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Diffusion and spatial correlations in suspensions of swimming particles
Patrick T Underhill1, Juan P Hernandez-Ortiz, Michael D Graham
1Department of Chemical and Biological Engineering, University of Wisconsin-Madison, 1415 Engineering Drive, Madison, Wisconsin 53706, USA.
Physical Review Letters
|July 23, 2008
Summary
Microscopic swimmers dramatically enhance particle diffusion through fluid motion. "Pusher" type swimmers, propelled from behind, show greater diffusion enhancement than "puller" types, a finding supported by simulations and theory.
Area of Science:
- Fluid dynamics
- Microbiology
- Statistical mechanics
Background:
- Microorganisms generate fluid flows influencing their environment.
- Enhanced diffusion of passive tracers is observed in active matter suspensions.
- Swimming mechanisms (pusher vs. puller) may differentially affect fluid dynamics.
Purpose of the Study:
- To investigate how different swimming modes of microorganisms affect tracer particle diffusion.
- To quantify the influence of swimmer propulsion type on diffusion enhancement.
- To explore the relationship between swimmer behavior, fluid motion, and tracer diffusivity.
Main Methods:
- Numerical simulations of swimming particle suspensions in a periodic domain.
- Analysis of tracer particle trajectories to determine diffusivity.
- Comparison of diffusion enhancement for 'pusher' and 'puller' swimmer models.
- Development of a mean field theory to explain observed phenomena.
Main Results:
- Swimming microorganisms significantly enhance the diffusion of tracer particles.
- Swimmers propelled from behind ('pushers') exhibit greater diffusion enhancement than those pulled from the front ('pullers').
- Diffusivity and velocity correlation length increase with domain size specifically for 'pusher' simulations.
- A physical argument and mean field theory explain these observed differences.
Conclusions:
- The mode of swimming is a critical factor in determining diffusion enhancement in micro-organism suspensions.
- 'Pusher' swimmers induce larger-scale fluid motions leading to enhanced tracer diffusion compared to 'pullers'.
- These findings provide insights into the collective behavior of active matter and its impact on transport phenomena.
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