Effective interactions between particles immersed in a fluid have been
the subject of extensive investigations in different regimes: from the
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. 
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
To achieve this goal, we developed a novel density functional technique
based on the weighted density concept . Coupling this approach with the
hierarchical reference theory of fluids (HRT) , 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 .
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