3,071 research outputs found
Critical behaviour in the nonlinear elastic response of hydrogels
In this paper we study the elastic response of synthetic hydrogels to an
applied shear stress. The hydrogels studied here have previously been shown to
mimic the behaviour of biopolymer networks when they are sufficiently far above
the gel point. We show that near the gel point they exhibit an elastic response
that is consistent with the predicted critical behaviour of networks near or
below the isostatic point of marginal stability. This point separates rigid and
floppy states, distinguished by the presence or absence of finite linear
elastic moduli. Recent theoretical work has also focused on the response of
such networks to finite or large deformations, both near and below the
isostatic point. Despite this interest, experimental evidence for the existence
of criticality in such networks has been lacking. Using computer simulations,
we identify critical signatures in the mechanical response of sub-isostatic
networks as a function of applied shear stress. We also present experimental
evidence consistent with these predictions. Furthermore, our results show the
existence of two distinct critical regimes, one of which arises from the
nonlinear stretch response of semi-flexible polymers.
Iceman Survived due to Cooling Device
Although mild hypothermia treatment is part of the standard postresuscitation care today, no standard method for treatment of accidental severe hypothermia has been yet established. Different strategies including invasive and noninvasive methods have been described in the literature. We present the case of a 75-year-old man with accidental severe hypothermia (23°C) and demonstrate that using a surface cooling device with automatic controlled temperature feedback mechanism (ArcticSun2000 Medivance, Louisville, Colorado, USA) is an effective and safe method for controlled rewarming in this life-threatening setting
Evaluation of SIR-A space radar for geologic interpretation: United States, Panama, Colombia, and New Guinea
Comparisons between LANDSAT MSS imagery, and aircraft and space radar imagery from different geologic environments in the United States, Panama, Colombia, and New Guinea demonstrate the interdependence of radar system geometry and terrain configuration for optimum retrieval of geologic information. Illustrations suggest that in the case of space radars (SIR-A in particular), the ability to acquire multiple look-angle/look-direction radar images of a given area is more valuable for landform mapping than further improvements in spatial resolution. Radar look-angle is concluded to be one of the most important system parameters of a space radar designed to be used for geologic reconnaissance mapping. The optimum set of system parameters must be determined for imaging different classes of landform features and tailoring the look-angle to local topography
Accurate determination of elastic parameters for multi-component membranes
Heterogeneities in the cell membrane due to coexisting lipid phases have been
conjectured to play a major functional role in cell signaling and membrane
trafficking. Thereby the material properties of multiphase systems, such as the
line tension and the bending moduli, are crucially involved in the kinetics and
the asymptotic behavior of phase separation. In this Letter we present a
combined analytical and experimental approach to determine the properties of
phase-separated vesicle systems. First we develop an analytical model for the
vesicle shape of weakly budded biphasic vesicles. Subsequently experimental
data on vesicle shape and membrane fluctuations are taken and compared to the
model. The combined approach allows for a reproducible and reliable
determination of the physical parameters of complex vesicle systems. The
parameters obtained set limits for the size and stability of nanodomains in the
plasma membrane of living cells.Comment: (*) authors contributed equally, 6 pages, 3 figures, 1 table; added
insets to figure
Monte Carlo study of multiply crosslinked semiflexible polymer networks
We present a method to generate realistic, three-dimensional networks of
crosslinked semiflexible polymers. The free energy of these networks is
obtained from the force-extension characteristics of the individual polymers
and their persistent directionality through the crosslinks. A Monte Carlo
scheme is employed to obtain isotropic, homogeneous networks that minimize the
free energy, and for which all of the relevant parameters can be varied: the
persistence length, the contour length as well as the crosslinking length may
be chosen at will. We also provide an initial survey of the mechanical
properties of our networks subjected to shear strains, showing them to display
the expected non-linear stiffening behavior. Also, a key role for non-affinity
and its relation to order in the network is uncovered.Comment: 11 pages, revised figures, added extra information about the network
Semiflexible Filamentous Composites
Inspired by the ubiquity of composite filamentous networks in nature we
investigate models of biopolymer networks that consist of interconnected floppy
and stiff filaments. Numerical simulations carried out in three dimensions
allow us to explore the microscopic partitioning of stresses and strains
between the stiff and floppy fractions c_s and c_f, and reveal a non-trivial
relationship between the mechanical behavior and the relative fraction of stiff
polymer: when there are few stiff polymers, non-percolated stiff ``inclusions``
are protected from large deformations by an encompassing floppy matrix, while
at higher fractions of stiff material the stiff network is independently
percolated and dominates the mechanical response.Comment: Phys. Rev. Lett, to appear (4 pages, 2 figures
Behavioural effects of pheromone-based control system, ExosexTM SPTab, on male Indianmeal moth, Plodia interpunctella
We report on laboratory studies to examine behavioural effects of a new pheromone-based control system for stored product moths, Exosex™ SPTab auto-confusion, on virgin male Plodia interpunctella. The SPTab comprises a compressed electrostatic powder tablet, containing the female sex pheromone (Z,E)-9,12-Tetradecadienyl acetate; designed to be an attractive source to males and disrupt the mate finding ability of several stored moth pest species. Our aims were to examine the effects of SPTab contact on male ability to locate females and to be attractive sources to other males. Observations of behavioural effects were made in a moth flight tunnel. Virgin male P. interpunctella were treated on SPTabs weathered from 1-8 weeks. After treatment, males were either immediately released downwind of calling females in the upwind section of the flight tunnel, or caged individually for 1 to 48 h before release. The ability of treated males to act as false lures for other males was measured by treating males on different age SPTabs as before, caging them and then placing them upwind of untreated males. In all studies males were scored on making source contact. Males treated on the SPTab and immediately released downwind of calling females showed a 96.7% reduction in their ability to contact the females. Males held for 48 h before release still showed a significant reduction in source contact compared to untreated controls. Treated males tested after 3 h, were as attractive as calling females with >75% of flights resulting in source contact. Significant reduction in female source contact was still evident when males were treated on SPTab weathered for 4 weeks. Males treated on SPTabs weathered for up to 8 weeks were significantly attractive to untreated males, and equal to calling females. The results indicate that contact with SPTab significantly reduces the ability of male P. interpunctella to locate females for up to two days. These males could increase the confusion effect by becoming competitive attractive point sources for other males. The Exosex SPTab has novel methods of causing mating disruption. Keywords: Mating disruption, Plodia interpunctella, Exosex SPTab, Sex pheromone, Flight tunnel
Enhanced persistence and collective migration in cooperatively aligning cell clusters
Most cells possess the capacity to locomote. Alone or collectively, this
allows them to adapt, to rearrange, and to explore their surroundings. The
biophysical characterization of such motile processes, in health and disease,
has so far focused mostly on two limiting cases: single-cell motility on the
one hand, and the dynamics of confluent tissues such as the epithelium on the
other. The in-between regime of clusters, composed of relatively few cells,
moving as a coherent unit has received less attention. Such small clusters are,
however, deeply relevant in development but also in cancer metastasis. In this
work, we use cellular Potts models and analytical active matter theory to
understand how the motility of small cell clusters changes with N, the number
of cells in the cluster. Modeling and theory reveal our two main findings:
Cluster persistence time increases with N while the intrinsic diffusivity
decreases with N. We discuss a number of settings in which the motile
properties of more complex clusters can be analytically understood, revealing
that the focusing effects of small-scale cooperation and cell-cell alignment
can overcome the increased bulkiness and internal disorder of multicellular
clusters to enhance overall migrational efficacy. We demonstrate this
enhancement for small-cluster collective durotaxis, which is shown to proceed
more effectively than for single cells. Our results may provide some novel
insights into the connection between single-cell and large-scale collective
motion and may point the way to the biophysical origins of the enhanced
metastatic potential of small tumor cell clusters
The distance to the LMC cluster Reticulum from the K-band Period-Luminosity-Metallicity relation of RR Lyrae stars
We present new and accurate Near-Infrared J and Ks-band data of the Large
Magellanic Cloud cluster Reticulum. Data were collected with SOFI available at
NTT and covering an area of approximately (5 x 5) arcmin^2 around the center of
the cluster. Current data allowed us to derive accurate mean K-band magnitudes
for 21 fundamental and 9 first overtone RR Lyrae stars. On the basis of the
semi-empirical K-band Period-Luminosity-Metallicity relation we have recently
derived, we find that the absolute distance to this cluster is 18.52 +- 0.005
(random) +- 0.117 (systematic). Note that the current error budget is dominated
by systematic uncertainty affecting the absolute zero-point calibration and the
metallicity scale.Comment: 14 pages, 2 figures, ApJ accepted. Full resolution figure 1 on
request ([email protected]
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