We report a numerical investigation of two colloids immersed in a critical
solvent, with the aim of quantifying the effective colloid-colloid interaction
potential. By turning on an attraction between the colloid and the solvent
particles we follow the evolution from the case in which the solvent density
close to the colloids changes from values smaller than the bulk to values
larger than the bulk. We thus effectively implement the so-called (+,+) and
(−,−) boundary conditions defined in field theoretical approaches focused on
the description of critical Casimir forces. We find that the effective
potential at large distances decays exponentially, with a characteristic decay
length compatible with the bulk critical correlation length, in full agreement
with theoretical predictions. We also investigate the case of (+,−) boundary
condition, where the effective potential becomes repulsive. Our study provides
a guidance for a design of the interaction potential which can be exploited to
control the stability of colloidal systems
