760 research outputs found
Differential growth of wrinkled biofilms
Biofilms are antibiotic-resistant bacterial aggregates that grow on moist
surfaces and can trigger hospital-acquired infections. They provide a classical
example in biology where the dynamics of cellular communities may be observed
and studied. Gene expression regulates cell division and differentiation, which
affect the biofilm architecture. Mechanical and chemical processes shape the
resulting structure. We gain insight into the interplay between cellular and
mechanical processes during biofilm development on air-agar interfaces by means
of a hybrid model. Cellular behavior is governed by stochastic rules informed
by a cascade of concentration fields for nutrients, waste and autoinducers.
Cellular differentiation and death alter the structure and the mechanical
properties of the biofilm, which is deformed according to Foppl-Von Karman
equations informed by cellular processes and the interaction with the
substratum. Stiffness gradients due to growth and swelling produce wrinkle
branching. We are able to reproduce wrinkled structures often formed by
biofilms on air-agar interfaces, as well as spatial distributions of
differentiated cells commonly observed with B. subtilis.Comment: 19 pages, 13 figure
Intrinsic viscosity of a suspension of weakly Brownian ellipsoids in shear
We analyze the angular dynamics of triaxial ellipsoids in a shear flow
subject to weak thermal noise. By numerically integrating an overdamped angular
Langevin equation, we find the steady angular probability distribution for a
range of triaxial particle shapes. From this distribution we compute the
intrinsic viscosity of a dilute suspension of triaxial particles. We determine
how the viscosity depends on particle shape in the limit of weak thermal noise.
While the deterministic angular dynamics depends very sensitively on particle
shape, we find that the shape dependence of the intrinsic viscosity is weaker,
in general, and that suspensions of rod-like particles are the most sensitive
to breaking of axisymmetry. The intrinsic viscosity of a dilute suspension of
triaxial particles is smaller than that of a suspension of axisymmetric
particles with the same volume, and the same ratio of major to minor axis
lengths.Comment: 14 pages, 6 figures, 1 table, revised versio
Settling of an asymmetric dumbbell in a quiescent fluid
We compute the hydrodynamic torque on a dumbbell (two spheres linked by a
massless rigid rod) settling in a quiescent fluid at small but finite Reynolds
number. The spheres have the same mass densities but different sizes. When the
sizes are quite different the dumbbell settles vertically, aligned with the
direction of gravity, the largest sphere first. But when the size difference is
sufficiently small then its steady-state angle is determined by a competition
between the size difference and the Reynolds number. When the sizes of the
spheres are exactly equal then fluid inertia causes the dumbbell to settle in a
horizontal orientation.Comment: 11 pages, 1 figure, as publishe
Effect of weak fluid inertia upon Jeffery orbits
We consider the rotation of small neutrally buoyant axisymmetric particles in
a viscous steady shear flow. When inertial effects are negligible the problem
exhibits infinitely many periodic solutions, the "Jeffery orbits". We compute
how inertial effects lift their degeneracy by perturbatively solving the
coupled particle-flow equations. We obtain an equation of motion valid at small
shear Reynolds numbers, for spheroidal particles with arbitrary aspect ratios.
We analyse how the linear stability of the \lq log-rolling\rq{} orbit depends
on particle shape and find it to be unstable for prolate spheroids. This
resolves a puzzle in the interpretation of direct numerical simulations of the
problem. In general both unsteady and non-linear terms in the Navier-Stokes
equations are important.Comment: 5 pages, 2 figure
Dynamics of bacterial aggregates in microflows
Depto. de Análisis Matemático y Matemática AplicadaFac. de Ciencias MatemáticasFALSEunpu
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