96 research outputs found
Dynamically Slow Processes in Supercooled Water Confined Between Hydrophobic Plates
We study the dynamics of water confined between hydrophobic flat surfaces at
low temperature. At different pressures, we observe different behaviors that we
understand in terms of the hydrogen bonds dynamics. At high pressure, the
formation of the open structure of the hydrogen bond network is inhibited and
the surfaces can be rapidly dehydrated by decreasing the temperature. At lower
pressure the rapid ordering of the hydrogen bonds generates heterogeneities
that are responsible for strong non-exponential behavior of the correlation
function, but with no strong increase of the correlation time. At very low
pressures, the gradual formation of the hydrogen bond network is responsible
for the large increase of the correlation time and, eventually, the dynamical
arrest of the system and of the dehydration process.Comment: 14 pages, 3 figure
Effect of hydrogen bond cooperativity on the behavior of water
Four scenarios have been proposed for the low--temperature phase behavior of
liquid water, each predicting different thermodynamics. The physical mechanism
which leads to each is debated. Moreover, it is still unclear which of the
scenarios best describes water, as there is no definitive experimental test.
Here we address both open issues within the framework of a microscopic cell
model by performing a study combining mean field calculations and Monte Carlo
simulations. We show that a common physical mechanism underlies each of the
four scenarios, and that two key physical quantities determine which of the
four scenarios describes water: (i) the strength of the directional component
of the hydrogen bond and (ii) the strength of the cooperative component of the
hydrogen bond. The four scenarios may be mapped in the space of these two
quantities. We argue that our conclusions are model-independent. Using
estimates from experimental data for H bond properties the model predicts that
the low-temperature phase diagram of water exhibits a liquid--liquid critical
point at positive pressure.Comment: 18 pages, 3 figure
Two-dimensional Packing in Prolate Granular Materials
We investigate the two-dimensional packing of extremely prolate (aspect ratio
) granular materials, comparing experiments with Monte-Carlo
simulations. The average packing fraction of particles with aspect ratio
is . We quantify the orientational correlation of
particles and find a correlation length of two particle lengths. The functional
form of the decay of orientational correlation is the same in both experiments
and simulations spanning three orders of magnitude in aspect ratio. This
function decays over a distance of two particle lengths. It is possible to
identify voids in the pile with sizes ranging over two orders of magnitude. The
experimental void distribution function is a power law with exponent
. Void distributions in simulated piles do not decay as a
power law, but do show a broad tail. We extend the simulation to investigate
the scaling at very large aspect ratios. A geometric argument predicts the pile
number density to scale as . Simulations do indeed scale this way,
but particle alignment complicates the picture, and the actual number densities
are quite a bit larger than predicted.Comment: 6 pages + 10 ps/eps figure
More than one dynamic crossover in protein hydration water
Studies of liquid water in its supercooled region have led to many insights
into the structure and behavior of water. While bulk water freezes at its
homogeneous nucleation temperature of approximately 235 K, for protein
hydration water, the binding of water molecules to the protein avoids
crystallization. Here we study the dynamics of the hydrogen bond (HB) network
of a percolating layer of water molecules, comparing measurements of a hydrated
globular protein with the results of a coarse-grained model that has been shown
to successfully reproduce the properties of hydration water. With dielectric
spectroscopy we measure the temperature dependence of the relaxation time of
protons charge fluctuations. These fluctuations are associated to the dynamics
of the HB network of water molecules adsorbed on the protein surface. With
Monte Carlo (MC) simulations and mean--field (MF) calculations we study the
dynamics and thermodynamics of the model. In both experimental and model
analyses we find two dynamic crossovers: (i) one at about 252 K, and (ii) one
at about 181 K. The agreement of the experiments with the model allows us to
relate the two crossovers to the presence of two specific heat maxima at
ambient pressure. The first is due to fluctuations in the HB formation, and the
second, at lower temperature, is due to the cooperative reordering of the HB
network
Astrometric and Photometric Analysis of the September 2008 ATV-1 Re-Entry Event
NASA utilized Image Intensified Video Cameras for ATV data acquisition from a jet flying at 12.8 km. Afterwards the video was digitized and then analyzed with a modified commercial software package, Image Systems Trackeye. Astrometric results were limited by saturation, plate scale, and imposed linear plate solution based on field reference stars. Time-dependent fragment angular trajectories, velocities, accelerations, and luminosities were derived in each video segment. It was evident that individual fragments behave differently. Photometric accuracy was insufficient to confidently assess correlations between luminosity and fragment spatial behavior (velocity, deceleration). Use of high resolution digital video cameras in future should remedy this shortcoming
X-ray Diffraction and Molecular Dynamics Study of Medium-range Order in Ambient and Hot Water
We have developed x-ray diffraction measurements with high energy-resolution
and accuracy to study water structure at three different temperatures (7, 25
and 66 C) under normal pressure. Using a spherically curved Ge crystal an
energy resolution better than 15 eV has been achieved which eliminates
influence from Compton scattering. The high quality of the data allows a
precise oxygen-oxygen pair correlation function (PCF) to be directly derived
from the Fourier transform of the experimental data resolving shell structure
out to ~12 {\AA}, i.e. 5 hydration shells. Large-scale molecular dynamics (MD)
simulations using the TIP4P/2005 force-field reproduce excellently the
experimental shell-structure in the range 4-12 {\AA} although less agreement is
seen for the first peak in the PCF. The Local Structure Index [J. Chem. Phys.
104, 7671 (1996)] identifies a tetrahedral minority giving the
intermediate-range oscillations in the PCF and a disordered majority providing
a more featureless background in this range. The current study supports the
proposal that the structure of liquid water, even at high temperatures, can be
described in terms of a two-state fluctuation model involving local structures
related to the high-density and low-density forms of liquid water postulated in
the liquid-liquid phase transition hypothesis.Comment: Submitted to Phys. Chem. Chem. Phy
Ab initio van der Waals interactions in simulations of water alter structure from mainly tetrahedral to high-density-like
The structure of liquid water at ambient conditions is studied in ab initio
molecular dynamics simulations using van der Waals (vdW) density-functional
theory, i.e. using the new exchange-correlation functionals optPBE-vdW and
vdW-DF2. Inclusion of the more isotropic vdW interactions counteracts highly
directional hydrogen-bonds, which are enhanced by standard functionals. This
brings about a softening of the microscopic structure of water, as seen from
the broadening of angular distribution functions and, in particular, from the
much lower and broader first peak in the oxygen-oxygen pair-correlation
function (PCF), indicating loss of structure in the outer solvation shells. In
combination with softer non-local correlation terms, as in the new
parameterization of vdW-DF, inclusion of vdW interactions is shown to shift the
balance of resulting structures from open tetrahedral to more close-packed. The
resulting O-O PCF shows some resemblance with experiment for high-density water
(A. K. Soper and M. A. Ricci, Phys. Rev. Lett., 84:2881, 2000), but not
directly with experiment for ambient water. However, an O-O PCF consisting of a
linear combination of 70% from vdW-DF2 and 30% from experiment on low-density
liquid water reproduces near-quantitatively the experimental O-O PCF for
ambient water, indicating consistency with a two-liquid model with fluctuations
between high- and low-density regions
Entropy-driven liquid-liquid separation in supercooled water
Twenty years ago Poole et al. (Nature 360, 324, 1992) suggested that the
anomalous properties of supercooled water may be caused by a critical point
that terminates a line of liquid-liquid separation of lower-density and
higher-density water. Here we present an explicit thermodynamic model based on
this hypothesis, which describes all available experimental data for
supercooled water with better quality and with fewer adjustable parameters than
any other model suggested so far. Liquid water at low temperatures is viewed as
an 'athermal solution' of two molecular structures with different entropies and
densities. Alternatively to popular models for water, in which the
liquid-liquid separation is driven by energy, the phase separation in the
athermal two-state water is driven by entropy upon increasing the pressure,
while the critical temperature is defined by the 'reaction' equilibrium
constant. In particular, the model predicts the location of density maxima at
the locus of a near-constant fraction (about 0.12) of the lower-density
structure.Comment: 7 pages, 6 figures. Version 2 contains an additional supplement with
tables for the mean-field equatio
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