From capillary condensation to interface localization transitions in colloid-polymer mixtures confined in thin-film geometry

Abstract

Monte Carlo simulations of the Asakura-Oosawa (AO) model for colloid-polymer mixtures confined between two parallel repulsive structureless walls are presented and analyzed in the light of current theories on capillary condensation and interface localization transitions. Choosing a polymer to colloid size ratio of q = 0.8 and studying ultrathin films in the range of D = 3 to D = 10 colloid diameters thickness, grand canonical Monte Carlo methods are used; phase transitions are analyzed via finite size scaling, as in previous work on bulk systems and under confinement between identical types of walls. Unlike the latter work, inequivalent walls are used here: while the left wall has a hard-core repulsion for both polymers and colloids, at the right wall an additional square-well repulsion of variable strength acting only on the colloids is present. We study how the phase separation into colloid-rich and colloid-poor phases occurring already in the bulk is modified by such a confinement. When the asymmetry of the wall-colloid interaction increases, the character of the transition smoothly changes from capillary condensation-type to interface localization-type. The critical behavior of these transitions is discussed, as well as the colloid and polymer density profiles across the film in the various phases, and the correlation of interfacial fluctuations in the direction parallel to the confining walls.The experimental observability of these phenomena also is briefly discussed.

Confocal Imaging of Confined Quiescent and Flowing Colloid-polymer Mixtures

“…Second, particle trajectories obtained from the confocal micrographs can be used to calculate dynamic and structural metrics for individual micron-sized colloidal particles. These measurements facilitate direct comparison with simulations 21,22 . Other image analysis algorithms, such as image correlation 37 based on particle image velocimetry 38 , can be applied to confocal micrographs in which the particle displacements are larger than the interparticle spacing to probe the dynamics.…”

“…For these fundamental studies, understanding the effect of interparticle attractions on confined phase behavior offers the opportunity to explore phenomena such as capillary condensation and evaporation 21,22,67 . In addition, confined attractive suspensions appear ubiquitously in industrial and technological applications.…”

“…Techniques used to probe the structure of micron-sized colloids in confined geometries, including electron microscopy 17,18 , x-ray microscopy 19 , and laser-diffraction microscopy 20 , can be used to measure the structure and dynamics of particles on the microscale. These techniques, however, do not allow access to the trajectories of individual particles, from which structural and dynamic metrics can be computed for direct comparison to numerical simulations 21,22 .…”

Phase transitions of fluids in heterogeneous pores

“…For L finite, the binding potential is non-vanishing in the middle of the pore which makes the delocalized state less stable. The grand potential per unit area for the localized state is given by 20) where loc is the minimum of the binding potential near the (left-hand) wall. In order to describe the binding potential for the case of first-order wetting (see figure 3), we should consider one more term in (2.3)…”

“…Secondly, ubiquity of dispersion forces in fluid systems is an important extra ingredient to be considered which, in fact, prevents complete drying. The latter problem can be avoided by considering binary (colloid-polymer) mixtures such as in references [18][19][20]. The long-range dispersion interaction was included in reference [21] by Stewart and Evans for a simple, one-component fluid; in this study, extensive density functional theory (DFT) calculations have been made to confirm scaling predictions for several 13604-2 thermodynamic quantities.…”