  We study the orientational and translational dynamics of spherical
microswimmers trapped at fluid interfaces, in terms of the force dipole and
source dipole components of their flow field. Using numerical simulations and
analytical calculations, we show that the force dipole exerts a torque,
orienting pushers parallel to the interface, and pullers in normal direction.
The source dipole results in particle rotation only for a finite viscosity
contrast between the two fluids, in agreement with previous studies. The
superposition of these two contributions leads to an rotational dynamics with a
steady-state orientation that depends on the relative magnitudes of the force
and source dipoles. In the general case, swimmers with weak force dipoles and
strong pullers are observed to align perpendicular to the interface and become
stationary, while strong pushers have a finite inclination angle towards the
lower viscosity fluid and swim along the interface.

Soft Condensed Matter;Fluid Dynamics