1,269 research outputs found
Vesicles in a Poiseuille flow
Vesicle dynamics in unbounded Poiseuille flow is analyzed using a
small-deformation theory. Our analytical results quantitatively describe
vesicle migration and provide new physical insights. At low ratio between the
inner and outer viscosity (i.e. in the tank-treading regime), the
vesicle always migrates towards the flow centerline, unlike other soft
particles such as drops. Above a critical , vesicle tumbles and
cross-stream migration vanishes. A novel feature is predicted, namely the
coexistence of two types of nonequilibrium configurations at the centreline, a
bullet-like and a parachute-like shapes.Comment: 4 pages and 5 figure
Bistability in a simple fluid network due to viscosity contrast
We study the existence of multiple equilibrium states in a simple fluid
network using Newtonian fluids and laminar flow. We demonstrate theoretically
the presence of hysteresis and bistability, and we confirm these predictions in
an experiment using two miscible fluids of different viscosity--sucrose
solution and water. Possible applications include bloodflow, microfluidics, and
other network flows governed by similar principles
Origins of Metabolic Signals
Diameters of microvessels undergo continuous structural adaptation in response
to hemodynamic and metabolic stimuli. To ensure adequate flow distribution,
metabolic responses are needed to increase diameters of vessels feeding poorly
perfused regions. Possible modes of metabolic control include release of
signaling substances from vessel walls, from the supplied tissue and from red
blood cells (RBC). Here, a theoretical model was used to compare the abilities
of these metabolic control modes to provide adequate tissue oxygenation, and
to generate blood flow velocities in agreement with experimental observations.
Structural adaptation of vessel diameters was simulated for an observed
mesenteric network structure in the rat with 576 vessel segments. For each
mode of metabolic control, resulting distributions of oxygen and deviations
between simulated and experimentally observed flow velocities were analyzed.
It was found that wall-derived and tissue-derived growth signals released in
response to low oxygen levels could ensure adequate oxygen supply, but RBC-
derived signals caused inefficient oxygenation. Closest agreement between
predicted and observed flow velocities was obtained with wall-derived growth
signals proportional to vessel length. Adaptation in response to oxygen-
independent release of a metabolic signal substance from vessel walls or the
supplied tissue was also shown to be effective for ensuring tissue oxygenation
due to a dilution effect if growth signal substances are released into the
blood. The present results suggest that metabolic signals responsible for
structural adaptation of microvessel diameters are derived from vessel walls
or from perivascular tissue
The integral monodromy of hyperelliptic and trielliptic curves
We compute the \integ/\ell and \integ_\ell monodromy of every irreducible
component of the moduli spaces of hyperelliptic and trielliptic curves. In
particular, we provide a proof that the \integ/\ell monodromy of the moduli
space of hyperelliptic curves of genus is the symplectic group
\sp_{2g}(\integ/\ell). We prove that the \integ/\ell monodromy of the
moduli space of trielliptic curves with signature is the special
unitary group \su_{(r,s)}(\integ/\ell\tensor\integ[\zeta_3])
GENETICS AND BIOCHEMISTRY OF DEHALOGENATING ENZYMES
Microorganisms that can utilize halogenated compounds as a growth substrate generally produce enzymes whose function is carbon-halogen bond cleavage. Based on substrate range, reaction type and gene sequences, the dehalogenating enzymes can be classified in different groups, including hydrolytic dehalogenases, glutathione transferases, monooxygenases and hydratases. X-ray crystallographic and biochemical studies have provided detailed mechanistic insight into the action of haloalkane dehalogenase. The essential features are nucleophilic substitution of the halogen by a carboxylate group and the presence of a distinct halogen binding site, formed by tryptophan residues. This review summaries current knowledge on a variety of other dehalogenating enzymes and indicates the existence of a widespread and diverse microbial potential for dechlorination of natural and xenobiotic halogenated compounds
A multiple scale model for tumor growth
We present a physiologically structured lattice model for vascular tumor growth which accounts for blood flow and structural adaptation of the vasculature, transport of oxygen, interaction between cancerous and normal tissue, cell division, apoptosis, vascular endothelial growth factor release, and the coupling between these processes. Simulations of the model are used to investigate the effects of nutrient heterogeneity, growth and invasion of cancerous tissue, and emergent growth laws
A simplified particulate model for coarse-grained hemodynamics simulations
Human blood flow is a multi-scale problem: in first approximation, blood is a
dense suspension of plasma and deformable red cells. Physiological vessel
diameters range from about one to thousands of cell radii. Current
computational models either involve a homogeneous fluid and cannot track
particulate effects or describe a relatively small number of cells with high
resolution, but are incapable to reach relevant time and length scales. Our
approach is to simplify much further than existing particulate models. We
combine well established methods from other areas of physics in order to find
the essential ingredients for a minimalist description that still recovers
hemorheology. These ingredients are a lattice Boltzmann method describing rigid
particle suspensions to account for hydrodynamic long range interactions
and---in order to describe the more complex short-range behavior of
cells---anisotropic model potentials known from molecular dynamics simulations.
Paying detailedness, we achieve an efficient and scalable implementation which
is crucial for our ultimate goal: establishing a link between the collective
behavior of millions of cells and the macroscopic properties of blood in
realistic flow situations. In this paper we present our model and demonstrate
its applicability to conditions typical for the microvasculature.Comment: 12 pages, 11 figure
- …