  The influence of a fluid-fluid interface on self-phoresis of chemically
active, axially symmetric, spherical colloids is analyzed. Distinct from the
studies of self-phoresis for colloids trapped at fluid interfaces or in the
vicinity of hard walls, here we focus on the issue of self-phoresis close to a
fluid-fluid interface. In order to provide physically intuitive results
highlighting the role played by the interface, the analysis is carried out for
the case that the symmetry axis of the colloid is normal to the interface;
moreover, thermal fluctuations are not taken into account. Similarly to what
has been observed near hard walls, we find that such colloids can be set into
motion even if their whole surface is homogeneously active. This is due to the
anisotropy along the direction normal to the interface owing to the
partitioning by diffusion, among the coexisting fluid phases, of the product of
the chemical reaction taking place at the colloid surface. Different from
results corresponding to hard walls, in the case of a fluid interface the
direction of motion, i.e., towards the interface or away from it, can be
controlled by tuning the physical properties of one of the two fluid phases.
This effect is analyzed qualitatively and quantitatively, both by resorting to
a far-field approximation and via an exact, analytical calculation which
provides the means for a critical assessment of the approximate analysis.
