5,965 research outputs found
Statistical Mechanics of Time Independent Non-Dissipative Nonequilibrium States
We examine the question of whether the formal expressions of equilibrium
statistical mechanics can be applied to time independent non-dissipative
systems that are not in true thermodynamic equilibrium and are nonergodic. By
assuming the phase space may be divided into time independent, locally ergodic
domains, we argue that within such domains the relative probabilities of
microstates are given by the standard Boltzmann weights. In contrast to
previous energy landscape treatments, that have been developed specifically for
the glass transition, we do not impose an a priori knowledge of the
inter-domain population distribution. Assuming that these domains are robust
with respect to small changes in thermodynamic state variables we derive a
variety of fluctuation formulae for these systems. We verify our theoretical
results using molecular dynamics simulations on a model glass forming system.
Non-equilibrium Transient Fluctuation Relations are derived for the
fluctuations resulting from a sudden finite change to the system's temperature
or pressure and these are shown to be consistent with the simulation results.
The necessary and sufficient conditions for these relations to be valid are
that the domains are internally populated by Boltzmann statistics and that the
domains are robust. The Transient Fluctuation Relations thus provide an
independent quantitative justification for the assumptions used in our
statistical mechanical treatment of these systems.Comment: 17 pages, 4 figures, minor amendment
The rheology of solid glass
As the glass transition is approached from the high temperature side, viewed as a liquid, the properties of the ever more viscous supercooled liquid are continuous functions of temperature and pressure. The point at which we decide to classify the fluid as a solid is therefore subjective. This subjective decision does, however, have discontinuous consequences for how we determine the rheological properties of the glass. We apply the recently discovered relaxation theorem to the time independent, nondissipative, nonergodic glassy state to derive an expression for the phase space distribution of an ensemble of glass samples. This distribution is then used to construct a time dependent linear response theory for aged glassysolids. The theory is verified using molecular dynamics simulations of oscillatory shear for a realistic model glass former with excellent agreement being obtained between the response theory calculations and direct nonequilibrium molecular dynamics calculations. Our numerical results confirm that unlike all the fluid states, including supercooled liquids, a solidglass (in common with crystalline states) has a nonzero value for the zero frequency shear modulus. Of all the states of matter, a supercooled fluid approaching the glass transition has the highest value for the limiting zero frequency shear viscosity. Finally, solidglasses like dilute gases and crystals have a positive temperature coefficient for the shear viscosity whereas supercooled and normal liquids have a negative temperature coefficient.We thank the National Computational Infrastructure
NCI for computational facilities and the Australian
Research Council ARC for funding
The release of a captive-raised female African Elephant <em>(Loxodonta africana</em>) in the Okavango Delta, Botswana
Wild female elephants live in close-knit matrilineal groups and housing captive elephants in artificial social groupings can cause significant welfare issues for individuals not accepted by other group members. We document the release of a captive-raised female elephant used in the safari industry because of welfare and management problems. She was fitted with a satellite collar, and spatial and behavioural data were collected over a 17-month period to quantify her interactions with the wild population. She was then monitored infrequently for a further five-and-a-half years. We observed few signs of aggression towards her from the wild elephants with which she socialized. She used an area of comparable size to wild female elephants, and this continued to increase as she explored new areas. Although she did not fully integrate into a wild herd, she had three calves of her own, and formed a social unit with another female and her calf that were later released from the same captive herd. We recommend that release to the wild be considered as a management option for other captive female elephants
Verification of time-reversibility requirementfor systems satisfying the Evans-Searles fluctuation theorem
The Evans-Searles fluctuation theorem (ESFT) has been shown to be applicable in the near- and far-from-equilibrium regimes for systems with both constant and time-dependent external fields. The derivations of the ESFT have assumed that the external field has a definite parity under a time-reversal mapping. In the present paper, we confirm that the time-reversibility of the system dynamics is a necessary condition for the ESFT to hold. The manner in which the ESFT fails for systems that are not time-reversible is presented, and results are shown which demonstrate that systems which fail to satisfy the ESFT may still satisfy the Crooks relation (CR)
On the relaxation to nonequilibrium steady states
The issue of relaxation has been addressed in terms of ergodic theory in the
past. However, the application of that theory to models of physical interest is
problematic, especially when dealing with relaxation to nonequilibrium steady
states. Here, we consider the relaxation of classical, thermostatted particle
systems to equilibrium as well as to nonequilibrium steady states, using
dynamical notions including decay of correlations. We show that the condition
known as {\Omega}T-mixing is necessary and sufficient to prove relaxation of
ensemble averages to steady state values. We then observe that the condition
known as weak T-mixing applied to smooth observables is sufficient for
relaxation to be independent of the initial ensemble. Lastly, weak T-mixing for
integrable functions makes relaxation independent of the ensemble member, apart
from a negligible set of members enabling the result to be applied to
observations from a single physical experiment. The results also allow us to
give a microscopic derivation of Prigogine's principle of minimum entropy
production in the linear response regime. The key to deriving these results
lies in shifting the discussion from characteristics of dynamical systems, such
as those related to metric transitivity, to physical measurements and to the
behaviour of observables. This naturally leads to the notion of physical
ergodicity.Comment: 44 pages, 1 figur
The Glass Transition and the Jarzynski Equality
A simple model featuring a double well potential is used to represent a
liquid that is quenched from an ergodic state into a history dependent glassy
state. Issues surrounding the application of the Jarzynski Equality to glass
formation are investigated. We demonstrate that the Jarzynski Equality gives
the free energy difference between the initial state and the state we would
obtain if the glass relaxed to true thermodynamic equilibrium. We derive new
variations of the Jarzynski Equality which are relevant to the history
dependent glassy state rather than the underlying equilibrium state. It is
shown how to compute the free energy differences for the nonequilibrium history
dependent glassy state such that it remains consistent with the standard
expression for the entropy and with the second law inequality.Comment: 16 pages, 5 figure
In situ apparatus for the study of clathrate hydrates relevant to solar system bodies using synchrotron X-ray diffraction and Raman spectroscopy
Clathrate hydrates are believed to play a significant role in various solar
system environments, e.g. comets, and the surfaces and interiors of icy
satellites, however the structural factors governing their formation and
dissociation are poorly understood. We demonstrate the use of a high pressure
gas cell, combined with variable temperature cooling and time-resolved data
collection, to the in situ study of clathrate hydrates under conditions
relevant to solar system environments. Clathrates formed and processed within
the cell are monitored in situ using synchrotron X-ray powder diffraction and
Raman spectroscopy. X-ray diffraction allows the formation of clathrate
hydrates to be observed as CO2 gas is applied to ice formed within the cell.
Complete conversion is obtained by annealing at temperatures just below the ice
melting point. A subsequent rise in the quantity of clathrate is observed as
the cell is thermally cycled. Four regions between 100-5000cm-1 are present in
the Raman spectra that carry features characteristic of both ice and clathrate
formation. This novel experimental arrangement is well suited to studying
clathrate hydrates over a range of temperature (80-500K) and pressure
(1-100bar) conditions and can be used with a variety of different gases and
starting aqueous compositions. We propose the increase in clathrate formation
observed during thermal cycling may be due to the formation of a quasi
liquid-like phase that forms at temperatures below the ice melting point, but
which allows easier formation of new clathrate cages, or the retention and
delocalisation of previously formed clathrate structures, possibly as amorphous
clathrate. The structural similarities between hexagonal ice, the quasi
liquid-like phase, and crystalline CO2 hydrate mean that differences in the
Raman spectrum are subtle; however, all features out to 5000cm-1 are diagnostic
of clathrate structure.Comment: Astronomy & Astrophysics, in press. 6 page
Sher 25: pulsating but apparently alone
The blue supergiant Sher25 is surrounded by an asymmetric, hourglass-shaped
circumstellar nebula, which shows similarities to the triple-ring structure
seen around SN1987A. From optical spectroscopy over six consecutive nights, we
detect periodic radial velocity variations in the stellar spectrum of Sher25
with a peak-to-peak amplitude of ~12 km/s on a timescale of about 6 days,
confirming the tentative detec-tion of similar variations by Hendry et al. From
consideration of the amplitude and timescale of the signal, coupled with
observed line profile variations, we propose that the physical origin of these
variations is related to pulsations in the stellar atmosphere, rejecting the
previous hypothesis of a massive, short-period binary companion. The radial
velocities of two other blue supergiants with similar bipolar nebulae, SBW1 and
HD 168625, were also monitored over the course of six nights, but these did not
display any significant radial velocity variations.Comment: 9 pages, 7 figures. Accepted for publication in MNRA
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