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We present a realization of a fast interfacial Marangoni microswimmer by ahalf-spherical alginate capsule at the air-water interface, which diffusivelyreleases water-soluble spreading molecules (weak surfactants such aspolyethylene glycol (PEG)), which act as "fuel" by modulating the air-waterinterfacial tension. For a number of different fuels, we can observe symmetrybreaking and spontaneous propulsion although the alginate particle and emissionare isotropic. The propulsion mechanism is similar to soap or camphor boats,which are, however, typically asymmetric in shape or emission to select aswimming direction. We develop a theory of Marangoni boat propulsion startingfrom low Reynolds numbers by analyzing the coupled problems of surfactantdiffusion and advection and fluid flow, which includes surfactant-induced fluidMarangoni flow, and surfactant adsorption at the air-water interface; we alsoinclude a possible evaporation of surfactant. The swimming velocity isdetermined by the balance of drag and Marangoni forces. We show thatspontaneous symmetry breaking resulting in propulsion is possible above acritical dimensionless surfactant emission rate (Peclet number). We derive therelation between Peclet number and swimming speed and generalize to higherReynolds numbers utilizing the concept of the Nusselt number. The theoryexplains the observed swimming speeds for PEG-alginate capsules, and we unravelthe differences to other Marangoni boat systems based on camphor, which aremainly caused by surfactant evaporation from the liquid-air interface. Thecapsule Marangoni microswimmers also exhibit surfactant-mediated repulsiveinteractions with walls, which can be qualitatively explained by surfactantaccumulation at the wall.

the european physical journal. e, soft matter and biological physics/the european physical journal. e, soft matter

Surfactant-loaded capsules as Marangoni microswimmers at the air-water  interface: Symmetry breaking and spontaneous propulsion by surfactant  diffusion and advection