65,393 research outputs found
A new approach for the limit to tree height using a liquid nanolayer model
Liquids in contact with solids are submitted to intermolecular forces
inferring density gradients at the walls. The van der Waals forces make liquid
heterogeneous, the stress tensor is not any more spherical as in homogeneous
bulks and it is possible to obtain stable thin liquid films wetting vertical
walls up to altitudes that incompressible fluid models are not forecasting.
Application to micro tubes of xylem enables to understand why the ascent of sap
is possible for very high trees like sequoias or giant eucalyptus.Comment: In the conclusion is a complementary comment to the Continuum
Mechanics and Thermodynamics paper. 21 pages, 4 figures. Continuum Mechanics
and Thermodynamics 20, 5 (2008) to appea
Liquid-solid interaction at nanoscale and its application in vegetal biology
The water ascent in tall trees is subject to controversy: the vegetal
biologists debate on the validity of the cohesion-tension theory which
considers strong negative pressures in microtubes of xylem carrying the crude
sap. This article aims to point out that liquids are submitted at the walls to
intermolecular forces inferring density gradients making heterogeneous liquid
layers and therefore disqualifying the Navier-Stokes equations for nanofilms.
The crude sap motion takes the disjoining pressure gradient into account and
the sap flow dramatically increases such that the watering of nanolayers may be
analogous to a microscopic flow. Application to microtubes of xylem avoids the
problem of cavitation and enables us to understand why the ascent of sap is
possible for very high trees.Comment: 16 pages 1 figure New modern concept of the sap ascent in high trees
taking into account the disjoining pressure in nanofilms of liquids. The
motion of the sap is related to thin films slippering on the xylem wall
Ecological implications of a flower size/number trade-off in tropical forest trees
Peer reviewedPublisher PD
A review of Monte Carlo simulations of polymers with PERM
In this review, we describe applications of the pruned-enriched Rosenbluth
method (PERM), a sequential Monte Carlo algorithm with resampling, to various
problems in polymer physics. PERM produces samples according to any given
prescribed weight distribution, by growing configurations step by step with
controlled bias, and correcting "bad" configurations by "population control".
The latter is implemented, in contrast to other population based algorithms
like e.g. genetic algorithms, by depth-first recursion which avoids storing all
members of the population at the same time in computer memory. The problems we
discuss all concern single polymers (with one exception), but under various
conditions: Homopolymers in good solvents and at the point, semi-stiff
polymers, polymers in confining geometries, stretched polymers undergoing a
forced globule-linear transition, star polymers, bottle brushes, lattice
animals as a model for randomly branched polymers, DNA melting, and finally --
as the only system at low temperatures, lattice heteropolymers as simple models
for protein folding. PERM is for some of these problems the method of choice,
but it can also fail. We discuss how to recognize when a result is reliable,
and we discuss also some types of bias that can be crucial in guiding the
growth into the right directions.Comment: 29 pages, 26 figures, to be published in J. Stat. Phys. (2011
Murray's law revisited: Qu\'emada's fluid model and fractal trees
In 1926, Murray proposed the first law for the optimal design of blood
vessels. He minimized the power dissipation arising from the trade-off between
fluid circulation and blood maintenance. The law, based on a constant fluid
viscosity, states that in the optimal configuration the fluid flow rate inside
the vessel is proportional to the cube of the vessel radius, implying that wall
shear stress is not dependent on the vessel radius. Murray's law has been found
to be true in blood macrocirculation, but not in microcirculation. In 2005,
Alarc\'on et al took into account the non monotonous dependence of viscosity on
vessel radius - F{\aa}hr{\ae}us-Lindqvist effect - due to phase separation
effect of blood. They were able to predict correctly the behavior of wall shear
stresses in microcirculation. One last crucial step remains however: to account
for the dependence of blood viscosity on shear rates. In this work, we
investigate how viscosity dependence on shear rate affects Murray's law. We
extended Murray's optimal design to the whole range of Qu\'emada's fluids, that
models pseudo-plastic fluids such as blood. Our study shows that Murray's
original law is not restricted to Newtonian fluids, it is actually universal
for all Qu\'emada's fluid as long as there is no phase separation effect. When
phase separation effect occurs, then we derive an extended version of Murray's
law. Our analyses are very general and apply to most of fluids with shear
dependent rheology. Finally, we study how these extended laws affect the
optimal geometries of fractal trees to mimic an idealized arterial network
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