  We present a realization of a fast interfacial Marangoni microswimmer by a
half-spherical alginate capsule at the air-water interface, which diffusively
releases water-soluble spreading molecules (weak surfactants such as
polyethylene glycol (PEG)), which act as "fuel" by modulating the air-water
interfacial tension. For a number of different fuels, we can observe symmetry
breaking and spontaneous propulsion although the alginate particle and emission
are isotropic. The propulsion mechanism is similar to soap or camphor boats,
which are, however, typically asymmetric in shape or emission to select a
swimming direction. We develop a theory of Marangoni boat propulsion starting
from low Reynolds numbers by analyzing the coupled problems of surfactant
diffusion and advection and fluid flow, which includes surfactant-induced fluid
Marangoni flow, and surfactant adsorption at the air-water interface; we also
include a possible evaporation of surfactant. The swimming velocity is
determined by the balance of drag and Marangoni forces. We show that
spontaneous symmetry breaking resulting in propulsion is possible above a
critical dimensionless surfactant emission rate (Peclet number). We derive the
relation between Peclet number and swimming speed and generalize to higher
Reynolds numbers utilizing the concept of the Nusselt number. The theory
explains the observed swimming speeds for PEG-alginate capsules, and we unravel
the differences to other Marangoni boat systems based on camphor, which are
mainly caused by surfactant evaporation from the liquid-air interface. The
capsule Marangoni microswimmers also exhibit surfactant-mediated repulsive
interactions with walls, which can be qualitatively explained by surfactant
accumulation at the wall.
