1,521 research outputs found
Capillary wave dynamics on supported viscoelastic films: Single and double layers
We study the capillary wave dynamics of a single viscoelastic supported film
and of a double layer of immiscible viscoelastic supported films. Using both
simple scaling arguments and a continuum hydrodynamic theory, we investigate
the effects of viscoelasticity and interfacial slip on the relaxation dynamics
of these capillary waves. Our results account for the recent observation of a
wavelength-independent decay rate for capillary waves in a supported
polystyrene/brominated polystyrene double layer [X. Hu {\em et al.}, Phys. Rev.
E {\bf 74}, 010602 (R) (2006)].Comment: 14 pages, 9 figure
The effect of curvature and topology on membrane hydrodynamics
We study the mobility of extended objects (rods) on a spherical liquid-liquid
interface to show how this quantity is modified in a striking manner by both
the curvature and the topology of the interface. We present theoretical
calculations and experimental measurements of the interfacial fluid velocity
field around a moving rod bound to the crowded interface of a water-in-oil
droplet. By using different droplet sizes, membrane viscosities, and rod
lengths, we show that the viscosity mismatch between the interior and exterior
fluids leads to a suppression of the fluid flow on small droplets that cannot
be captured by the flat interface predictions.Comment: 4 pages, 3 figure
Efficiency of encounter-controlled reaction between diffusing reactants in a finite lattice: topology and boundary effects
The role of dimensionality (Euclidean versus fractal), spatial extent,
boundary effects and system topology on the efficiency of diffusion-reaction
processes involving two simultaneously-diffusing reactants is analyzed. We
present numerically-exact values for the mean time to reaction, as gauged by
the mean walklength before reactive encounter, obtained via application of the
theory of finite Markov processes, and via Monte Carlo simulation. As a general
rule, we conclude that for sufficiently large systems, the efficiency of
diffusion-reaction processes involving two synchronously diffusing reactants
(two-walker case) relative to processes in which one reactant of a pair is
anchored at some point in the reaction space (one walker plus trap case) is
higher, and is enhanced the lower the dimensionality of the system. This
differential efficiency becomes larger with increasing system size and, for
periodic systems, its asymptotic value may depend on the parity of the lattice.
Imposing confining boundaries on the system enhances the differential
efficiency relative to the periodic case, while decreasing the absolute
efficiencies of both two-walker and one walker plus trap processes. Analytic
arguments are presented to provide a rationale for the results obtained. The
insights afforded by the analysis to the design of heterogeneous catalyst
systems are also discussed.Comment: 15 pages, 8 figures, uses revtex4, accepted for publication in
Physica
Polyelectrolyte Bundles
Using extensive Molecular Dynamics simulations we study the behavior of
polyelectrolytes with hydrophobic side chains, which are known to form
cylindrical micelles in aqueous solution. We investigate the stability of such
bundles with respect to hydrophobicity, the strength of the electrostatic
interaction, and the bundle size. We show that for the parameter range relevant
for sulfonated poly-para-phenylenes (PPP) one finds a stable finite bundle
size. In a more generic model we also show the influence of the length of the
precursor oligomer on the stability of the bundles. We also point out that our
model has close similarities to DNA solutions with added condensing agents,
hinting to the possibility that the size of DNA aggregates is under certain
circumstances thermodynamically limited.Comment: 10 pages, 8 figure
Single vesicle imaging indicates distinct modes of rapid membrane retrieval during nerve growth
<p>Abstract</p> <p>Background</p> <p>During nerve growth, cytoplasmic vesicles add new membrane preferentially to the growth cone located at the distal tip of extending axons. Growth cone membrane is also retrieved locally, and asymmetric retrieval facilitates membrane remodeling during growth cone repulsion by a chemorepellent gradient. Moreover, growth inhibitory factors can stimulate bulk membrane retrieval and induce growth cone collapse. Despite these functional insights, the processes mediating local membrane remodeling during axon extension remain poorly defined.</p> <p>Results</p> <p>To investigate the spatial and temporal dynamics of membrane retrieval in actively extending growth cones, we have used a transient labeling and optical recording method that can resolve single vesicle events. Live-cell confocal imaging revealed rapid membrane retrieval by distinct endocytic modes based on spatial distribution in <it>Xenopus </it>spinal neuron growth cones. These modes include endocytic "hot-spots" triggered at the base of filopodia, at the lateral margins of lamellipodia, and along dorsal ridges of the growth cone. Additionally, waves of endocytosis were induced when individual filopodia detached from the substrate and fused with the growth cone dorsal surface or with other filopodia. Vesicle formation at sites of membrane remodeling by self-contact required F-actin polymerization. Moreover, bulk membrane retrieval by macroendocytosis correlated positively with the substrate-dependent rate of axon extension and required the function of Rho-family GTPases.</p> <p>Conclusions</p> <p>This study provides insight into the dynamic membrane remodeling processes essential for nerve growth by identifying several distinct modes of rapid membrane retrieval in the growth cone during axon extension. We found that endocytic membrane retrieval is intensified at specific subdomains and may drive the dynamic membrane ruffling and re-absorption of filopodia and lamellipodia in actively extending growth cones. The findings offer a platform for determining the molecular mechanisms of distinct endocytic processes that may remodel the surface distribution of receptors, ion channels and other membrane-associated proteins locally to drive growth cone extension and chemotactic guidance.</p
Stress Induced Protein Changes in Tall Fescue
Tall fescue (Festuca arundinacea Schreb.), the most important pasture grass in Arkansas, exhibits different agricultural properties when it is infected by its mutualistic endophyte Acremonium coenophialum Morgan-Jones and Gams. We postulate that the presence of endophyte exerts a stress on the host that enhances or detracts from the host\u27s ability to express specific genes. We tested this hypothesis by heat stressing infected and non-infected, juvenile and mature tall fescue, and examining their protein profiles by SDS-PAGE analysis. The results indicate that mature, infected, stressed grass produced greater amounts of Rubisco (ribulose bisphosphate carboxylase-oxygenase) than all other treatments. Additionally, the mature, infected, stressed grass exhibited a 20 k Dalton protein band which was not apparent in other treatments. These observations support the possibility that the endophyte prestresses the grass, and they suggest a molecular mechanism for this response
Effective Viscosity of a Dilute Suspension of Membrane-bound Inclusions
When particulate suspensions are sheared, perturbations in the shear flows
around the rigid particles increase the local energy dissipation, so that the
viscosity of the suspension is effectively higher than that of the solvent. For
bulk (three-dimensional) fluids, understanding this viscosity enhancement is a
classic problem in hydrodynamics that originated over a century ago with
Einstein's study of a dilute suspension of spherical particles.
\cite{Einstein1} In this paper, we investigate the analogous problem of the
effective viscosity of a suspension of disks embedded in a two-dimensional
membrane or interface. Unlike the hydrodynamics of bulk fluids, low-Reynolds
number membrane hydrodynamics is characterized by an inherent length scale
generated by the coupling of the membrane to the bulk fluids that surround it.
As a result, we find that the size of the particles in the suspension relative
to this hydrodynamic length scale has a dramatic effect on the effective
viscosity of the suspension. Our study also helps to elucidate the mathematical
tools needed to solve the mixed boundary value problems that generically arise
when considering the motion of rigid inclusions in fluid membranes.Comment: 33 pages, 4 figures (preprint); submitted to Physics of Fluid
- …