https://doi.org/10.1140/epje/s10189-021-00026-9
Regular Article - Soft Matter
Shaping the gradients driving phoretic micro-swimmers: influence of swimming speed, budget of carbonic acid and environment
1
Institute of Condensed Matter Physics, Johannes Gutenberg Universität, Staudinger Weg 7, 55128, Mainz, Germany
2
Max Planck Graduade Center, Institute of Physics, Johannes Gutenberg Universität, Staudinger Weg 7, 55128, Mainz, Germany
3
Institute for Condensed Matter Physics, Technische Universität Darmstadt, Hochschulstr. 8, 64289, Darmstadt, Germany
Received:
5
October
2020
Accepted:
22
January
2021
Published online:
23
March
2021
pH gradient-driven modular micro-swimmers are investigated as a model for a large variety of quasi-two-dimensional chemi-phoretic self-propelled entities. Using three-channel micro-photometry, we obtain a precise large field mapping of pH at a spatial resolution of a few microns and a pH resolution of 
units for swimmers of different velocities propelling on two differently charged substrates. We model our results in terms of solutions of the three-dimensional advection–diffusion equation for a 1:1 electrolyte, i.e. carbonic acid, which is produced by ion exchange and consumed by equilibration with dissolved 
. We demonstrate the dependence of gradient shape and steepness on swimmer speed, diffusivity of chemicals, as well as the fuel budget. Moreover, we experimentally observe a subtle, but significant feedback of the swimmer’s immediate environment in terms of a substrate charge-mediated solvent convection. We discuss our findings in view of different recent results from other micro-fluidic or active matter investigations. We anticipate that they are relevant for quantitative modelling and targeted applications of diffusio-phoretic flows in general and artificial micro-swimmers in particular.
© The Author(s) 2021
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