10,662 research outputs found
Wicking through a confined micropillar array
This study considers the spreading of a Newtonian and perfectly wetting
liquid in a square array of cylindric micropillars confined between two plates.
We show experimentally that the dynamics of the contact line follows a
Washburn-like law which depends on the characteristics of the micropillar array
(height, diameter and pitch). The presence of pillars can either enhanced or
slow down the motion of the contact line. A theoretical model based on
capillary and viscous forces has been developed in order to rationalize our
observations. Finally, the impact of pillars on the volumic flow rate of liquid
which is pumped in the microchannel is inspected
Depinning and plasticity of driven disordered lattices
We review in these notes the dynamics of extended condensed matter systesm,
such as vortex lattices in type-II superconductors and charge density waves in
anisotropic metals, driven over quenched disorder. We focus in particular on
the case of strong disorder, where topological defects are generated in the
driven lattice. In this case the repsonse is plastic and the depinning
transition may become discontinuous and hysteretic.Comment: 21 pages, 6 figures. Proceedings the XIX Sitges Conference on
Jamming, Yielding, and Irreversible Deformations in Condensed Matter, Sitges,
Barcelona, Spain, June 14-18, 200
Order in driven vortex lattices in superconducting Nb films with nanostructured pinning potentials
Driven vortex lattices have been studied in a material with strong pinning,
such as Nb films. Samples in which natural random pinning coexists with
artificial ordered arrays of defects (submicrometric Ni dots) have been
fabricated with different geometries (square, triangular and rectangular).
Three different dynamic regimes are found: for low vortex velocities, there is
a plastic regime in which random defects frustrate the effect of the ordered
array; then, for vortex velocities in the range 1-100 m/s, there is a sudden
increase in the interaction between the vortex lattice and the ordered dot
array, independent on the geometry. This effect is associated to the onset of
quasi long range order in the vortex lattice leading to an increase in the
overlap between the vortex lattice and the magnetic dots array. Finally, at
larger velocities the ordered array-vortex lattice interaction is suppresed
again, in agreement with the behavior found in numerical simulations.Comment: 8 text pages + 4 figure
Dynamics of stable viscous displacement in porous media
We investigate the stabilization mechanisms of the invasion front in
two-dimensional drainage displacement in porous media by using a network
simulator. We focus on the process when the front stabilizes due to the viscous
forces in the liquids. We find that the capillary pressure difference between
two different points along the front varies almost linearly as function of
height separation in the direction of the displacement. The numerical results
support arguments that differ from those suggested earlier for viscous
stabilization. Our arguments are based upon the observation that nonwetting
fluid flows in loopless strands (paths) and we conclude that earlier suggested
theories are not suitable to drainage when nonwetting strands dominate the
displacement process. We also show that the arguments might influence the
scaling behavior between the front width and the injection rate and compare
some of our results to experimental work.Comment: The paper has been substantially revised. 12 papes, 10 figure
A novel model for one-dimensional morphoelasticity. Part I - Theoretical foundations
While classical continuum theories of elasticity and viscoelasticity have long been used to describe the mechanical behaviour of solid biological tissues, they are of limited use for the description of biological tissues that undergo continuous remodelling. The structural changes to a soft tissue associated with growth and remodelling require a mathematical theory of ‘morphoelasticity’ that is more akin to plasticity than elasticity. However, previously-derived mathematical models for plasticity are difficult to apply and interpret in the context of growth and remodelling: many important concepts from the theory of plasticity do not have simple analogues in biomechanics.\ud
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In this work, we describe a novel mathematical model that combines the simplicity and interpretability of classical viscoelastic models with the versatility of plasticity theory. While our focus here is on one-dimensional problems, our model builds on earlier work based on the multiplicative decomposition of the deformation gradient and can be adapted to develop a three-dimensional theory. The foundation of this work is the concept of ‘effective strain’, a measure of the difference between the current state and a hypothetical state where the tissue is mechanically relaxed. We develop one-dimensional equations for the evolution of effective strain, and discuss a number of potential applications of this theory. One significant application is the description of a contracting fibroblast-populated collagen lattice, which we further investigate in Part II
Nonlinear Viscous Vortex Motion in Two-Dimensional Josephson-Junction Arrays
When a vortex in a two-dimensional Josephson junction array is driven by a
constant external current it may move as a particle in a viscous medium. Here
we study the nature of this viscous motion. We model the junctions in a square
array as resistively and capacitively shunted Josephson junctions and carry out
numerical calculations of the current-voltage characteristics. We find that the
current-voltage characteristics in the damped regime are well described by a
model with a {\bf nonlinear} viscous force of the form , where is the vortex velocity,
is the velocity dependent viscosity and and are
constants for a fixed value of the Stewart-McCumber parameter. This result is
found to apply also for triangular lattices in the overdamped regime. Further
qualitative understanding of the nature of the nonlinear friction on the vortex
motion is obtained from a graphic analysis of the microscopic vortex dynamics
in the array. The consequences of having this type of nonlinear friction law
are discussed and compared to previous theoretical and experimental studies.Comment: 14 pages RevTex, 9 Postscript figure
Driven depinning of strongly disordered media and anisotropic mean-field limits
Extended systems driven through strong disorder are modeled generically using
coarse-grained degrees of freedom that interact elastically in the directions
parallel to the driving force and that slip along at least one of the
directions transverse to the motion. A realization of such a model is a
collection of elastic channels with transverse viscous couplings. In the
infinite range limit this model has a tricritical point separating a region
where the depinning is continuous, in the universality class of elastic
depinning, from a region where depinning is hysteretic. Many of the collective
transport models discussed in the literature are special cases of the generic
model.Comment: 4 pages, 2 figure
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