model to
quantify structural order in both the first shell (defined by four nearest
neighbors)and second shell (defined by twelve next-nearest neighbors) of a
central water molecule. We find the anomalous decrease of orientational order
upon compression occurs in both shells, but the anomalous decrease of
translational order upon compression occurs {\it mainly in the second shell}.
The decreases of translational and orientational orders upon compression
("structural anomaly") are thus correlated only in the second shell. Our
findings quantitatively confirm the qualitative idea that the thermodynamic,
dynamic and structural anomalies of water are related to changes in the second
shell upon compression.Comment: 12 pages, 5 figure
A Family of Tunable Spherically-Symmetric Potentials that Span the Range from Hard Spheres to Water-like Behavior
We investigate the equation of state, diffusion coefficient, and structural
order of a family of spherically-symmetric potentials consisting of a hard core
and a linear repulsive ramp. This generic potential has two characteristic
length scales: the hard and soft core diameters. The family of potentials is
generated by varying their ratio, . We find negative thermal expansion
(thermodynamic anomaly) and an increase of the diffusion coefficient upon
isothermal compression (dynamic anomaly) for . As in water,
the regions where these anomalies occur are nested domes in the () or
() planes, with the thermodynamic anomaly dome contained entirely within
the dynamic anomaly dome. We calculate translational and orientational order
parameters ( and ), and project equilibrium state points onto the () plane, or order map. The order map evolves from water-like behavior to
hard-sphere-like behavior upon varying between 4/7 and 6/7. Thus, we
traverse the range of liquid behavior encompassed by hard spheres ()
and water-like () with a family of tunable
spherically-symmetric potentials by simply varying the ratio of hard to
soft-core diameters. Although dynamic and thermodynamic anomalies occur almost
across the entire range , water-like structural anomalies
(i.e., decrease in both and upon compression and strictly correlated
and in the anomalous region) occur only around .
Water-like anomalies in structure, dynamics and thermodynamics arise solely due
to the existence of two length scales, orientation-dependent interactions being
absent by design.Comment: total 21 pages, 6 figure
Liquid-Liquid Phase Transition and Glass Transition in a Monoatomic Model System
We review our recent study on the polyamorphism of the liquid and glass states in a monatomic system, a two-scale spherical-symmetric Jagla model with both attractive and repulsive interactions. This potential with a parametrization for which crystallization can be avoided and both the glass transition and the liquid-liquid phase transition are clearly separated, displays water-like anomalies as well as polyamorphism in both liquid and glassy states, providing a unique opportunity to study the interplay between the liquid-liquid phase transition and the glass transition. Our study on a simple model may be useful in understanding recent studies of polyamorphism in metallic glasses
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