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Issue 8, 2016
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Particle diffusion in active fluids is non-monotonic in size

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We experimentally investigate the effect of particle size on the motion of passive polystyrene spheres in suspensions of Escherichia coli. Using particles covering a range of sizes from 0.6 to 39 microns, we probe particle dynamics at both short and long time scales. In all cases, the particles exhibit super-diffusive ballistic behavior at short times before eventually transitioning to diffusive behavior. Surprisingly, we find a regime in which larger particles can diffuse faster than smaller particles: the particle long-time effective diffusivity exhibits a peak in particle size, which is a deviation from classical thermal diffusion. We also find that the active contribution to particle diffusion is controlled by a dimensionless parameter, the Péclet number. A minimal model qualitatively explains the existence of the effective diffusivity peak and its dependence on bacterial concentration. Our results have broad implications on characterizing active fluids using concepts drawn from classical thermodynamics.

Graphical abstract: Particle diffusion in active fluids is non-monotonic in size

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The article was received on 14 Nov 2015, accepted on 14 Jan 2016 and first published on 21 Jan 2016

Article type: Paper
DOI: 10.1039/C5SM02800K
Author version available: Download Author version (PDF)
Citation: Soft Matter, 2016,12, 2365-2372
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    Particle diffusion in active fluids is non-monotonic in size

    A. E. Patteson, A. Gopinath, P. K. Purohit and P. E. Arratia, Soft Matter, 2016, 12, 2365
    DOI: 10.1039/C5SM02800K

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