4,421 research outputs found
Polyelectrolyte Networks: Elasticity, Swelling, and the Violation of the Flory - Rehner Hypothesis
This paper discusses the elastic behavior of polyelectrolyte networks. The
deformation behavior of single polyelectrolyte chains is discussed. It is shown
that a strong coupling between interactions and chain elasticity exists. The
theory of the complete crosslinked networks shows that the Flory - Rehner -
Hypothesis (FRH) does not hold. The modulus contains contributions from the
classical rubber elasticity and from the electrostatic interactions. The
equilibrium degree of swelling is estimated by the assumption of a
-network.Comment: submitted to Computational and Theoretical Polymer Scienc
Elasticity in strongly interacting soft solids: polyelectrolyte network
This paper discusses the elastic behavior of a very long crosslinked
polyelectrolyte chain (Debye-H\"uckel chain), which is weakly charged.
Therefore the response of the crosslinked chain (network) on an external
constant force acting on the ends of the chain is considered. A
selfconsistent variational computation of an effective field theory is
employed. It is shown, that the modulus of the polyelectrolyte network has two
parts: the first term represents the usual entropy elasticity of connected
flexible chains and the second term takes into account the electrostatic
interaction of the monomers. It is proportional to the squared crosslink
density and the Debye - screening parameter.Comment: submitted for publication to PR
The Effects of Owl Decoys and Non-threatening Objects on Bird Feeding Behavior
As traditional agricultural practices change, the scarecrow has also been renovated and modernized to include mock natural predators, such as owls, hawks, and falcons instead of comical human representations. These facsimiles represent an excellent opportunity to examine anti-predatory tactics and vigilance behavior in birds as a response to perceived threats. In this study, we tested songbird reaction to an owl decoy that mimicked the presence of a predator and to a non-threatening object placed in an oak woodland within Oregon’s Willamette Valley. Frequency of bird visitations to bird feeders when either a plastic owl or a cardboard box of similar size was used to examine the effect of the presence of a predator on bird feeding behavior. We hypothesized that introduction of a model owl would reduce the number of birds observed at a nearby feeder, but a cardboard box would not have a significant effect on bird presence. Using paired t-tests, we determined that a false predator was effective in deterring bird species from feeding, while a box was not
Effects of a Novel Dental Gel on Plaque and Gingivitis: A Comparative Study.
ObjectivesThe goal of this prospective, randomized, controlled, double-blinded study was to evaluate the effects of a novel dental gel on plaque and gingival health. The dental gel was designed to (1) break up and prevent re-accumulation of microbial biofilm, and (2) inhibit metal mediated inflammation.Materials and methodsTwenty-five subjects with moderate gingival inflammation (Löe and Silness Gingival Index ≥2) and pocket depths <4 were randomly assigned to brush twice daily for 21 days with the test or the control dental gel. On Days 0, 7, 14 and 21, plaque levels (Quigley-Hein, Turesky Modification Plaque Index), gingival inflammation (Löe and Silness Gingival Index) and gingival bleeding (modified Sulcus Bleeding Index) were determined by one blinded, investigator using a pressure sensitive probe.ResultsAfter 3 weeks, all 3 clinical indices were significantly improved in both groups (P<0.05) and significantly lower in the test group (P<0.05).ConclusionThe novel dental gel formulation was provided effective plaque control and reduced gingival inflammation.Clinical relevanceA novel dentifrice formulation may be an effective tool for plaque removal and maintaining gingival health
Modal Analysis with the Mobile Modal Testing Unit
Recently, National Aeronautics and Space Administration's (NASA's) White Sands Test Facility (WSTF) has tested rocket engines with high pulse frequencies. This has resulted in the use of some of WSTF's existing thrust stands, which were designed for static loading, in tests with large dynamic forces. In order to ensure that the thrust stands can withstand the dynamic loading of high pulse frequency engines while still accurately reporting the test data, their vibrational modes must be characterized. If it is found that they have vibrational modes with frequencies near the pulsing frequency of the test, then they must be modified to withstand the dynamic forces from the pulsing rocket engines. To make this determination the Mobile Modal Testing Unit (MMTU), a system capable of determining the resonant frequencies and mode shapes of a structure, was used on the test stands at WSTF. Once the resonant frequency has been determined for a test stand, it can be compared to the pulse frequency of a test engine to determine whether or not that stand can avoid resonance and reliably test that engine. After analysis of test stand 406 at White Sands Test Facility, it was determined that natural frequencies for the structure are located around 75, 125, and 240 Hz, and thus should be avoided during testing
Should you test for H pylori in patients with nonulcer dyspepsia?
Helicobacter pylori in creases the risk of developing peptic ulcer disease (strength of recommendation [SOR]: B, cohort study), but there is no evidence that treating H pylori reduces that risk. Treating H pylori in patients with nonulcer dyspepsia reduces symptoms, but doesn't improve quality of life in the short term (SOR: B, systematic review of randomized controlled trials [RCTs] with inconsistent results), but may alleviate symptoms in the long term (SOR: B, RCT with methodological flaws). Eradicating H pylori is relatively inexpensive (SOR: A, systematic review of economic analyses)
Convective Fingering of an Autocatalytic Reaction Front
We report experimental observations of the convection-driven fingering
instability of an iodate-arsenous acid chemical reaction front. The front
propagated upward in a vertical slab; the thickness of the slab was varied to
control the degree of instability. We observed the onset and subsequent
nonlinear evolution of the fingers, which were made visible by a {\it p}H
indicator. We measured the spacing of the fingers during their initial stages
and compared this to the wavelength of the fastest growing linear mode
predicted by the stability analysis of Huang {\it et. al.} [{\it Phys. Rev. E},
{\bf 48}, 4378 (1993), and unpublished]. We find agreement with the thickness
dependence predicted by the theory.Comment: 11 pages, RevTex with 3 eps figures. To be published in Phys Rev E,
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