35,145 research outputs found
Thrifty swimming with shear-thinning
Microscale propulsion is integral to numerous biomedical systems, for example
biofilm formation and human reproduction, where the surrounding fluids comprise
suspensions of polymers. These polymers endow the fluid with non-Newtonian
rheological properties, such as shear-thinning and viscoelasticity. Thus, the
complex dynamics of non-Newtonian fluids presents numerous modelling
challenges, strongly motivating experimental study. Here, we demonstrate that
failing to account for "out-of-plane" effects when analysing experimental data
of undulatory swimming through a shear-thinning fluid results in a significant
overestimate of fluid viscosity around the model swimmer C. elegans. This
miscalculation of viscosity corresponds with an overestimate of the power the
swimmer expends, a key biophysical quantity important for understanding the
internal mechanics of the swimmer. As experimental flow tracking techniques
improve, accurate experimental estimates of power consumption using this
technique will arise in similar undulatory systems, such as the planar beating
of human sperm through cervical mucus, will be required to probe the
interaction between internal power generation, fluid rheology, and the
resulting waveform
An Analysis of a Proposed New Economic Development Initiative
This report contains an analysis of a new economic development incentive that has been proposed as an addition to the existing BEST program. Report #8
The chiral condensate in a constant electromagnetic field
We study the shift of the chiral condensate in a constant electromagnetic
field in the context of chiral perturbation theory. Using the Schwinger
proper-time formalism, we derive a one-loop expression correct to all orders in
. Our result correctly reproduces a previously derived
``low-energy theorem'' for . We show that it is essential to include
corrections due to non-vanishing in order for a low energy theorem to
have any approximate regime of validity in the physical universe. We generalize
these results to systems containing electric fields, and discuss the regime of
validity for the results. In particular, we discuss the circumstances in which
the method formally breaks down due to pair creation in an electric field.Comment: 9 pages, 6 figures, LaTeX; removed extraneous section + minor
revision
Thermally-activated Non-Schmid Glide of Screw Dislocations in W using Atomistically-informed Kinetic Monte Carlo Simulations
Thermally-activated \small{\nicefrac{1}{2}} screw dislocation motion
is the controlling plastic mechanism at low temperatures in body-centered cubic
(bcc) crystals. Motion proceeds by the nucleation and propagation of
atomic-sized kink pairs susceptible of being studied using molecular dynamics
(MD). However, MD's natural inability to properly sample thermally-activated
processes as well as to capture screw dislocation glide calls for the
development of other methods capable of overcoming these limitations. Here we
develop a kinetic Monte Carlo (kMC) approach to study single screw dislocation
dynamics from room temperature to and at stresses
, where and are the melting point and
the Peierls stress. The method is entirely parameterized with atomistic
simulations using an embedded atom potential for tungsten. To increase the
physical fidelity of our simulations, we calculate the deviations from Schmid's
law prescribed by the interatomic potential used and we study single
dislocation kinetics using both projections. We calculate dislocation
velocities as a function of stress, temperature, and dislocation line length.
We find that considering non-Schmid effects has a strong influence on both the
magnitude of the velocities and the trajectories followed by the dislocation.
We finish by condensing all the calculated data into effective stress and
temperature dependent mobilities to be used in more homogenized numerical
methods
An idiotypic cross-reaction between allotype a3 and allotype a negative rabbit antibodies to streptococcal carbohydrates
Two antibodies to Group C streptococcal carbohydrate isolated from an individual rabbit had similar relative binding affinities for a Group C immuno-adsorbent column. Their light chains were similar, if not identical, as were the constant regions of their heavy chains. Differences in the variable regions of the H chains of the two antibodies were detected by chemical analysis. The two antibodies had serologically identical idiotypic determinants although one antibody possessed the a3 allotype and the other had no detectable group a marker. The occurrence of such antibodies indicates the absence of obligatory associations between group a allotypes and idiotypic specificities, despite the fact that both determinants have antigenic components in the VH region of the H chain
Shape oscillations of human neutrophil leukocytes: characterization and relationship to cell motility
When neutrophil leukocytes are stimulated by chemotactic factors or by substratum contact, they change their shape. Shape changes are a prerequisite for cellular migration and typically involve the extrusion of thin, veil-like lamellipods and the development of morphological polarity. Stimulation also leads to changes in the neutrophil content of filamentous actin (F-actin), which is the major cytoskeletal component. Suspensions of human neutrophils stimulated with chemoattractants exhibit sinusoidal light-scattering oscillations with a period of approximately 8 s at 37°C. These oscillations arise from periodic fluctuations in the cell body size caused by lamellipod extension and retraction cycles. The light-scattering oscillations are paralleled by corresponding oscillations in F-actin content. This raises the interesting possibility that cyclic actin polymerization constitutes the driving force for shape oscillations of suspended neutrophils. Similar periodic shape changes are present in neutrophils crawling on a surface, suggesting that shape oscillations are important for neutrophil motion. This review summarizes our present knowledge about shape oscillations in suspended and crawling neutrophils and discusses a possible role for these oscillations in neutrophil motility
Chaos in cylindrical stadium billiards via a generic nonlinear mechanism
We describe conditions under which higher-dimensional billiard models in
bounded, convex regions are fully chaotic, generalizing the Bunimovich stadium
to dimensions above two. An example is a three-dimensional stadium bounded by a
cylinder and several planes; the combination of these elements may give rise to
defocusing, allowing large chaotic regions in phase space. By studying families
of marginally-stable periodic orbits that populate the residual part of phase
space, we identify conditions under which a nonlinear instability mechanism
arises in their vicinity. For particular geometries, this mechanism rather
induces stable nonlinear oscillations, including in the form of
whispering-gallery modes.Comment: 4 pages, 4 figure
- …