1,579 research outputs found
Detecting short periods of elevated workload. A compariÂson of nine workload assessment techniques
The present experiment tested the merits of 9 common workload assessment techniques with relatively short periods of workload in a car-driving task. Twelve participants drove an instrumented car and performed a visually loading task and a mentally loading task for 10, 30, and 60 s. The results show that 10-s periods of visual and mental workload can be measured successfully with subjective ratings and secondary task performance. With respect to longer loading periods (30 and 60 s), steering frequency was found to be sensitive to visual workload, and skin conductance response (SCR) was sensitive to mental workload. The results lead to preliminary guidelines that will help applied researchers to determine which techniques are best suited for assessing visual and mental workload
Effective Electrostatic Interactions in Suspensions of Polyelectrolyte Brush-Coated Colloids
Effective electrostatic interactions between colloidal particles, coated with
polyelectrolyte brushes and suspended in an electrolyte solvent, are described
via linear response theory. The inner cores of the macroions are modeled as
hard spheres, the outer brushes as spherical shells of continuously distributed
charge, the microions (counterions and salt ions) as point charges, and the
solvent as a dielectric continuum. The multi-component mixture of macroions and
microions is formally mapped onto an equivalent one-component suspension by
integrating out from the partition function the microion degrees of freedom.
Applying second-order perturbation theory and a random phase approximation,
analytical expressions are derived for the effective pair interaction and a
one-body volume energy, which is a natural by-product of the one-component
reduction. The combination of an inner core and an outer shell, respectively
impenetrable and penetrable to microions, allows the interactions between
macroions to be tuned by varying the core diameter and brush thickness. In the
limiting cases of vanishing core diameter and vanishing shell thickness, the
interactions reduce to those derived previously for star polyelectrolytes and
charged colloids, respectively.Comment: 20 pages, 5 figures, Phys. Rev. E (in press
Electrostatic Disorder-Induced Interactions in Inhomogeneous Dielectrics
We investigate the effect of quenched surface charge disorder on
electrostatic interactions between two charged surfaces in the presence of
dielectric inhomogeneities and added salt. We show that in the linear
weak-coupling regime (i.e., by including mean-field and Gaussian-fluctuations
contributions), the image-charge effects lead to a non-zero disorder-induced
interaction free energy between two surfaces of equal mean charge that can be
repulsive or attractive depending on the dielectric mismatch across the
bounding surfaces and the exact location of the disordered charge distribution.Comment: 7 pages, 2 figure
Detecting short periods of elevated workload: A comparison of nine workload assessment techniques.
Charge and orbital order in Fe_3O_4
Charge and orbital ordering in the low-temperature monoclinic structure of
magnetite (Fe_3O_4) is investigated using LSDA+U. While the difference between
t_{2g} minority occupancies of Fe^{2+}_B and Fe^{3+}_B cations is large and
gives direct evidence for charge ordering, the screening is so effective that
the total 3d charge disproportion is rather small. The charge order has a
pronounced [001] modulation, which is incompatible with the Anderson criterion.
The orbital order agrees with the Kugel-Khomskii theory.Comment: 4 pages, 2 figure
A closed form for the electrostatic interaction between two rod-like charged objects
We have calculated the electrostatic interaction between two rod-like charged
objects with arbitrary orientations in three dimensions. we obtained a closed
form formula expressing the interaction energy in terms of the separation
distance between the centers of the two rod-like objects, , their lengths
(denoted by and ), and their relative orientations (indicated by
and ). When the objects have the same length (),
for particular values of separations, i.e for , two types of
minimum are appeared in the interaction energy with respect to . By
employing the closed form formula and introducing a scaled temperature , we
have also studied the thermodynamic properties of a one dimensional system of
rod-like charged objects. For different separation distances, the dependence of
the specific heat of the system to the scaled temperature has been studied. It
is found that for , the specific heat has a maximum.Comment: 10 pages, 9 figures, 1 table, Accepted by J. Phys.: Condens. Matte
Order parameters in the Verwey phase transition
The Verwey phase transition in magnetite is analyzed on the basis of the
Landau theory. The free energy functional is expanded in a series of components
belonging to the primary and secondary order parameters. A low-temperature
phase with the monoclinic P2/c symmetry is a result of condensation of two
order parameters X_3 and \Delta_5 . The temperature dependence of the shear
elastic constant C_44 is derived and the mechanism of its softening is
discussed.Comment: 4 pages, 1 figur
Discrete charge patterns, Coulomb correlations and interactions in protein solutions
The effective Coulomb interaction between globular proteins is calculated as
a function of monovalent salt concentration , by explicit Molecular
Dynamics simulations of pairs of model proteins in the presence of microscopic
co and counterions. For discrete charge patterns of monovalent sites on the
surface, the resulting osmotic virial coefficient is found to be a
strikingly non-monotonic function of . The non-monotonicity follows from a
subtle Coulomb correlation effect which is completely missed by conventional
non-linear Poisson-Boltzmann theory and explains various experimental findings.Comment: 4 twocolumn pages with 4 figure
Electromagnetic vacuum of complex media II: the Lamb shift and the total vacuum energy
We study the physical content of the electromagnetic vacuum energy of a
random medium made of atomic electric dipoles. First, we evaluate the
contribution of statistical fluctuations to the average total vacuum energy,
which is made out of the integration of the variations of the Lamb shift with
respect to the coupling constant. While the Lamb shift is a function of the
electrical susceptibility only, the vacuum energy is generally not. Second, we
make clear why the effective medium bulk energy does not account for the total
vacuum energy of a molecular dielectric. Consequently, the Lamb shift does not
derive from the effective medium bulk energy except at leading order in the
molecular density. The local field factors provide natural cutoffs for the
spectrum of the total vacuum energy at a wavelength of the order of the
correlation length. Third, we investigate to what extent shifts in the spectrum
of the dielectric constant may be attributed to the binding energy of a
dielectric. In particular, in the continuum approximation we have found a
relation between the electrostatic binding energy and the Lorentz-Lorenz shift.
Nonetheless, we conclude that the knowledge of the spectrum of the refractive
index is insufficient either to quantify the energy of radiative modes or to
estimate the electrostatic binding energy of molecular clusters.Comment: Comments added, some sections rewritte
Nonlinear effects in charge stabilized colloidal suspensions
Molecular Dynamics simulations are used to study the effective interactions
in charged stabilized colloidal suspensions. For not too high macroion charges
and sufficiently large screening, the concept of the potential of mean force is
known to work well. In the present work, we focus on highly charged macroions
in the limit of low salt concentrations. Within this regime, nonlinear
corrections to the celebrated DLVO theory [B. Derjaguin and L. Landau, Acta
Physicochem. USSR {\bf 14}, 633 (1941); E.J.W. Verwey and J.T.G. Overbeck, {\em
Theory of the Stability of Lyotropic Colloids} (Elsevier, Amsterdam, 1948)]
have to be considered. For non--bulklike systems, such as isolated pairs or
triples of macroions, we show, that nonlinear effects can become relevant,
which cannot be described by the charge renormalization concept [S. Alexander
et al., J. Chem. Phys. {\bf 80}, 5776 (1984)]. For an isolated pair of
macroions, we find an almost perfect qualitative agreement between our
simulation data and the primitive model. However, on a quantitative level,
neither Debye-H\"uckel theory nor the charge renormalization concept can be
confirmed in detail. This seems mainly to be related to the fact, that for
small ion concentrations, microionic layers can strongly overlap, whereas,
simultaneously, excluded volume effects are less important. In the case of
isolated triples, where we compare between coaxial and triangular geometries,
we find attractive corrections to pairwise additivity in the limit of small
macroion separations and salt concentrations. These triplet interactions arise
if all three microionic layers around the macroions exhibit a significant
overlap. In contrast to the case of two isolated colloids, the charge
distribution around a macroion in a triple is found to be anisotropic.Comment: 10 pages, 9 figure
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