  Within mean-field theory we determine the universal scaling function for the
effective force acting on a single colloid located near the interface between
two coexisting liquid phases of a binary liquid mixture close to its critical
consolute point. This is the first study of critical Casimir forces emerging
from the confinement of a fluctuating medium by at least one soft interface,
instead by rigid walls only as studied previously. For this specific system,
our semi-analytical calculation illustrates that knowledge of the
colloid-induced, deformed shape of the interface allows one to accurately
describe the effective interaction potential between the colloid and the
interface. Moreover, our analysis demonstrates that the critical Casimir force
involving a deformable interface is accurately described by a universal scaling
function, the shape of which differs from that one for rigid walls.
