31,330 research outputs found
Deformation of crosslinked semiflexible polymer networks
Networks of filamentous proteins play a crucial role in cell mechanics. These
cytoskeletal networks, together with various crosslinking and other associated
proteins largely determine the (visco)elastic response of cells. In this letter
we study a model system of crosslinked, stiff filaments in order to explore the
connection between the microstructure under strain and the macroscopic response
of cytoskeletal networks. We find two distinct regimes as a function primarily
of crosslink density and filament rigidity: one characterized by affine
deformation and one by non-affine deformation. We characterize the crossover
between these two.Comment: Typos fixed and some technical details clarified. To appear in Phys.
Rev. Let
Rate statistics for radio noise from lightning
Radio frequency noise from lightning was measured at several frequencies in the HF - VHF range at the Kennedy Space Center, Florida. The data were examined to determine flashing rate statistics during periods of strong activity from nearby storms. It was found that the time between flashes is modeled reasonably well by a random variable with a lognormal distribution
Non-equilibrium mechanics and dynamics of motor activated gels
The mechanics of cells is strongly affected by molecular motors that generate
forces in the cellular cytoskeleton. We develop a model for cytoskeletal
networks driven out of equilibrium by molecular motors exerting transient
contractile stresses. Using this model we show how motor activity can
dramatically increase the network's bulk elastic moduli. We also show how motor
binding kinetics naturally leads to enhanced low-frequency stress fluctuations
that result in non-equilibrium diffusive motion within an elastic network, as
seen in recent \emph{in vitro} and \emph{in vivo} experiments.Comment: 21 pages, 8 figure
Detection of Prolonged Diapause of Northern Corn Rootworm in Michigan (Coleoptera: Chrysomelidae)
Prolonged diapause of northern corn rootworm, while known from other Midwestern states, has not previously been reported in Michigan. Populations of northern corn rootworm, (Diabrotica barberi) from two first-year corn fields in Genesee County, Michigan were examined for prolonged egg diapause. Pro- longed diapause was suspected in these populations due to an unusually high proportion of northern versus western corn rootworms in these fields. Eggs obtained from females collected at these sites were reared in the laboratory for two years. The presence of the prolonged diapause trait was confirmed in one population by eggs which hatched following two simulated winters (7.3%). None of the eggs m the second population hatched following the second chill period, however, some eggs in this population remained in apparent diapause at the end of two years. The potential for using observed population shifts in favor of D. barberi as an early warning of the expansion of prolonged diapause in a population is discussed
Ceramics for engines
The NASA Lewis Research Center's Ceramic Technology Program is focused on aerospace propulsion and power needs. Thus, emphasis is on high-temperature ceramics and their structural and environmental durability and reliability. The program is interdisciplinary in nature with major emphasis on materials and processing, but with significant efforts in design methodology and life prediction
Identifying and Indexing Icosahedral Quasicrystals from Powder Diffraction Patterns
We present a scheme to identify quasicrystals based on powder diffraction
data and to provide a standardized indexing. We apply our scheme to a large
catalog of powder diffraction patterns, including natural minerals, to look for
new quasicrystals. Based on our tests, we have found promising candidates
worthy of further exploration.Comment: 4 pages, 1 figur
Ceramics for engines
Structural ceramics were under nearly continuous development for various heat engine applications since the early 1970s. These efforts were sustained by the properties that ceramics offer in the areas of high-temperature strength, environmental resistance, and low density and the large benefits in system efficiency and performance that can result. The promise of ceramics was not realized because their brittle nature results in high sensitivity to microscopic flaws and catastrophic fracture behavior. This translated into low reliability for ceramic components and thus limited their application in engines. For structural ceramics to successfully make inroads into the terrestrial heat engine market requires further advances in low cost, net shape fabrication of high reliability components, and improvements in properties such as toughness, and strength. These advances will lead to very limited use of ceramics in noncritical applications in aerospace engines. For critical aerospace applications, an additional requirement is that the components display markedly improved toughness and noncatastrophic or graceful fracture. Thus the major emphasis is on fiber-reinforced ceramics
The mechanical response of semiflexible networks to localized perturbations
Previous research on semiflexible polymers including cytoskeletal networks in
cells has suggested the existence of distinct regimes of elastic response, in
which the strain field is either uniform (affine) or non-uniform (non-affine)
under external stress. Associated with these regimes, it has been further
suggested that a new fundamental length scale emerges, which characterizes the
scale for the crossover from non-affine to affine deformations. Here, we extend
these studies by probing the response to localized forces and force dipoles. We
show that the previously identified nonaffinity length [D.A. Head et al. PRE
68, 061907 (2003).] controls the mesoscopic response to point forces and the
crossover to continuum elastic behavior at large distances.Comment: 16 pages, 18 figures; substantial changes to text and figures to
clarify the crossover to continuum elasticity and the role of finite-size
effect
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