3,356 research outputs found
Turbulent fluid acceleration generates clusters of gyrotactic microorganisms
The motility of microorganisms is often biased by gradients in physical and
chemical properties of their environment, with myriad implications on their
ecology. Here we show that fluid acceleration reorients gyrotactic plankton,
triggering small-scale clustering. We experimentally demonstrate this
phenomenon by studying the distribution of the phytoplankton Chlamydomonas
augustae within a rotating tank and find it to be in good agreement with a new,
generalized model of gyrotaxis. When this model is implemented in a direct
numerical simulation of turbulent flow, we find that fluid acceleration
generates multi-fractal plankton clustering, with faster and more stable cells
producing stronger clustering. By producing accumulations in high-vorticity
regions, this process is fundamen- tally different from clustering by
gravitational acceleration, expanding the range of mechanisms by which
turbulent flows can impact the spatial distribution of active suspensions.Comment: 5 pages, 4 figure
Gyrotactic phytoplankton in laminar and turbulent flows: a dynamical systems approach
Gyrotactic algae are bottom heavy, motile cells whose swimming direction is
determined by a balance between a buoyancy torque directing them upwards and
fluid velocity gradients. Gyrotaxis has, in recent years, become a paradigmatic
model for phytoplankton motility in flows. The essential attractiveness of this
peculiar form of motility is the availability of a mechanistic description
which, despite its simplicity, revealed predictive, rich in phenomenology,
easily complemented to include the effects of shape, feed-back on the fluid and
stochasticity (e.g. in cell orientation). In this review we consider recent
theoretical, numerical and experimental results to discuss how, depending on
flow properties, gyrotaxis can produce inhomogeneous phytoplankton
distributions on a wide range of scales, from millimeters to kilometers, in
both laminar and turbulent flows. In particular, we focus on the phenomenon of
gyrotactic trapping in nonlinear shear flows and in fractal clustering in
turbulent flows. We shall demonstrate the usefulness of ideas and tools
borrowed from dynamical systems theory in explaining and interpreting these
phenomena
Galaxy alignments: An overview
The alignments between galaxies, their underlying matter structures, and the
cosmic web constitute vital ingredients for a comprehensive understanding of
gravity, the nature of matter, and structure formation in the Universe. We
provide an overview on the state of the art in the study of these alignment
processes and their observational signatures, aimed at a non-specialist
audience. The development of the field over the past one hundred years is
briefly reviewed. We also discuss the impact of galaxy alignments on
measurements of weak gravitational lensing, and discuss avenues for making
theoretical and observational progress over the coming decade.Comment: 43 pages excl. references, 16 figures; minor changes to match version
published in Space Science Reviews; part of a topical volume on galaxy
alignments, with companion papers at arXiv:1504.05546 and arXiv:1504.0546
The 'Cheerios effect'
Objects that float at the interface between a liquid and a gas interact
because of interfacial deformation and the effect of gravity. We highlight the
crucial role of buoyancy in this interaction, which, for small particles,
prevails over the capillary suction that is often assumed to be the dominant
effect. We emphasize this point using a simple classroom demonstration, and
then derive the physical conditions leading to mutual attraction or repulsion.
We also quantify the force of interaction in some particular instances and
present a simple dynamical model of this interaction. The results obtained from
this model are then validated by comparison to experimental results for the
mutual attraction of two identical spherical particles. We conclude by looking
at some of the applications of the effect that can be found in the natural and
manmade worlds.Comment: 10 pages, 12 figures. (Typos in eqs 7 and 8 corrected
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