800 research outputs found
Electrostatic Cancellation of Gravity Effects in Liquid Mixtures
We point out that a spatially-varying electric field can be used to cancel
the effect of gravity in liquid mixtures by coupling to the different
components' permittivities. Cancellation occurs if the system under
consideration is small enough. For a simple ``wedge'' electrode geometry we
show that the required system size and voltage are practical, easily realizable
in the Lab. Thus this setup might be a simple alternative to more expensive or
hazardous options such as the space-shuttle, drop-tower, or magnetic levitation
experiments.Comment: 1.5 pages, one figure. Accepted to PRE brief report
Influence of M-phase chromatin on the anisotropy of microtubule asters
In many eukaryotic cells going through M-phase, a bipolar spindle is formed by microtubules nucleated from centrosomes. These microtubules, in addition to being "captured" by kinetochores, may be stabilized by chromatin in two different ways: short-range stabilization effects may affect microtubules in close contact with the chromatin, while long-range stabilization effects may "guide" microtubule growth towards the chromatin (e.g., by introducing a diffusive gradient of an enzymatic activity that affects microtubule assembly). Here, we use both meiotic and mitotic extracts from Xenopus laevis eggs to study microtubule aster formation and microtubule dynamics in the presence of chromatin. In "low-speed" meiotic extracts, in the presence of salmon sperm chromatin, we find that short-range stabilization effects lead to a strong anisotropy of the microtubule asters. Analysis of the dynamic parameters of microtubule growth show that this anisotropy arises from a decrease in the catastrophe frequency, an increase in the rescue frequency and a decrease in the growth velocity. In this system we also find evidence for long-range "guidance" effects, which lead to a weak anisotropy of the asters. Statistically relevant results on these long-range effects are obtained in "high-speed" mitotic extracts in the presence of artificially constructed chromatin stripes. We find that aster anisotropy is biased in the direction of the chromatin and that the catastrophe frequency is reduced in its vicinity. In this system we also find a surprising dependence of the catastrophe and the rescue frequencies on the length of microtubules nucleated from centrosomes: the catastrophe frequency increase and the rescue frequency decreases with microtubule length
Dynamics of active membranes with internal noise
We study the time-dependent height fluctuations of an active membrane
containing energy-dissipating pumps that drive the membrane out of equilibrium.
Unlike previous investigations based on models that neglect either curvature
couplings or random fluctuations in pump activities, our formulation explores
two new models that take both of these effects into account. In the first
model, the magnitude of the nonequilibrium forces generated by the pumps is
allowed to fluctuate temporally. In the second model, the pumps are allowed to
switch between "on" and "off" states. We compute the mean squared displacement
of a membrane point for both models, and show that they exhibit distinct
dynamical behaviors from previous models, and in particular, a superdiffusive
regime specifically arising from the shot noise.Comment: 7 pages, 4 figure
Design of Copolymeric Materials
We devise a method for designing materials that will have some desired
structural characteristics. We apply it to multiblock copolymers that have two
different types of monomers, A and B. We show how to determine what sequence of
A's and B's should be synthesised in order to give a particular structure and
morphology. %For example in a melt of such %polymers, one may wish to engineer
a body-centered %cubic structure. Using this method in conjunction with the
theory of microphase separation developed by Leibler, we show it is possible to
efficiently search for a desired morphology. The method is quite general and
can be extended to design isolated heteropolymers, such as proteins, with
desired structural characteristics. We show that by making certain
approximations to the exact algorithm, a method recently proposed by
Shakhnovich and Gutin is obtained. The problems with this method are discussed
and we propose an improved approximate algorithm that is computationally
efficient.Comment: 15 pages latex 2.09 and psfig, 1 postscript figure
Fluctuation induced interactions between domains in membranes
We study a model lipid bilayer composed of a mixture of two incompatible
lipid types which have a natural tendency to segregate in the absence of
membrane fluctuations. The membrane is mechanically characterized by a local
bending rigidity which varies with the average local lipid
composition . We show, in the case where varies weakly with
, that the effective interaction between lipids of the same type can
either be everywhere attractive or can have a repulsive component at
intermediate distances greater than the typical lipid size. When this
interaction has a repulsive component, it can prevent macro-phase separation
and lead to separation in mesophases with a finite domain size. This effect
could be relevant to certain experimental and numerical observations of
mesoscopic domains in such systems.Comment: 9 pages RevTex, 1 eps figur
A Geometric Theory of Diblock Copolymer Phases
We analyze the energetics of sphere-like micellar phases in diblock
copolymers in terms of well-studied, geometric quantities for their lattices.
We argue that the A15 lattice with Pm3n symmetry should be favored as the
blocks become more symmetric and corroborate this through a self-consistent
field theory. Because phases with columnar or bicontinuous topologies
intervene, the A15 phase, though metastable, is not an equilibrium phase of
symmetric diblocks. We investigate the phase diagram of branched diblocks and
find thatthe A15 phase is stable.Comment: 4 pages, RevTeX, 3 eps figures include
"Wet-to-Dry" Conformational Transition of Polymer Layers Grafted to Nanoparticles in Nanocomposite
The present communication reports the first direct measurement of the
conformation of a polymer corona grafted around silica nano-particles dispersed
inside a nanocomposite, a matrix of the same polymer. This measurement
constitutes an experimental breakthrough based on a refined combination of
chemical synthesis, which permits to match the contribution of the neutron
silica signal inside the composite, and the use of complementary scattering
methods SANS and SAXS to extract the grafted polymer layer form factor from the
inter-particles silica structure factor. The modelization of the signal of the
grafted polymer on nanoparticles inside the matrix and the direct comparison
with the form factor of the same particles in solution show a clear-cut change
of the polymer conformation from bulk to the nanocomposite: a transition from a
stretched and swollen form in solution to a Gaussian conformation in the matrix
followed with a compression of a factor two of the grafted corona. In the
probed range, increasing the interactions between the grafted particles (by
increasing the particle volume fraction) or between the grafted and the free
matrix chains (decreasing the grafted-free chain length ratio) does not
influence the amplitude of the grafted brush compression. This is the first
direct observation of the wet-to-dry conformational transition theoretically
expected to minimize the free energy of swelling of grafted chains in
interaction with free matrix chains, illustrating the competition between the
mixing entropy of grafted and free chains, and the elastic deformation of the
grafted chains. In addition to the experimental validation of the theoretical
prediction, this result constitutes a new insight for the nderstanding of the
general problem of dispersion of nanoparticles inside a polymer matrix for the
design of new nanocomposites materials
Interfaces of Modulated Phases
Numerically minimizing a continuous free-energy functional which yields
several modulated phases, we obtain the order-parameter profiles and
interfacial free energies of symmetric and non-symmetric tilt boundaries within
the lamellar phase, and of interfaces between coexisting lamellar, hexagonal,
and disordered phases. Our findings agree well with chevron, omega, and
T-junction tilt-boundary morphologies observed in diblock copolymers and
magnetic garnet films.Comment: 4 page
Shear induced grain boundary motion for lamellar phases in the weakly nonlinear regime
We study the effect of an externally imposed oscillatory shear on the motion
of a grain boundary that separates differently oriented domains of the lamellar
phase of a diblock copolymer. A direct numerical solution of the
Swift-Hohenberg equation in shear flow is used for the case of a
transverse/parallel grain boundary in the limits of weak nonlinearity and low
shear frequency. We focus on the region of parameters in which both transverse
and parallel lamellae are linearly stable. Shearing leads to excess free energy
in the transverse region relative to the parallel region, which is in turn
dissipated by net motion of the boundary toward the transverse region. The
observed boundary motion is a combination of rigid advection by the flow and
order parameter diffusion. The latter includes break up and reconnection of
lamellae, as well as a weak Eckhaus instability in the boundary region for
sufficiently large strain amplitude that leads to slow wavenumber readjustment.
The net average velocity is seen to increase with frequency and strain
amplitude, and can be obtained by a multiple scale expansion of the governing
equations
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