2,106 research outputs found
Predicting the progress of diffusively limited chemical reactions in the presence of chaotic advection
The effects of chaotic advection and diffusion on fast chemical reactions in
two-dimensional fluid flows are investigated using experimentally measured
stretching fields and fluorescent monitoring of the local concentration. Flow
symmetry, Reynolds number, and mean path length affect the spatial distribution
and time dependence of the reaction product. A single parameter \lambda*N,
where \lambda is the mean Lyapunov exponent and N is the number of mixing
cycles, can be used to predict the time-dependent total product for flows
having different dynamical features.Comment: 4 pages, 4 figures, updated reference
Undulatory swimming in fluids with polymer networks
The motility behavior of the nematode Caenorhabditis elegans in polymeric
solutions of varying concentrations is systematically investigated in
experiments using tracking and velocimetry methods. As the polymer
concentration is increased, the solution undergoes a transition from the
semi-dilute to the concentrated regime, where these rod-like polymers entangle,
align, and form networks. Remarkably, we find an enhancement in the nematode's
swimming speed of approximately 65% in concentrated solutions compared to
semi-dilute solutions. Using velocimetry methods, we show that the undulatory
swimming motion of the nematode induces an anisotropic mechanical response in
the fluid. This anisotropy, which arises from the fluid micro-structure, is
responsible for the observed increase in swimming speed.Comment: Published 1 November 2013 in Europhysics Letter
Induced representations of quantum kinematical algebras
We construct the induced representations of the null-plane quantum Poincar\'e
and quantum kappa Galilei algebras in (1+1) dimensions. The induction procedure
makes use of the concept of module and is based on the existence of a pair of
Hopf algebras with a nondegenerate pairing and dual bases.Comment: 8 pages,LaTeX2e, to be published in the Proceedings of XXIII
International Colloquium on Group-Theoretical Methods in Physics, Dubna
(Russia), 31.07--05.08, 200
Polymeric filament thinning and breakup in microchannels
The effects of elasticity on filament thinning and breakup are investigated
in microchannel cross flow. When a viscous solution is stretched by an external
immiscible fluid, a low 100 ppm polymer concentration strongly affects the
breakup process, compared to the Newtonian case. Qualitatively, polymeric
filaments show much slower evolution, and their morphology features multiple
connected drops. Measurements of filament thickness show two main temporal
regimes: flow- and capillary-driven. At early times both polymeric and
Newtonian fluids are flow-driven, and filament thinning is exponential. At
later times, Newtonian filament thinning crosses over to a capillary-driven
regime, in which the decay is algebraic. By contrast, the polymeric fluid first
crosses over to a second type of flow-driven behavior, in which viscoelastic
stresses inside the filament become important and the decay is again
exponential. Finally, the polymeric filament becomes capillary-driven at late
times with algebraic decay. We show that the exponential flow thinning behavior
allows a novel measurement of the extensional viscosities of both Newtonian and
polymeric fluids.Comment: 7 pages, 7 figure
Motility of small nematodes in disordered wet granular media
The motility of the worm nematode \textit{Caenorhabditis elegans} is
investigated in shallow, wet granular media as a function of particle size
dispersity and area density (). Surprisingly, we find that the nematode's
propulsion speed is enhanced by the presence of particles in a fluid and is
nearly independent of area density. The undulation speed, often used to
differentiate locomotion gaits, is significantly affected by the bulk material
properties of wet mono- and polydisperse granular media for .
This difference is characterized by a change in the nematode's waveform from
swimming to crawling in dense polydisperse media \textit{only}. This change
highlights the organism's adaptability to subtle differences in local structure
and response between monodisperse and polydisperse media
Microfluidic rheology of soft colloids above and below jamming
The rheology near jamming of a suspension of soft colloidal spheres is
studied using a custom microfluidic rheometer that provides stress versus
strain rate over many decades. We find non-Newtonian behavior below the jamming
concentration and yield stress behavior above it. The data may be collapsed
onto two branches with critical scaling exponents that agree with expectations
based on Hertzian contacts and viscous drag. These results support the
conclusion that jamming is similar to a critical phase transition, but with
interaction-dependent exponents.Comment: 4 pages, experimen
Running and tumbling with E. coli in polymeric solutions
Run-and-tumble motility is widely used by swimming microorganisms including
numerous prokaryotic eukaryotic organisms. Here, we experimentally investigate
the run-and-tumble dynamics of the bacterium E. coli in polymeric solutions. We
find that even small amounts of polymer in solution can drastically change E.
coli dynamics: cells tumble less and their velocity increases, leading to an
enhancement in cell translational diffusion and a sudden decline in rotational
diffusion. We show that suppression of tumbling is due to fluid viscosity while
the enhancement in swimming speed is mainly due to fluid elasticity.
Visualization of single fluorescently labeled DNA polymers reveals that the
flow generated by individual E. coli is sufficiently strong to stretch polymer
molecules and induce elastic stresses in the fluid, which in turn can act on
the cell in such a way to enhance its transport. Our results show that the
transport and spread of chemotactic cells can be independently modified and
controlled by the fluid material properties
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