Pietro Anzini  — Università degli Studi dell'Insubria - Como #
Solvent mediated forces in critical fluids #
Effective interactions between particles immersed in a fluid have been the subject of extensive investigations in different regimes: from th Asakura-Oosawa short range attraction present in the ideal gas limit, to the oscillatory behavior induced by repulsive forces. These depletion interactions represent a particular limit of the more general concept of solvent mediated forces, which are known to drive several important phenomena in
soft matter, leading either to clustering, colloidal aggregation or dynamical arrest.
Solvent mediated forces undergo a significant change when long range correlations are present in the host fluid due to the proximity of a second order phase transition. The universal properties of critical phenomena reflect in the structure of the effective interactions which acquire a scaling form. [1]
The transition between the depletion and the critical Casimir regimes is a particularly challenging problem from the theoretical point of view because it requires the accurate description of inhomogeneous critical fluids at a microscopic level.
To achieve this goal, we developed a novel density functional technique based on the weighted density concept [2]. Coupling this approach with the hierarchical reference theory of fluids (HRT) [3], which provides a microscopic description of fluids accurate also in the critical region, we performed a detailed investigation of the effective interactions between two hard walls immersed in a Yukawa fluid by varying the thermodynamic state.
The evolution of the effective potential as a function of temperature and density is discussed, emphasizing the smooth transition between the high temperature, entropy-dominated, limit and the critical regime. Furthermore this approach allows a direct investigation of the universal properties both in the critical and in the pre-critical regime and these results are compared with predictions obtained by numerical simulations [4].
References:
[1] M. E. Fisher and P. G. De Gennes, Comptes rendus de l’Academie des sciences, Serie B 287, 207 (1978).
[2] P. Anzini and A. Parola, Physical Review E 94, 052113 (2016).
[3] A. Parola and L. Reatto, Advances in Physics 44, 211 (1995).
[4] N. Gnan, E. Zaccarelli, P. Tartaglia, and F. Sciortino, Soft Matter 8, 1991 (2012).