Confinement Effects on Phase Behavior of Soft Matter Systems

K. BINDER; J. HORBACH; R. L. C. VINK; A. DE VIRGILIIS

Soft Matter

RSC Publishing

Año: 2008 vol. 4 p. 1555 - 1568

When systems that can undergo phase separationbetween two coexisting phases in thebulk are confined in thin film geometry betweenparallel walls, the phase behavior can be profoundlymodified. These phenomena shall bedescribed and exemplified by computer simulationsof the Asakura-Oosawa model for colloidpolymermixtures, but applications to other softmatter systems (e.g. confined polymer blends)will also be mentioned. Typically a wall willprefer one of the phases, and hence the compositionof the system in the direction perpendicularto the walls will not be homogeneous. If bothwalls are of the same kind, this effect leads to adistortion of the phase diagram of the system inthin film geometry, in comparison with the bulk,analogous to the phenomenon of “capillary condensation”of simple fluids in thin capillaries. Inthe case of “competing walls”, where both wallsprefer different phases of the two phases coexistingin the bulk, a state with an interface parallelto the walls gets stabilized. The transition fromthe disordered phase to this “soft mode phase”is rounded by the finite thickness of the film andnot a sharp phase transition. However, a sharptransition can occur where this interface getslocalized at (one of) the walls. The relation ofthis interface localization transition to wettingphenomena is discussed. Finally, an outlook torelated phenomena is given, such as the effectsof confinement in cylindrical pores on the phasebehavior, and more complicated ordering phenomena(lamellar mesophases of block copolymersor nematic phases of liquid crystals underconfinement).