4,711 research outputs found
Crystallization of the Wahnstr\"om Binary Lennard-Jones Liquid
We report observation of crystallization of the glass-forming binary
Lennard-Jones liquid first used by Wahnstr\"om [G. Wahnstr\"om, Phys. Rev. A
44, 3752 (1991)]. Molecular dynamics simulations of the metastable liquid on a
timescale of microseconds were performed. The liquid crystallized
spontaneously. The crystal structure was identified as MgZn_2. Formation of
transient crystallites is observed in the liquid. The crystallization is
investigate at different temperatures and compositions. At high temperature the
rate of crystallite formation is the limiting factor, while at low temperature
the limiting factor is growth rate. The melting temperature of the crystal is
estimated to be T_m=0.93 at rho=0.82. The maximum crystallization rate of the
A_2B composition is T=0.60+/-0.02.Comment: 4 pages, 4 figures; corrected typo
Exponential distributions of collective flow-event properties in viscous liquid dynamics
We study the statistics of flow events in the inherent dynamics in
supercooled two- and three-dimensional binary Lennard-Jones liquids.
Distributions of changes of the collective quantities energy, pressure and
shear stress become exponential at low temperatures, as does that of the event
"size" . We show how the -distribution controls the
others, while itself following from exponential tails in the distributions of
(1) single particle displacements , involving a Lindemann-like length
and (2) the number of active particles (with ).Comment: Accepter version (PRL
Strong pressure-energy correlations in liquids as a configuration space property: Simulations of temperature down jumps and crystallization
Computer simulations recently revealed that several liquids exhibit strong
correlations between virial and potential energy equilibrium fluctuations in
the NVT ensemble [U. R. Pedersen {\it et al.}, Phys. Rev. Lett. {\bf 100},
015701 (2008)]. In order to investigate whether these correlations are present
also far from equilibrium constant-volume aging following a temperature down
jump from equilibrium was simulated for two strongly correlating liquids, an
asymmetric dumbbell model and Lewis-Wahnstr{\"o}m OTP, as well as for SPC water
that is not strongly correlating. For the two strongly correlating liquids
virial and potential energy follow each other closely during the aging towards
equilibrium. For SPC water, on the other hand, virial and potential energy vary
with little correlation as the system ages towards equilibrium. Further proof
that strong pressure-energy correlations express a configuration space property
comes from monitoring pressure and energy during the crystallization (reported
here for the first time) of supercooled Lewis-Wahnstr{\"o}m OTP at constant
temperature
Estimating the density scaling exponent of viscous liquids from specific heat and bulk modulus data
It was recently shown by computer simulations that a large class of liquids
exhibits strong correlations in their thermal fluctuations of virial and
potential energy [Pedersen et al., Phys. Rev. Lett. 100, 015701 (2008)]. Among
organic liquids the class of strongly correlating liquids includes van der
Waals liquids, but excludes ionic and hydrogen-bonding liquids. The present
note focuses on the density scaling of strongly correlating liquids, i.e., the
fact their relaxation time tau at different densities rho and temperatures T
collapses to a master curve according to the expression tau propto
F(rho^gamma/T) [Schroder et al., arXiv:0803.2199]. We here show how to
calculate the exponent gamma from bulk modulus and specific heat data, either
measured as functions of frequency in the metastable liquid or extrapolated
from the glass and liquid phases to a common temperature (close to the glass
transition temperature). Thus an exponent defined from the response to highly
nonlinear parameter changes may be determined from linear response
measurements
Pressure-energy correlations in liquids. V. Isomorphs in generalized Lennard-Jones systems
This series of papers is devoted to identifying and explaining the properties
of strongly correlating liquids, i.e., liquids with more than 90% correlation
between their virial W and potential energy U fluctuations in the NVT ensemble.
Paper IV [N. Gnan et al., J. Chem. Phys. v131, 234504 (2009)] showed that
strongly correlating liquids have "isomorphs", which are curves in the phase
diagram along which structure, dynamics, and some thermodynamic properties are
invariant in reduced units. In the present paper, using the fact that
reduced-unit radial distribution functions are isomorph invariant, we derive an
expression for the shapes of isomorphs in the WU phase diagram of generalized
Lennard-Jones systems of one or more types of particles. The isomorph shape
depends only on the Lennard-Jones exponents; thus all isomorphs of standard
Lennard-Jones systems (with exponents 12 and 6) can be scaled onto to a single
curve. Two applications are given. One is testing the prediction that the
solid-liquid coexistence curve follows an isomorph by comparing to recent
simulations by Ahmed and Sadus [J. Chem. Phys. v131, 174504 (2009)]. Excellent
agreement is found on the liquid side of the coexistence, whereas the agreement
is worse on the solid side. A second application is the derivation of an
approximate equation of state for generalized Lennard-Jones systems by
combining the isomorph theory with the Rosenfeld-Tarazona expression for the
temperature dependence of potential energy on isochores. It is shown that the
new equation of state agrees well with simulations.Comment: 12 pages, 14 figures, Section on solid-liquid coexistence expande
Strong pressure-energy correlations in van der Waals liquids
Strong correlations between equilibrium fluctuations of the configurational
parts of pressure and energy are found in the Lennard-Jones liquid and other
simple liquids, but not in hydrogen-bonding liquids like methanol and water.
The correlations, that are present also in the crystal and glass phases,
reflect an effective inverse power-law repulsive potential dominating
fluctuations, even at zero and slightly negative pressure. In experimental data
for supercritical Argon, the correlations are found to be approximately 96%.
Consequences for viscous liquid dynamics are discussed.Comment: Phys. Rev. Lett., in pres
A stable but reversible integrated surrogate reporter for assaying CRISPR/Cas9-stimulated homology-directed repair
Pressure-energy correlations in liquids. IV. "Isomorphs" in liquid state diagrams
This paper is the fourth in a series devoted to identifying and explaining
the properties of strongly correlating liquids, i.e., liquids where virial and
potential energy correlate better than 90% in their thermal equilibrium
fluctuations in the NVT ensemble. For such liquids we here introduce the
concept of "isomorphic" curves in the state diagram. A number of thermodynamic,
static, and dynamic isomorph invariants are identified. These include the
excess entropy, the isochoric specific heat, reduced-unit static and dynamic
correlation functions, as well as reduced-unit transport coefficients. The
dynamic invariants apply for both Newtonian and Brownian dynamics. It is shown
that after a jump between isomorphic state points the system is instantaneously
in thermal equilibrium; consequences of this for generic aging experiments are
discussed. Selected isomorph predictions are validated by computer simulations
of the Kob-Andersen binary Lennard-Jones mixture, which is a strongly
correlating liquid. The final section of the paper relates the isomorph concept
to phenomenological melting rules, Rosenfeld's excess entropy scaling, Young
and Andersen's approximate scaling principle, and the two-order parameter maps
of Debenedetti and coworkers. This section also shows how the existence of
isomorphs implies an "isomorph filter" for theories for the non-Arrhenius
temperature dependence of viscous liquids' relaxation time, as well as explains
isochronal superposition for strongly correlating viscous liquids
Variation of the dynamic susceptibility along an isochrone
Koperwas {\it et al.} showed in a recent paper, Phys. Rev. Lett. {\bf 111},
125701 (2013), that the dynamic susceptibility as estimated by
dielectric measurements for certain glass-forming liquids decreases
substantially with increasing pressure along a curve of constant relaxation
time. This observation is at odds with other measures of dynamics being
invariant and seems to pose a problem for theories of glass formation. We show
that this variation is in fact consistent with predictions for liquids with
hidden scale invariance: measures of dynamics at constant volume are invariant
along isochrones, called isomorphs in such liquids, but contributions to
fluctuations from long-wavelength fluctuations can vary. This is related to the
known non-invariance of the isothermal bulk modulus. Considering the version of
defined for the NVT ensemble, data from simulations of a binary
Lennard-Jones liquid show in fact a slight increase with increasing density.
This is a true departure from the formal invariance expected for this quantity.Comment: Expanded to include simulation data, and accepted for publication in
Phys. Rev.
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