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It is known that effective forces can deeply modify the phase diagram of colloidal particles. This happens, for instance, in the presence of depletion forces, i.e. effective forces that originate from the presence of a co-solute (polymers, surfactants) in the suspension.
Beside short-range depletion, also long-range effective interactions can be exploited for controlling the behaviour of a colloidal solution. The most famous example is that of the critical Casimir force[1], which arises when two colloids are immersed in a solvent close to its critical point. In such a case, the confinement of the density critical fluctuations between the colloids surfaces give rise to a long-range force[2], that can be controlled through a tiny variation of the temperature close to the critical point. In this talk I will discuss the results of numerical studies of the effective potential $V_{eff}$ between two hard-sphere colloids dispersed in an implicit solvent in the presence of interacting depletant particles approaching the depletant gas-liquid critical point. We find that $V_{eff}$ decays exponentially at long distances, with a characteristic decay length compatible with the bulk critical correlation length. By investigating the stability of the particles interacting via $V_{eff}$, the locus of colloidal aggregation in the phase-diagram of the depletant is evaluated, assessing under which conditions critical Casimir forces can be used to manipulate colloidal aggregation[3,4]. In addition, the variation of colloid-solute interactions allows to tune the effective long-range potential from attractive to repulsive[5], in agreement with theoretical calculations and experimental results. Finally, building on the analogy between critical phenomena and the percolation transition[6], I will also discuss a Casimir-like potential between colloidal particles dispersed in a solvent close to percolation. In this case, the range of the effective interaction is controlled by the connectivity length of the solvent and diverges at the percolation transition[7]. These results provide the geometric analogue of the critical Casimir force, opening the way for a fine tuning of colloidal interactions by controlling also the clustering properties of the solvent. [1] M.E. Fisher, P.G. de Gennes. C. R. Acad. Sc. Paris B 287, 207 (1978) [2] C. Hertlein et al., Nature 451, 172 (2008); A.Gambassi et al, Phys. Rev. E 80, 061143 (2009) [3] S. Buzzaccaro, J. Colombo, A. Parola, R. Piazza, Phys. Rev. Lett. 105, 198301 (2010) [4] N. Gnan, E. Zaccarelli, P. Tartaglia, F. Sciortino, Soft Matter 8, 1991 (2012) [5] N. Gnan, E. Zaccarelli, F. Sciortino, J. Chem. Phys. 137, 084903 (2012) [6] M. Daoud, A. Coniglio, J. Phys. A: Math. Gen. 14, L301 (1981) [7] N. Gnan, E. Zaccarelli, F. Sciortino, Nature Comm. 5, 3267 (2014) |