629 research outputs found
Statics and Dynamics of Strongly Charged Soft Matter
Soft matter materials, such as polymers, membranes, proteins, are often
electrically charged. This makes them water soluble, which is of great
importance in technological application and a prerequisite for biological
function. We discuss a few static and dynamic systems that are dominated by
charge effects. One class comprises complexation between oppositely charged
objects, for example the adsorption of charged ions or charged polymers (such
as DNA) on oppositely charged substrates of different geometry. The second
class comprises effective interactions between similarly charged objects. Here
the main theme is to understand the experimental finding that similarly and
highly charged bodies attract each other in the presence of multi-valent
counterions. This is demonstrated using field-theoretic arguments as well as
Monte-Carlo simulations for the case of two homogeneously charged bodies.
Realistic surfaces, on the other hand, are corrugated and also exhibit
modulated charge distributions, which is important for static properties such
as the counterion-density distribution, but has even more pronounced
consequences for dynamic properties such as the counterion mobility. More
pronounced dynamic effects are obtained with highly condensed charged systems
in strong electric fields. Likewise, an electrostatically collapsed highly
charged polymer is unfolded and oriented in strong electric fields. At the end
of this review, we give a very brief account of the behavior of water at planar
surfaces and demonstrate using ab-initio methods that specific interactions
between oppositely charged groups cause ion-specific effects that have recently
moved into the focus of interest.Comment: 61 pages, 31 figures, Physics Reports (2005)-in press (high quality
figures available from authors
The Dynamics of Food Web Model with Defensive Switching Property
In this paper, a food web model consisting of two-predator one-prey with the defensive switching of predation avoidance is proposed and analyzed. It is assumed that the prey growth logistically in the absence of predators and defends itself from relatively abundant predator species by switching to another habitat with relatively rare predator species. Sufficient conditions for the stability of the non-trivial equilibrium point are obtained. The Lyapunov function is constructed to establish the global asymptotic stability of the non-trivial equilibrium point when the intensity of defensive switching equal one. Numerical simulations for different sets of parameter values and for different sets of initial conditions are carried out. It has been shown that the system has a globally asymptotically stable non-trivial point when the two predators have the same mortality rates
Molecular Dynamics Simulation of Semiflexible Polyampholyte Brushes - The Effect of Charged Monomers Sequence
Planar brushes formed by end-grafted semiflexible polyampholyte chains, each
chain containing equal number of positively and negatively charged monomers is
studied using molecular dynamics simulations. Keeping the length of the chains
fixed, dependence of the average brush thickness and equilibrium statistics of
the brush conformations on the grafting density and the salt concentration are
obtained with various sequences of charged monomers. When similarly charged
monomers of the chains are arranged in longer blocks, the average brush
thickness is smaller and dependence of brush properties on the grafting density
and the salt concentration is stronger. With such long blocks of similarly
charged monomers, the anchored chains bond to each other in the vicinity of the
grafting surface at low grafting densities and buckle toward the grafting
surface at high grafting densities.Comment: 8 pages,7 figure
Exact asymptotic expansions for the cylindrical Poisson-Boltzmann equation
The mathematical theory of integrable Painleve/Toda type systems sheds new
light on the behavior of solutions to the Poisson-Boltzmann equation for the
potential due to a long rod-like macroion. We investigate here the case of
symmetric electrolytes together with that of 1:2 and 2:1 salts. Short and large
scale features are analyzed, with a particular emphasis on the low salinity
regime. Analytical expansions are derived for several quantities relevant for
polyelectrolytes theory, such as the Manning radius. In addition, accurate and
practical expressions are worked out for the electrostatic potential, which
improve upon previous work and cover the full range of radial distances
Drag forces on inclusions in classical fields with dissipative dynamics
We study the drag force on uniformly moving inclusions which interact
linearly with dynamical free field theories commonly used to study soft
condensed matter systems. Drag forces are shown to be nonlinear functions of
the inclusion velocity and depend strongly on the field dynamics. The general
results obtained can be used to explain drag forces in Ising systems and also
predict the existence of drag forces on proteins in membranes due to couplings
to various physical parameters of the membrane such as composition, phase and
height fluctuations.Comment: 14 pages, 7 figure
Scaling and Universality in the Counterion-Condensation Transition at Charged Cylinders
We address the critical and universal aspects of counterion-condensation
transition at a single charged cylinder in both two and three spatial
dimensions using numerical and analytical methods. By introducing a novel
Monte-Carlo sampling method in logarithmic radial scale, we are able to
numerically simulate the critical limit of infinite system size (corresponding
to infinite-dilution limit) within tractable equilibration times. The critical
exponents are determined for the inverse moments of the counterionic density
profile (which play the role of the order parameters and represent the inverse
localization length of counterions) both within mean-field theory and within
Monte-Carlo simulations. In three dimensions (3D), correlation effects
(neglected within mean-field theory) lead to an excessive accumulation of
counterions near the charged cylinder below the critical temperature
(condensation phase), while surprisingly, the critical region exhibits
universal critical exponents in accord with the mean-field theory. In two
dimensions (2D), we demonstrate, using both numerical and analytical
approaches, that the mean-field theory becomes exact at all temperatures
(Manning parameters), when number of counterions tends to infinity. For finite
particle number, however, the 2D problem displays a series of peculiar singular
points (with diverging heat capacity), which reflect successive de-localization
events of individual counterions from the central cylinder. In both 2D and 3D,
the heat capacity shows a universal jump at the critical point, and the energy
develops a pronounced peak. The asymptotic behavior of the energy peak location
is used to locate the critical temperature, which is also found to be universal
and in accordance with the mean-field prediction.Comment: 31 pages, 16 figure
Effects of Phytosterol in Feed on Growth and Related Gene Expression in Muscles of Broiler Chickens
Purpose: To investigate the effect of phytosterol in a feed on growth and gene expression of broiler chickens.Methods: Seven isonitrogenous diets with graded levels of polyhydroxy phytosterol(Castastesrone) (15, 20, and 25 g/kg diet) and hydroxyphytosterol (â-sitosterol) (25, 50, and 75 g/kg diet) were used to feed broiler chickens for 45 d.Results: The dietary phytosterol significantly improved (p < 0.05) the body weight and feed intake of broiler chickens, especially with the 75 g/kg diet. In contrast, phytosterol supplementation was associated with significant (p < 0.05) hypotriglyceridemic effects with concurrent modifications ofdepressed antioxidant defence systems in the broiler chickens. Myogen, eIF4E, and S6k1 gene expression levels in tissues were significantly (p < 0.05) improved by dietary phytosterol. mTOR gene expression levels in muscle tissues were increased significantly (p < 0.05), but myostatin (GDF-8) and ubiquitin levels were significantly decreased (p < 0.05).Conclusion: These results suggest that giving immediate phytosterol feeding regimen to chicks is a good feeding program for appropriate morphological development of the pectoralis major muscle and the expression of genes necessary for muscle cell proliferation and  differentiation. Furthermore, the data suggest that developmental decline in skeletal muscle protein synthesis, may be partly attributed to developmental regulation of the activation of growth factor and nutrient components.Keywords: Broiler chicken, Polyhydroxy phytosterol; Hydroxyphytosterol, Feed, Antioxidant status, Gene expression
Partially Annealed Disorder and Collapse of Like-Charged Macroions
Charged systems with partially annealed charge disorder are investigated
using field-theoretic and replica methods. Charge disorder is assumed to be
confined to macroion surfaces surrounded by a cloud of mobile neutralizing
counterions in an aqueous solvent. A general formalism is developed by assuming
that the disorder is partially annealed (with purely annealed and purely
quenched disorder included as special cases), i.e., we assume in general that
the disorder undergoes a slow dynamics relative to fast-relaxing counterions
making it possible thus to study the stationary-state properties of the system
using methods similar to those available in equilibrium statistical mechanics.
By focusing on the specific case of two planar surfaces of equal mean surface
charge and disorder variance, it is shown that partial annealing of the
quenched disorder leads to renormalization of the mean surface charge density
and thus a reduction of the inter-plate repulsion on the mean-field or
weak-coupling level. In the strong-coupling limit, charge disorder induces a
long-range attraction resulting in a continuous disorder-driven collapse
transition for the two surfaces as the disorder variance exceeds a threshold
value. Disorder annealing further enhances the attraction and, in the limit of
low screening, leads to a global attractive instability in the system.Comment: 21 pages, 2 figure
Thermal Casimir effect between random layered dielectrics
We study the thermal Casimir effect between two thick slabs composed of
plane-parallel layers of random dielectric materials interacting across an
intervening homogeneous dielectric. It is found that the effective interaction
at long distances is self averaging and is given by a description in terms of
effective dielectric functions. The behavior at short distances becomes random
(sample dependent) and is dominated by the local values of the dielectric
function proximal to each other across the dielectrically homogeneous slab.Comment: 4 pages RevTex, 1 pdf figur
Self-Consistent Field study of Polyelectrolyte Brushes
We formulate a self-consistent field theory for polyelectrolyte brushes in
the presence of counterions. We numerically solve the self-consistent field
equations and study the monomer density profile, the distribution of
counterions, and the total charge distribution. We study the scaling relations
for the brush height and compare them to the prediction of other theories. We
find a weak dependence of the brush height on the grafting density.We fit the
counterion distribution outside the brush by the Gouy-Chapman solution for a
virtual charged wall. We calculate the amount of counterions outside the brush
and find that it saturates as the charge of the polyelectrolytes increases
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