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Chemically-powered swimming and diffusion in the microscopic world


The past decade has seen intriguing reports and heated debates concerning the chemically-driven enhanced motion of objects ranging from small molecules to millimetre-size synthetic robots. These objects, in solutions in which chemical reactions were occurring, were observed to diffuse (spread non-directionally) or swim (move directionally) at rates exceeding those expected from Brownian motion alone. The debates have focused on whether observed enhancement is an experimental artefact or a real phenomenon. If the latter were true, then we would also need to explain how the chemical energy is converted into mechanical work. In this Perspective, we summarize and discuss recent observations and theories of active diffusion and swimming. Notably, the chemomechanical coupling and magnitude of diffusion enhancement are strongly size-dependent and should vanish as the size of the swimmers approaches the molecular scale. We evaluate the reliability of common techniques to measure diffusion coefficients and finish by considering the potential applications and chemical to mechanical energy conversion efficiencies of typical nanoswimmers and microswimmers.

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Fig. 1: Passive and active motion of a particle.
Fig. 2: Self-propulsion mechanisms of enhanced diffusion.
Fig. 3: Enhanced diffusion of a particle under catalytic and non-catalytic conditions.
Fig. 4: Reported example of boosted molecular diffusion during azide–alkyne cycloaddition.
Fig. 5: Enhanced transport of tracers.
Fig. 6: Efficiencies of typical microscopic motors.


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Y.Z. acknowledges start-up funds from Beijing Advanced Innovation Center for Soft Matter Science and Engineering at Beijing University of Chemical Technology (BAIC202103). H.H. acknowledges financial support from National Science Foundation Division of Materials Research (NSF-DMR) grant 1807514.

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Correspondence to Yifei Zhang or Henry Hess.

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Zhang, Y., Hess, H. Chemically-powered swimming and diffusion in the microscopic world. Nat Rev Chem 5, 500–510 (2021).

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