Contact Angle of the Colloidal Liquid-Gas Interface and a Hard Wall

Abstract

We consider the Asakura-Oosawa-Vrij model of hard sphere colloids and ideal polymer coils in contact with a planar hard wall at (colloidal) liquid-gas coexistence. Using extensive numerical density functional calculations, the liquid-gas, wall-liquid and wall-gas interfacial free energies are calculated. The results are inserted into Young's equation to obtain the contact angle between the liquid-gas interface and the wall. As a function of polymer fugacity this angle exhibits discontinuities of slope (``kinks'') upon crossing first-order surface phase transitions located on the gas branch of the bulk binodal. Each kink corresponds to a transition from n-1 to $n$ colloid layers adsorbed at the wall, referred to as the n'th layering transition. The corresponding adsorption spinodal points from n-1 to n layers upon reducing the polymer fugacity along the bulk binodal were found in a previous study (J. M. Brader et al. J. Phys.: Cond. Matt., 14: L1, 2002; Mol. Phys., 101: 3349, 2003). Remarkably, we find desorption spinodal points from n to n-1 layers to be absent upon increasing polymer fugacity at bulk coexistence, and many branches (containing up to 7 colloid layers) remain metastable. Results for the first layering binodal and both spinodal branches off-bulk coexistence hint at a topology of the surface phase diagram consistent with these findings. Both the order of the transition to complete wetting and whether it is preceded by a finite or an infinite number of layering transitions remain open questions. We compare the locations of the first layering binodal line and of the second layering binodal point at bulk coexistence with recent computer simulation results by Dijkstra and van Roij (Phys. Rev. Lett., 89: 208303, 2002) and discuss our results for the contact angle in the light of recent experiments.