8,825 research outputs found
A continuum treatment of growth in biological tissue: The coupling of mass transport and mechanics
Growth (and resorption) of biological tissue is formulated in the continuum
setting. The treatment is macroscopic, rather than cellular or sub-cellular.
Certain assumptions that are central to classical continuum mechanics are
revisited, the theory is reformulated, and consequences for balance laws and
constitutive relations are deduced. The treatment incorporates multiple
species. Sources and fluxes of mass, and terms for momentum and energy transfer
between species are introduced to enhance the classical balance laws. The
transported species include: (\romannumeral 1) a fluid phase, and
(\romannumeral 2) the precursors and byproducts of the reactions that create
and break down tissue. A notable feature is that the full extent of coupling
between mass transport and mechanics emerges from the thermodynamics.
Contributions to fluxes from the concentration gradient, chemical potential
gradient, stress gradient, body force and inertia have not emerged in a unified
fashion from previous formulations of the problem. The present work
demonstrates these effects via a physically-consistent treatment. The presence
of multiple, interacting species requires that the formulation be consistent
with mixture theory. This requirement has far-reaching consequences. A
preliminary numerical example is included to demonstrate some aspects of the
coupled formulation.Comment: 29 pages, 11 figures, accepted for publication in Journal of the
Mechanics and Physics of Solids. See journal for final versio
Stellar Mixing and the Primordial Lithium Abundance
We compare the properties of recent samples of the lithium abundances in halo
stars to one another and to the predictions of theoretical models including
rotational mixing, and we examine the data for trends with metal abundance. We
find from a KS test that in the absence of any correction for chemical
evolution, the Ryan, Norris, & Beers (1999} sample is fully consistent with
mild rotational mixing induced depletion and, therefore, with an initial
lithium abundance higher than the observed value. Tests for outliers depend
sensitively on the threshold for defining their presence, but we find a
1045% probability that the RNB sample is drawn from the rotationally mixed
models with a 0.2 dex median depletion (with lower probabilities corresponding
to higher depletion factors). When chemical evolution trends (Li/H versus Fe/H)
are treated in the linear plane we find that the dispersion in the RNB sample
is not explained by chemical evolution; the inferred bounds on lithium
depletion from rotational mixing are similar to those derived from models
without chemical evolution. We find that differences in the equivalent width
measurements are primarily responsible for different observational conclusions
concerning the lithium dispersion in halo stars. The standard Big Bang
Nucleosynthesis predicted lithium abundance which corresponds to the deuterium
abundance inferred from observations of high-redshift, low-metallicity QSO
absorbers requires halo star lithium depletion in an amount consistent with
that from our models of rotational mixing, but inconsistent with no depletion.Comment: 39 pages, 9 figures; submitted Ap
Biological remodelling: Stationary energy, configurational change, internal variables and dissipation
Remodelling is defined as an evolution of microstructure or variations in the
configuration of the underlying manifold. The manner in which a biological
tissue and its subsystems remodel their structure is treated in a continuum
mechanical setting. While some examples of remodelling are conveniently
modelled as evolution of the reference configuration (Case I), others are more
suited to an internal variable description (Case II). In this paper we explore
the applicability of stationary energy states to remodelled systems. A
variational treatment is introduced by assuming that stationary energy states
are attained by changes in microstructure via one of the two mechanisms--Cases
I and II. An example is presented to illustrate each case. The example
illustrating Case II is further studied in the context of the thermodynamic
dissipation inequality.Comment: 24 pages, 4 figures. Replaced version has corrections to typos in
equations, and the corresponding correct plot of the solution--all in Section
Domain-wall fermions with dynamical gauge fields
We have carried out a numerical simulation of a domain-wall model in
-dimensions, in the presence of a dynamical gauge field only in an extra
dimension, corresponding to the weak coupling limit of a ( 2-dimensional )
physical gauge coupling. Using a quenched approximation we have investigated
this model at 0.5 ( ``symmetric'' phase),
1.0, and 5.0 (``broken'' phase), where is the gauge coupling constant of
the extra dimension. We have found that there exists a critical value of a
domain-wall mass which separates a region with a fermionic zero
mode on the domain-wall from the one without it, in both symmetric and broken
phases. This result suggests that the domain-wall method may work for the
construction of lattice chiral gauge theories.Comment: 27 pages (11 figures), latex (epsf style-file needed
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