146 research outputs found
Hydrodynamic Irreversibility in Particle Suspensions with Non-Uniform Strain
A dynamical phase transition from reversible to irreversible behavior occurs
when particle suspensions are subjected to uniform oscillatory shear, even in
the Stokes flow limit. We consider a more general situation with non-uniform
strain (e.g. oscillatory channel flow), which is observed to exhibit markedly
different dynamics. Self-organization and shear-induced migration only
partially explain the delayed, simultaneous onset of irreversibility across the
channel. The onset of irreversibility is accompanied by long-range correlated
particle motion. This motion leads to particle activity even at the channel
center, where the strain is negligible, and prevents the system from evolving
into a reversible state
Mixing by Swimming Algae
In this fluid dynamics video, we demonstrate the microscale mixing
enhancement of passive tracer particles in suspensions of swimming microalgae,
Chlamydomonas reinhardtii. These biflagellated, single-celled eukaryotes (10
micron diameter) swim with a "breaststroke" pulling motion of their flagella at
speeds of about 100 microns/s and exhibit heterogeneous trajectory shapes.
Fluorescent tracer particles (2 micron diameter) allowed us to quantify the
enhanced mixing caused by the swimmers, which is relevant to suspension feeding
and biogenic mixing. Without swimmers present, tracer particles diffuse slowly
due solely to Brownian motion. As the swimmer concentration is increased, the
probability density functions (PDFs) of tracer displacements develop strong
exponential tails, and the Gaussian core broadens. High-speed imaging (500 Hz)
of tracer-swimmer interactions demonstrates the importance of flagellar beating
in creating oscillatory flows that exceed Brownian motion out to about 5 cell
radii from the swimmers. Finally, we also show evidence of possible cooperative
motion and synchronization between swimming algal cells.Comment: 1 page, APS-DFD 2009 Gallery of Fluid Motio
Oscillatory Flows Induced by Microorganisms Swimming in Two-dimensions
We present the first time-resolved measurements of the oscillatory velocity
field induced by swimming unicellular microorganisms. Confinement of the green
alga C. reinhardtii in stabilized thin liquid films allows simultaneous
tracking of cells and tracer particles. The measured velocity field reveals
complex time-dependent flow structures, and scales inversely with distance. The
instantaneous mechanical power generated by the cells is measured from the
velocity fields and peaks at 15 fW. The dissipation per cycle is more than four
times what steady swimming would require.Comment: 4 pages, 4 figure
Oscillatory Flows Induced by Microoganisms Swimming in Two Dimensions
We present the first time-resolved measurements of the oscillatory velocity field induced by swimming unicellular microorganisms. Confinement of the green alga C. reinhardtii in stabilized thin liquid films allows simultaneous tracking of cells and tracer particles. The measured velocity field reveals complex time-dependent flow structures, and scales inversely with distance. The instantaneous mechanical power generated by the cells is measured from the velocity fields and peaks at 15 fW. The dissipation per cycle is more than 4 times what steady swimming would require
Measuring Oscillatory Velocity Fields Due to Swimming Algae
Single cells exhibit a diverse array of swimming strategies at low Reynolds number to search for nutrients, light, and other organisms. The fluid flows generated by their locomotion are important to understanding biomixing and interactions between cells in suspension..
Direct measurement of the flow field around swimming microorganisms
Swimming microorganisms create flows that influence their mutual interactions
and modify the rheology of their suspensions. While extensively studied
theoretically, these flows have not been measured in detail around any
freely-swimming microorganism. We report such measurements for the microphytes
Volvox carteri and Chlamydomonas reinhardtii. The minute ~0.3% density excess
of V. carteri over water leads to a strongly dominant Stokeslet contribution,
with the widely-assumed stresslet flow only a correction to the subleading
source dipole term. This implies that suspensions of V. carteri have features
similar to suspensions of sedimenting particles. The flow in the region around
C. reinhardtii where significant hydrodynamic interaction is likely to occur
differs qualitatively from a "puller" stresslet, and can be described by a
simple three-Stokeslet model.Comment: 4 pages, 4 figures, accepted for publication in PR
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