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Issue 38, 2020
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Quantifying active diffusion in an agitated fluid

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Mixing of reactants in microdroplets predominantly relies on diffusional motion due to small Reynolds numbers and the resulting absence of turbulent flows. Enhancing diffusion in microdroplets by an auxiliary noise source is therefore a topical problem. Here we report on how the diffusional motion of tracer beads is enhanced upon agitating the surrounding aqueous fluid with miniaturized magnetic stir bars that are compatible with microdroplets and microfluidic devices. Using single-particle tracking, we demonstrate via a broad palette of measures that local stirring of the fluid at different frequencies leads to an enhanced but apparently normal and homogenous diffusion process, i.e. diffusional steps follow the anticipated Gaussian distribution and no ballistic motion is observed whereas diffusion coefficients are significantly increased. The signature of stirring is, however, visible in the power-spectral density and in the velocity autocorrelation function of trajectories. Our data therefore demonstrate that diffusive mixing can be locally enhanced with miniaturized stir bars while only moderately affecting the ambient noise properties. The latter may also facilitate the controlled addition of nonequilibrium noise to complex fluids in future applications.

Graphical abstract: Quantifying active diffusion in an agitated fluid

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Supplementary files

Article information

07 Jul 2020
11 Sep 2020
First published
11 Sep 2020

Phys. Chem. Chem. Phys., 2020,22, 21678-21684
Article type

Quantifying active diffusion in an agitated fluid

P. Gires, M. Thampi and M. Weiss, Phys. Chem. Chem. Phys., 2020, 22, 21678
DOI: 10.1039/D0CP03629C

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