33 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
Equilibrium Bundle Size of Rodlike Polyelectrolytes with Counterion-Induced Attractive Interactions
Multivalent counterions can induce an effective attraction between
like-charged rodlike polyelectrolytes, leading to the formation of
polelectrolyte bundles. In this paper, we calculate the equilibrium bundle size
using a simple model in which the attraction between polyelectrolytes (assumed
to be pairwise additive) is treated phenomenologically. If the counterions are
point-like, they almost completely neutralize the charge of the bundle, and the
equilibrium bundle size diverges. When the counterions are large, however,
steric and short-range electrostatic interactions prevent charge neutralization
of the bundle, thus forcing the equilibrium bundle size to be finite. We also
consider the possibility that increasing the number of nearest neighbors for
each rod in the bundle frustrates the attractive interaction between the rods.
Such a frustration leads to the formation of finite size bundles as well, even
when the counterions are small.Comment: 4 pages, 2 figures; v2: typos corrected, references added, minor
changes made to conten
Diffusion and binding of finite-size particles in confined geometries
Describing the diffusion of particles through crowded, confined environments
with which they can interact is of considerable biological and technological
interest. Under conditions where the confinement dimensions become comparable
to the particle dimensions, steric interactions between particles, as well as
particle-wall interactions, will play a crucial role in determining transport
properties. To elucidate the effects of these interactions on particle
transport, we consider the diffusion and binding of finite-size particles
within a channel whose diameter is comparable to the size of the particles.
Using a simple lattice model of this process, we calculate the steady-state
current and density profiles of both bound and free particles in the channel.
We show that the system can exhibit qualitatively different behavior depending
on the ratio of the channel width to the particle size. We also perform
simulations of this system, and find excellent agreement with our analytic
results.Comment: 11 pages, 5 figures, Phys. Rev. E (accepted
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
Photography-based taxonomy is inadequate, unnecessary, and potentially harmful for biological sciences
The question whether taxonomic descriptions naming new animal species without type specimen(s) deposited in collections should be accepted for publication by scientific journals and allowed by the Code has already been discussed in Zootaxa (Dubois & NemĂ©sio 2007; Donegan 2008, 2009; NemĂ©sio 2009aâb; Dubois 2009; Gentile & Snell 2009; Minelli 2009; Cianferoni & Bartolozzi 2016; Amorim et al. 2016). This question was again raised in a letter supported
by 35 signatories published in the journal Nature (Pape et al. 2016) on 15 September 2016. On 25 September 2016, the following rebuttal (strictly limited to 300 words as per the editorial rules of Nature) was submitted to Nature, which on
18 October 2016 refused to publish it. As we think this problem is a very important one for zoological taxonomy, this text is published here exactly as submitted to Nature, followed by the list of the 493 taxonomists and collection-based
researchers who signed it in the short time span from 20 September to 6 October 2016
Measurement of the nonlinear elasticity of red blood cell membranes
The membranes of human red blood cells (RBCs) are a composite of a fluid lipid bilayer and a triangular network of semiflexible filaments (spectrin). We perform cellular microrheology using the dynamic membrane fluctuations of the RBCs to extract the elastic moduli of this composite membrane. By applying known osmotic stresses, we measure the changes in the elastic constants under imposed strain and thereby determine the nonlinear elastic properties of the membrane. We find that the elastic nonlinearities of the shear modulus in tensed RBC membranes can be well understood in terms of a simple wormlike chain model. Our results show that the elasticity of the spectrin network can mostly account for the area compression modulus at physiological osmolality, suggesting that the lipid bilayer has significant excess area. As the cell swells, the elastic contribution from the now tensed lipid membrane becomes dominant.National Institutes of Health (Grant No. P41-RR02594-18-24)National Science Foundation (U.S.) (Grant No. 08-46660 CAREER)National Science Foundation (U.S.) (Grant No. NSF-DMR-0907212)National Cancer Institute (U.S.) (Grant No. R21 CA147967-01