Attractive interaction and bridging transition between neutral colloidal particles due to preferential adsorption in a near-critical binary mixture

We examine the solvent-mediated interaction between two neutral colloidal particles due to preferential adsorption in a near-critical binary mixture. We take into account the renormalization effect due to the critical fluctuations using the recent local functional theory. We calculate the free energy and the force between two colloidal particles as functions of the temperature T, the composition far from the colloidal particles c_∞, and the colloid separation ℓ. The interaction is much enhanced when the component favored by the colloid surfaces is poor in the reservoir. For such off-critical compositions, we find a surface of a first-order bridging transition ℓ=ℓ_cx(T,c_∞) in the T-c_∞- ℓ space in a universal, scaled form, across which a discontinuous change occurs between separated and bridged states. This surface starts from the bulk coexistence surface (CX) and ends at a bridging critical line where ℓ is determined by T as ℓ=ℓ_c(T). On approaching the critical line, the discontinuity vanishes and the derivatives of the force with respect to T and ℓ both diverge. Furthermore, bridged states continuously change into separated states if c_∞ (or T) is varied from a value on CX to a value far from CX with ℓ kept smaller than ℓ_c(T).