358 research outputs found
Microwave Heating of Water, Ice and Saline Solution: Molecular Dynamics Study
In order to study the heating process of water by the microwaves of 2.5-20GHz
frequencies, we have performed molecular dynamics simulations by adopting a
non-polarized water model that have fixed point charges on rigid-body
molecules. All runs are started from the equilibrated states derived from the
I ice with given density and temperature. In the presence of microwaves,
the molecules of liquid water exhibit rotational motion whose average phase is
delayed from the microwave electric field. Microwave energy is transferred to
the kinetic and inter-molecular energies of water, where one third of the
absorbed microwave energy is stored as the latter energy. The water in ice
phase is scarcely heated by microwaves because of the tight hydrogen-bonded
network of water molecules. Addition of small amount of salt to pure water
substantially increases the heating rate because of the weakening by defects in
the water network due to sloshing large-size negative ions.Comment: 21 pages, 13 figure
Spectral responses in granular compaction
The slow compaction of a gently tapped granular packing is reminiscent of the
low-temperature dynamics of structural and spin glasses. Here, I probe the
dynamical spectrum of granular compaction by measuring a complex
(frequency-dependent) volumetric susceptibility . While the
packing density displays glass-like slow relaxations (aging) and
history-dependence (memory) at low tapping amplitudes, the susceptibility
displays very weak aging effects, and its spectrum shows no
sign of a rapidly growing timescale. These features place in
sharp contrast to its dielectric and magnetic counterparts in structural and
spin glasses; instead, bears close similarities to the complex
specific heat of spin glasses. This, I suggest, indicates the glass-like
dynamics in granular compaction are governed by statistically rare relaxation
processes that become increasingly separated in timescale from the typical
relaxations of the system. Finally, I examine the effect of finite system size
on the spectrum of compaction dynamics. Starting from the ansatz that low
frequency processes correspond to large scale particle rearrangements, I
suggest the observed finite size effects are consistent with the suppression of
large-scale collective rearrangements in small systems.Comment: 18 pages, 17 figures. Submitted to PR
Power-law decay in first-order relaxation processes
Starting from a simple definition of stationary regime in first-order
relaxation processes, we obtain that experimental results are to be fitted to a
power-law when approaching the stationary limit. On the basis of this result we
propose a graphical representation that allows the discrimination between
power-law and stretched exponential time decays. Examples of fittings of
magnetic, dielectric and simulated relaxation data support the results.Comment: to appear in Phys. Rev. B; 4 figure
Origin of temperature dependent conductivity of -polyvinylidene fluoride
The conductivity of -polyvinylidene fluoride (-PVDF) is
obtained from dielectric measurements performed in the frequency domain at
several temperatures. At temperatures above the glass-transition, the
conductivity can be interpreted as an ionic conductivity, which confirms
earlier results reported in the literature. Our investigation shows that the
observed ionic conductivity is closely related to the amorphous phase of the
polymer
Dielectric relaxation of DNA aqueous solutions
We report on a detailed characterization of complex dielectric response of
Na-DNA aqueous solutions by means of low-frequency dielectric spectroscopy (40
Hz - 110 MHz). Results reveal two broad relaxation modes of strength
20<\Delta\epsilon_LF<100 and 5<\Delta\epsilon_HF<20, centered at 0.5
kHz<\nu_LF<70 kHz and 0.1 MHz<\nu_HF<15 MHz. The characteristic length scale of
the LF process, 50<L_LF<750nm, scales with DNA concentration as
c_DNA^{-0.29\pm0.04} and is independent of the ionic strength in the low added
salt regime. Conversely, the measured length scale of the LF process does not
vary with DNA concentration but depends on the ionic strength of the added salt
as I_s^{-1} in the high added salt regime. On the other hand, the
characteristic length scale of the HF process, 3<L_HF<50 nm, varyes with DNA
concentration as c_DNA^{-0.5} for intermediate and large DNA concentrations. At
low DNA concentrations and in the low added salt limit the characteristic
length scale of the HF process scales as c_DNA^{-0.33}. We put these results in
perspective regarding the integrity of the double stranded form of DNA at low
salt conditions as well as regarding the role of different types of counterions
in different regimes of dielectric dispersion. We argue that the free DNA
counterions are primarily active in the HF relaxation, while the condensed
counterions play a role only in the LF relaxation. We also suggest theoretical
interpretations for all these length scales in the whole regime of DNA and salt
concentrations and discuss their ramifications and limitations.Comment: 15 pages, 9 figure
Experimental search for dynamic heterogeneities in molecular glass formers
We have measured the linear dielectric susceptibility of two molecular glass
formers close to Tg in order to estimate the size of the dynamically correlated
clusters of molecules which are expected to govern the physics of glass
formation. This size has been shown to be related to the dynamic dielectric
susceptibility dEps(w)/dT (Eps : dielectric susceptibility, T : temperature,
w: frequency). To allow for an accurate determination of the T
derivative, we scanned the interval 192 < T < 232 K every 1 K for glycerol and
159 < T < 179 K every 0.5 K for propylene carbonate. The resolution on T
variations was about 1 mK. The result for glycerol is that the number of
correlated molecules increases by a factor 3 when T goes from 226 to 195 K. It
has been shown that the non-linear susceptibility provides a direct measurement
of dynamic correlations. To measure it, we used a standard Lockin technique
yielding the third harmonic of the current flowing out of a capacitor. We
obtained only an upper limit on the ratio of the third to the first harmonic,
due to the non-linear response of standard electronics.Comment: 7 page
Signs of low frequency dispersions in disordered binary dielectric mixtures (50-50)
Dielectric relaxation in disordered dielectric mixtures are presented by
emphasizing the interfacial polarization. The obtained results coincide with
and cause confusion with those of the low frequency dispersion behavior. The
considered systems are composed of two phases on two-dimensional square and
triangular topological networks. We use the finite element method to calculate
the effective dielectric permittivities of randomly generated structures. The
dielectric relaxation phenomena together with the dielectric permittivity
values at constant frequencies are investigated, and significant differences of
the square and triangular topologies are observed. The frequency dependent
properties of some of the generated structures are examined. We conclude that
the topological disorder may lead to the normal or anomalous low frequency
dispersion if the electrical properties of the phases are chosen properly, such
that for ``slightly'' {\em reciprocal mixture}--when , and
--normal, and while for ``extreme'' {\em reciprocal
mixture}--when , and --anomalous
low frequency dispersions are obtained. Finally, comparison with experimental
data indicates that one can obtain valuable information from simulations when
the material properties of the constituents are not available and of
importance.Comment: 13 pages, 7 figure
Out of equilibrium thermal Casimir effect in a model polarizable material
Relaxation of the thermal Casimir or van der Waals force for a model
dielectric medium is investigated. We start with a model of interacting
polarization fields with a dynamics that leads to a frequency dependent
dielectric constant of the Debye form. In the static limit the usual zero
frequency Matsubara mode component of the Casimir force is recovered. We then
consider the out of equilibrium relaxation of the van der Waals force to its
equilibrium value when two initially uncorrelated dielectric bodies are brought
into sudden proximity. It is found that the spatial dependence of the out of
equilibrium force is the same as the equilibrium one but it has a time
dependent amplitude, or Hamaker coefficient, which increases in time to its
equilibrium value. The final relaxation to the equilibrium value is exponential
in systems with a single or finite number of polarization field relaxation
times. However, in systems, such as those described by the Havriliak-Negami
dielectric constant, with a broad distribution of relaxation times, we observe
a much slower power law decay to the equilibrium value.Comment: 15 pages RevTex, 4 figure
Minimal model for beta relaxation in viscous liquids
Contrasts between beta relaxation in equilibrium viscous liquids and glasses
are rationalized in terms of a double-well potential model with
structure-dependent asymmetry, assuming structure is described by a single
order parameter. The model is tested for tripropylene glycol where it accounts
for the hysteresis of the dielectric beta loss peak frequency and magnitude
during cooling and reheating through the glass transition.Comment: Phys. Rev. Lett. (in press
Transitions/relaxations in polyester adhesive/PET system
The correlations between the transitions and the dielectric relaxation processes of the oriented poly(ethylene terephthalate) (PET) pre-impregnated of the polyester thermoplastic adhesive have been investigated by differential scanning calorimetry (DSC) and dynamic dielectric spectroscopy (DDS). The thermoplastic polyester adhesive and the oriented PET films have been studied as reference samples. This study evidences that the adhesive chain segments is responsible for the physical structure evolution in the PET-oriented film. The transitions and dielectric relaxation modesâ evolutions in the glass transition region appear characteristic of the interphase between adhesive and PET film, which is discussed in terms of molecular mobility. The storage at room temperature of the adhesive tape involves the heterogeneity of the physical structure, characterized by glass transition dissociation. Thus, the correlation between the transitions and the dielectric relaxation processes evidences a segregation of the amorphous phases. Therefore, the physical structure and the properties of the material have been linked to the chemical characteristics
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