12 research outputs found
Orientational dynamics in supercooled glycerol computed from MD simulations: self and cross contributions
The orientational dynamics of supercooled glycerol using molecular dynamics
simulations for temperatures ranging from 323 K to 253 K, is probed through
correlation functions of first and second ranks of Legendre polynomials,
pertaining respectively to dielectric spectroscopy (DS) and depolarized dynamic
light scattering (DDLS). The self, cross, and total correlation functions are
compared with relevant experimental data. The computations reveal the low
sensitivity of DDLS to cross-correlations, in agreement with what is found in
experimental work, and strengthen the idea of directly comparing DS and DDLS
data to evaluate the effect of cross-correlations in polar liquids. The
analysis of the net static cross-correlations and their spatial decomposition
shows that, although cross-correlations extend over nanometric distances, their
net magnitude originates, in the case of glycerol, from the first shell of
neighbouring molecules. Accessing the angular dependence of the static
correlation allows us to get a microscopic understanding of why the rank-1
correlation function is more sensitive to cross-correlation than its rank-2
counterpart.Comment: 9 pages, 6 figure
Magnetic relaxation of a system of superparamagnetic particles weakly coupled by dipole-dipole interactions
The effect of long range dipole-dipole interactions on the thermal
fluctuations of the magnetization of an assembly of single-domain ferromagnetic
particles is considered. If orientational correlations between the particles
are neglected, the evolution of the magnetization orientations may be described
by a nonlinear Fokker-Planck equation (FPE) reducing to the usual linear one in
the limit of infinite dilution [W.F. Brown Jr, Phys. Rev. 130, 1677 (1963)].
The thermally activated relaxation time scale of the assembly is estimated,
leading to a simple modification of the axially symmetric asymptotes for the
superparamagnetic relaxation time.Comment: 31 pages, 3 figures, regular articl
Temperature dependence of the static permittivity andintegral formula for the Kirkwood correlation factor ofsimple polar fluids
An exact integral formula for the Kirkwood correlation factor of isotropic
polar fluids is derived from the equilibrium averaged rotational Dean
equation, which as compared to previous approaches easily lends itself to
further approximations. The static linear permittivity of polar fluids
is calculated as a function of temperature, density and molecular
dipole moment in vacuo for arbitrary pair interaction potentials. Then, using
the Kirkwood superposition approximation for the three-body orientational
distribution function, we suggest a simple way to construct model potentials of
mean torques considering permanent and induced dipole moments. We successfully
compare the theory with the experimental temperature dependence of the static
linear permittivity of various polar fluids such as a series of linear
monohydroxy alcohols, water, tributyl phosphate, acetonitrile, acetone,
nitrobenzene and dimethyl sulfoxide, by fitting only one single parameter,
which describes the induction to dipole-dipole energy strength ratio. We
demonstrate that comparing the value of with unity in order to deduce the
alignment state of permanent dipole pairs, as is currently done is in many
situations, is a misleading oversimplification, while the correct alignment
state is revealed when considering the proper interaction potential. Moreover
we show, that picturing H-bonding polar fluids as polar molecules with
permanent and induced dipole moments without invoking any specific H-bonding
mechanism is in many cases sufficient to explain experimental data of the
static dielectric constant. In this light, the failure of the theory to
describe the experimental temperature dependence of the static dielectric
constant of glycerol, a non-rigid polyalcohol, is not due to the lack of
specific H-bonding mechanisms, but rather to an oversimplified model potential
for that particular molecule
Derivation of magnetic inertial effects from the classical mechanics of a circular current loop
International audienceThe dynamical equation of a single magnetic moment constituted by a rigid circular current loop is derived from the mechanical Lagrange equations of motion, introducing the Lorentz force and the damping process, described by a well defined dissipative mechanism. It is demonstrated that magnetic inertial effects arise naturally by simple mechanical considerations and superimpose onto Gilbert original dynamical equation. The comparison with models proposed in the recent literature is drawn and discussed
Orientational dynamics in supercooled glycerol computed from MD simulations: self and cross contributions
The orientational dynamics of supercooled glycerol is probed using molecular dynamics simulations for temperatures ranging from 323 K to 253 K, through correlation functions of first and second ranks of Legendre polynomials, pertaining respectively to dielectric spectroscopy (DS) and depolarized dynamic light scattering (DDLS). The self, cross, and total correlation functions are compared with relevant experimental data. The computations reveal the low sensitivity of DDLS to cross-correlations, in agreement with what is found in experimental work, and strengthen the idea of directly comparing DS and DDLS data to evaluate the effect of cross-correlations in polar liquids. The analysis of the net static cross-correlations and their spatial decomposition shows that, although cross-correlations extend over nanometric distances, their net magnitude originates, in the case of glycerol, from the first shell of neighbouring molecules. Accessing the angular dependence of the static correlation allows us to get a microscopic understanding of why the rank-1 correlation function is more sensitive to cross-correlation than its rank-2 counterpart
Temperature dependence of the Kirkwood correlation factor and linear dielectric constant of simple isotropic polar fluids.
The theory developed in an accompanying paper [Déjardin, Phys. Rev. E 105, 024109 (2022)10.1103/PhysRevE.105.024109] is used to compute the Kirkwood correlation factor of simple polar fluids of different nature. From this calculation, the theoretical static permittivity is readily obtained, which is compared with experimental values. This is accomplished by fitting only one parameter accounting for induction or dispersion forces and torques, which is necessarily connected with the individual molecular polarizability but not explicitly related to the physical properties due to the nonadditivity of such energies. Excellent agreement between theoretical and experimental static permittivities is obtained over a very broad temperature range for a number of associated and nonassociated liquids. Finally, limitations of the present theory are given