5,138 research outputs found
Axisymmetric bifurcations of thick spherical shells under inflation and compression
Incremental equilibrium equations and corresponding boundary conditions for an isotropic, hyperelastic and incompressible material are summarized and then specialized to a form suitable for the analysis of a spherical shell subject to an internal or an external pressure. A thick-walled spherical shell during inflation is analyzed using four different material models. Specifically, one and two terms in the Ogden energy formulation, the Gent model and an I1 formulation recently proposed by Lopez-Pamies. We investigate the existence of local pressure maxima and minima and the dependence of the corresponding stretches on the material model and on shell thickness. These results are then used to investigate axisymmetric bifurcations of the inflated shell. The analysis is extended to determine the behavior of a thick-walled spherical shell subject to an external pressure. We find that the results of the two terms Ogden formulation, the Gent and the Lopez-Pamies models are very similar, for the one term Ogden material we identify additional critical stretches, which have not been reported in the literature before
Experimental evidence of accelerated energy transfer in turbulence
We investigate the vorticity dynamics in a turbulent vortex using scattering
of acoustic waves. Two ultrasonic beams are adjusted to probe simultaneously
two spatial scales in a given volume of the flow, thus allowing a dual channel
recording of the dynamics of coherent vorticity structures. Our results show
that this allows to measure the average energy transfer time between different
spatial length scales, and that such transfer goes faster at smaller scales.Comment: 5 pages, 5 figure
Epidemic model on a network: analysis and applications to COVID-19
We analyze an epidemic model on a network consisting of
susceptible-infected-recovered equations at the nodes coupled by diffusion
using a graph Laplacian. We introduce an epidemic criterion and examine
different vaccination/containment strategies: we prove that it is most
effective to vaccinate a node of highest degree. The model is also useful to
evaluate deconfinement scenarios and prevent a so-called second wave. The model
has few parameters enabling fitting to the data and the essential ingredient of
importation of infected; these features are particularly important for the
current COVID-19 epidemic
Force induced stretched state: Effects of temperature
A model of self avoiding walks with suitable constraint has been developed to
study the effect of temperature on a single stranded DNA (ssDNA) in the
constant force ensemble. Our exact calculations for small chains show that the
extension (reaction co-ordinate) may increase or decrease with the temperature
depending upon the applied force. The simple model developed here which
incorporates semi-microscopic details of base direction provide an explanation
of the force induced transitions in ssDNA as observed in experiments.Comment: 5 pages, 8 figures, RevTex
Condensation transition in DNA-polyaminoamide dendrimer fibers studied using optical tweezers
When mixed together, DNA and polyaminoamide (PAMAM) dendrimers form fibers
that condense into a compact structure. We use optical tweezers to pull
condensed fibers and investigate the decondensation transition by measuring
force-extension curves (FECs). A characteristic plateau force (around 10 pN)
and hysteresis between the pulling and relaxation cycles are observed for
different dendrimer sizes, indicating the existence of a first-order transition
between two phases (condensed and extended) of the fiber. The fact that we can
reproduce the same FECs in the absence of additional dendrimers in the buffer
medium indicates that dendrimers remain irreversibly bound to the DNA backbone.
Upon salt variation FECs change noticeably confirming that electrostatic forces
drive the condensation transition. Finally, we propose a simple model for the
decondensing transition that qualitatively reproduces the FECs and which is
confirmed by AFM images.Comment: Latex version, 4 pages+3 color figure
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