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Physical and Theoretical Chemistry
Liquid State Chemical Physics
Professor Denis Evans
http://rsc.anu.edu.au/research/evans.php
Our research interests include
nonequilibrium statistical mechanics and thermodynamics. We have
been involved in the development of nearly all of the computer
simulation algorithms used in the calculation of transport properties
of classical atomic, molecular, short-chain polymeric fluids and
lubricants. Algorithms that we have proposed are used to compute the
viscosities, thermal conductivities, and diffusion coefficients for
molecular fluids and fluid mixtures, and are presently being used in
thermophysical data correlation packages provided to the chemical
industry by the National Institute of Standards and Technology
(NIST), Boulder, Colorado.
These
practical applications are based on the theory of nonequilibrium
steady states, also developed by this group. Our work on the theory
of thermostatted nonequilibrium systems provides a framework within
which exact relationships between nonequilibrium fluctuations and
measurable thermophysical properties have been established.
We
derived the first exact, practical link between the theory of chaos,
dynamical systems theory, and thermophysical properties. This link
shows that a transport coefficient like shear viscosity is related in
a direct, quantitative way to the stability of molecular
trajectories. Later we derived the so-called Fluctuation Theorem
(FT). This remarkable theorem gives an analytic expression for the
probability that in a nonequilibrium steady state of finite size,
observed for a finite time, the dissipative flux flows in the reverse
direction to that required by the Second Law of Thermodynamics.
Close to equilibrium the FT can be used to derive both Einstein and
Green-Kubo relations for transport coefficients. In a collaboration
with members of the Polymers and Soft Condensed Matter Group,
the FT has been verified experimentally.
Fluctuation Theorem
Considerable
progress was made in understanding the basis of the Fluctuation
Theorem. A major review was published late in the year in Advances in
Physics. During the year we proposed a version of the FT, that
applies to thermostatted dissipative systems which respond to time
dependent dissipative fields. In testing the time dependent
Fluctuation Theorem we provided for the first time, convincing
evidence that sets of trajectories with conjugate values for the time
integrated entropy production, (±A±dA), are indeed,
time reversed images of one another. "The famous problem,
the creation of anti-events from events has no solution. Although,
using simple instructions, the [solution] may be put into words:
reverse the instantaneous velocities of all of the atoms in the
Universe" - Loschmidt, 1876. For an ensemble of experiments
we now know that we can observe conjugate pairs of time reversed
responses without intervening and actually reversing particle
velocities. All one has to do is to sort the ensemble of responses
on the basis of their time integrated dissipation functions (entropy
production in thermostatted systems), and to compare those responses
with complementary values of total dissipation. These responses will
be time reversed mappings of each other. The ratio of probabilities
of observing these complementary time integrated values of
dissipation are given by the Fluctuation Theorem, with Second Law
satisfying responses being exponentially dominant.
Configurational Temperature
Work
continued towards understanding how to measure and how to control the
temperature of a system through purely configurational means - with
no knowledge of, or changes to, molecular velocities. The emphasis in
this work has moved to molecular rather than atomic systems. (with
J. Delhomelle [U. Henri Poincaré, France])
Transient Time Correlation Function
Work
was completed on a demonstration of the utility of the Transient Time
Correlation Function formalism for calculating transport coefficients
over an extremely wide range of external field strengths. (with I.
Borzsak, [Hungarian Academy Sc], P. Cummings, [Oak Ridge Nat. Lab.
and Vanderbilt USA.])
Transport Properties of Molten Salts
We
investigated the transport properties of molten salts, such as shear
viscosity and electrical conductivity in dc and ac fields using
non-equilibrium molecular dynamics. In particular, we investigated
the influence of the thermostat used in simulation on the results in
strong external fields. Molten salts were shown to have a larger
Newtonian region of shear viscosity than simple liquids, and the
choice of thermostatting method had little influence on the results
for the investigated range of shear rates. In an electric field,
dependence of the results on thermostat becomes apparent only at
extremely high fields (greater than 0.5¥109 V/m). For this range
of fields, quantitative differences of unexpected size can be seen in
the melt. In the supercritical fluid, different thermostats predict
qualitatively very different behaviour and structure. While the
kinetic-type thermostats predict increased association of ions in the
field, configurational thermostat predicts enhanced dissociation. The
anomalous behaviour of "configurational" temperature helped
us gain new insights into its physical meaning. (with J.
Delhomelle [U. Henri Poincaré, France])
Non-equilibrium Simulations of a Hard Sphere Fluid
Analytic
solutions for free trajectories and collisions of a hard sphere fluid
under shear and in a constant colour field with a Gauss kinetic
thermostat have been found. In both systems, the solutions provided
surprising insights into the mechanical aspects of thermostatting a
system in an external field. For the sheared fluid, the equivalence
of constant temperature and constant energy ensembles in the
thermodynamic limit in equilibrium, the conditions for the nature of
heat exchange with the environment, and the condition for
appearance of the artificial
string phase followed from the solution. The colour field solution
permitted us to comment on the similarities and differences between
an ionic and a "coloured" liquid. In this case, the first
non-equilibrium hard sphere simulation with curved trajectories was
performed in order to verify the analytic predictions. (with O.G.
Jepps, [Griffith U.])
Soft Condensed Matter
This
project continues a collaboration between the RSC, The Australian
Nuclear Science and Technology Organisation (ANSTO) and the National
Institute of Standards and Technology (NIST), USA. The objective is
to understand better the relationship between the properties of a
system and its structure. Soft condensed disordered systems are
emphasised - for example: gels, precipitates in petroleum fluids,
polymeric solutions and melts, micelles and macromolecules in
solutions, and inorganic/organic complexes. The project has promoted
and encouraged the use of the small angle neutron scattering
instruments at the ANSTO facility and the NIST Center for Neutron
Research, under an agreement between ANSTO and NIST. (with H.J.M.
Hanley)
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