The confinement-induced sponge to lamellar phase transition

The behavior of an isotropic fluid near its first-order phase transition to an ordered phase is investigated when confined between two macroscopic wal ls. The sponge (L-3) phase of the system sodium bis(2-ethylhexyl)sulfosucci nate/brine was used as the isotropic phase since it displays a bulk first-o rder transformation to the lamellar (L-alpha) phase upon reaching the trans ition temperature T-L alpha. When confined, measurement of the interactions between the two confining walls with a surface force apparatus showed that over a limited temperature range below T-L alpha the confined L-3 phase tr ansformed to the L-alpha phase at a specific wall separation. This confinem ent-induced transformation was signaled by the existence of two distinct os cillatory force regimes. Structural characteristics of either the L-3 or th e L-alpha phase were observed in the oscillatory profiles including the cha racteristic length or reticular spacing, the nature and topology of structu ral defects, the order parameter correlation length (for the L-3 phase), an d the elastic compressibility modulus (for the L-alpha phase). The force-di stance profiles also allowed the interfacial tension of the L-3/L-alpha int erface to be evaluated.