  We derive an analytical expression for the effective force between a pair of
macrospheres immersed in a sea of microspheres, in the case where the
interaction between the two unlike species is assumed to be a square well or a
square shoulder of given range and depth (or height). This formula extends a
similar one developed in the case of hard core interactions only. Qualitative
features of such effective force and the resulting phase diagram are then
analyzed in the limit of no interaction between the small particles.
Approximate force profiles are then obtained by means of integral equation
theories (PY and HNC) combined with the superposition approximation and
compared with exact ones from direct Monte Carlo simulations.
