9 research outputs found
Steering in computational science: mesoscale modelling and simulation
This paper outlines the benefits of computational steering for high
performance computing applications. Lattice-Boltzmann mesoscale fluid
simulations of binary and ternary amphiphilic fluids in two and three
dimensions are used to illustrate the substantial improvements which
computational steering offers in terms of resource efficiency and time to
discover new physics. We discuss details of our current steering
implementations and describe their future outlook with the advent of
computational grids.Comment: 40 pages, 11 figures. Accepted for publication in Contemporary
Physic
Transport properties of supercooled confined water
This article presents an overview of recent experiments
performed on transport properties of water in the deeply supercooled
region, a temperature region of fundamental importance in the science of water.
We report data of nuclear magnetic resonance, quasi-elastic neutron scattering,
Fourier-transform infrared spectroscopy, and Raman spectroscopy, studying
water confined in nanometer-scale environments. When contained within small
pores, water does not crystallise, and can be supercooled well below its
homogeneous nucleation temperature Th. On this basis it is possible to
carry out a careful analysis of the well known thermodynamical anomalies of
water. Studying the temperature and pressure dependencies of water
dynamics, we show that the liquid-liquid phase transition (LLPT) hypothesis
represents a reliable model for describing liquid water. In this
model, water in the liquid state is a mixture of two different local
structures, characterised by different densities, namely the low density
liquid (LDL) and the high-density liquid (HDL). The LLPT line should terminate
at a special transition point: a low-T liquid-liquid
critical point. We discuss the following experimental findings on
liquid water: (i) a crossover from non-Arrhenius behaviour
at high T to Arrhenius behaviour at low T in transport parameters;
(ii) a breakdown of the Stokes-Einstein relation;
(iii) the existence of a Widom line, which is the
locus of points corresponding to maximum correlation length in the p-T phase
diagram and which ends in the liquid-liquid critical point; (iv) the direct
observation of the LDL phase; (v) a
minimum in the density at approximately 70 K below the
temperature of the density maximum. In our opinion these results represent
the experimental proofs of the validity of the LLPT hypothesis
NMR evidence of a sharp change in a measure of local order in deeply supercooled confined water
Using NMR, we measure the proton chemical shift δ, of supercooled nanoconfined water in the temperature range 195 K < T < 350 K. Because δ is directly connected to the magnetic shielding tensor, we discuss the data in terms of the local hydrogen bond geometry and order. We argue that the derivative −(∂ ln δ/∂T)P should behave roughly as the constant pressure specific heat CP(T), and we confirm this argument by detailed comparisons with literature values of CP(T) in the range 290–370 K. We find that −(∂ ln δ/∂T)P displays a pronounced maximum upon crossing the locus of maximum correlation length at ≈240 K, consistent with the liquid-liquid critical point hypothesis for water, which predicts that CP(T) displays a maximum on crossing the Widom line