2,368 research outputs found
Short Time Behavior in De Gennes' Reptation Model
To establish a standard for the distinction of reptation from other modes of
polymer diffusion, we analytically and numerically study the displacement of
the central bead of a chain diffusing through an ordered obstacle array for
times . Our theory and simulations agree quantitatively and show
that the second moment approaches the often viewed as signature of
reptation only after a very long transient and only for long chains (N > 100).
Our analytically solvable model furthermore predicts a very short transient for
the fourth moment. This is verified by computer experiment.Comment: 4 pages, revtex, 4 ps file
Loss of solutions in shear banding fluids in shear banding fluids driven by second normal stress differences
Edge fracture occurs frequently in non-Newtonian fluids. A similar
instability has often been reported at the free surface of fluids undergoing
shear banding, and leads to expulsion of the sample. In this paper the
distortion of the free surface of such a shear banding fluid is calculated by
balancing the surface tension against the second normal stresses induced in the
two shear bands, and simultaneously requiring a continuous and smooth meniscus.
We show that wormlike micelles typically retain meniscus integrity when shear
banding, but in some cases can lose integrity for a range of average applied
shear rates during which one expects shear banding. This meniscus fracture
would lead to ejection of the sample as the shear banding region is swept
through. We further show that entangled polymer solutions are expected to
display a propensity for fracture, because of their much larger second normal
stresses. These calculations are consistent with available data in the
literature. We also estimate the meniscus distortion of a three band
configuration, as has been observed in some wormlike micellar solutions in a
cone and plate geometry.Comment: 23 pages, to be published in Journal of Rheolog
Crayfish feeding preferences for fresh water macrophytes: The influence of plant structure and chemistry
The omnivorous crayfish Procambarus clarkii fed selectively on several species of macrophytes, preferring delicate fresh plants that had filamentous or finely-branched architectures. When the macrophytes were dried, powdered, and reconstituted into an alginate gel (thus eliminating among-species differences in physical characteristics). crayfish preferences were altered; previously tough plants that were high in nitrogen and protein were preferred over previously delicate plants that were low in nitrogen and protein. Even though plant structure influences feeding decision of crayfish, the structurally identical macrophyte gels were fed upon differently, demonstrating that nonstructural traits are important feeding determinants. However, plant tissue constituents such as nitrogen, protein, phenolics, lignin, cellulose, or ash were not significantly con-elated with feeding preferences. Two high-nitrogen plants that were avoided by crayfish as fresh and as reconstituted tissue (Nuphar luteum macrophyllum and Alternanthera philoxeroides) possessed extracts that reduced crayfish feeding in laboratory assays, demonstrating that macrophyte metabolites can deter some herbivores. As is often observed with large generalist herbivores and omnivores in terrestrial and marine systems, the freshwater crayfish made feeding decisions based upon multiple plant cues (structure, nutrition, chemical defenses)
Dark-field transmission electron microscopy and the Debye-Waller factor of graphene
Graphene's structure bears on both the material's electronic properties and
fundamental questions about long range order in two-dimensional crystals. We
present an analytic calculation of selected area electron diffraction from
multi-layer graphene and compare it with data from samples prepared by chemical
vapor deposition and mechanical exfoliation. A single layer scatters only 0.5%
of the incident electrons, so this kinematical calculation can be considered
reliable for five or fewer layers. Dark-field transmission electron micrographs
of multi-layer graphene illustrate how knowledge of the diffraction peak
intensities can be applied for rapid mapping of thickness, stacking, and grain
boundaries. The diffraction peak intensities also depend on the mean-square
displacement of atoms from their ideal lattice locations, which is
parameterized by a Debye-Waller factor. We measure the Debye-Waller factor of a
suspended monolayer of exfoliated graphene and find a result consistent with an
estimate based on the Debye model. For laboratory-scale graphene samples,
finite size effects are sufficient to stabilize the graphene lattice against
melting, indicating that ripples in the third dimension are not necessary.Comment: 10 pages, 4 figure
The Sagnac Phase Shift suggested by the Aharonov-Bohm effect for relativistic matter beams
The phase shift due to the Sagnac Effect, for relativistic matter beams
counter-propagating in a rotating interferometer, is deduced on the bases of a
a formal analogy with the the Aharonov-Bohm effect. A procedure outlined by
Sakurai, in which non relativistic quantum mechanics and newtonian physics
appear together with some intrinsically relativistic elements, is generalized
to a fully relativistic context, using the Cattaneo's splitting technique. This
approach leads to an exact derivation, in a self-consistently relativistic way,
of the Sagnac effect. Sakurai's result is recovered in the first order
approximation.Comment: 18 pages, LaTeX, 2 EPS figures. To appear in General Relativity and
Gravitatio
Self-diffusion in binary blends of cyclic and linear polymers
A lattice model is used to estimate the self-diffusivity of entangled cyclic
and linear polymers in blends of varying compositions. To interpret simulation
results, we suggest a minimal model based on the physical idea that constraints
imposed on a cyclic polymer by infiltrating linear chains have to be released,
before it can diffuse beyond a radius of gyration. Both, the simulation, and
recently reported experimental data on entangled DNA solutions support the
simple model over a wide range of blend compositions, concentrations, and
molecular weights.Comment: 10 pages, 2 figure
Whitehall in the Caribbean? The legacy of colonial administration for post-colonial democratic development
Colonial-era administrative institutions and doctrines are fundamental to any analysis of Westminsterâs legacy in the Caribbean. Applying the lens of âPublic Service Bargainsâ (PSBs) â the formal and informal understandings of reward, competence and loyalty of public servants âwe first examine constitutional and administrative doctrines regarding the public service of Crown Colonies, before analysing how these worked themselves out in Jamaica. Our analysis reveals a number of perceived deficiencies in the PSB in the pre-independence period that cast a shadow on future relations in the post-independence period
Breastfeeding and the risk of dental caries: a systematic review and meta-analysis
Aim To synthesise the current evidence for the associations between breastfeeding and dental caries, with respect to specific windows of early childhood caries risk. Methods Systematic review, meta-analyses and narrative synthesis following searches of PubMed, CINAHL and EMBASE databases. Results Sixty-three papers included. Children exposed to longer versus shorter duration of breastfeeding up to age 12 months (more versus less breastfeeding), had a reduced risk of caries (OR 0.50; 95%CI 0.25, 0.99, I2 86.8%). Children breastfed > 12 months had an increased riskof caries when compared with children breastfed 12 months, those fed nocturnally or more frequently had afurther increased caries risk (five studies, OR 7.14; 3.14, 16.23, I2 77.1%). There was a lack of studies on children aged > 12 months simultaneously assessing caries risk in breastfed, bottle-fed and children not bottle or breastfed, alongside specific breastfeeding practices, consuming sweet drinks and foods, and oral hygiene practices limiting our ability to tease out the risks attributable to each. Conclusion Breastfeeding in infancy may protect against dental caries. Further research needed to understand the increased risk of caries in children breastfed after 12 month
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