Dynamics and mechanism of a deswelling transition of the sponge phase in a bilayer membrane system

Abstract

In surfactant solutions, membranes are self-organized and divide water into two domains, an “inside” and an “outside.” When the membrane phase transforms into a dilute sponge (L_{1}) phase, the L_{1} phase is also partitioned into inside and outside domains. Such systems are critical not only for material exchange mechanisms in biology but also for the formation of boundaries between topologically distinct regions. Nevertheless, the mechanism and associated behavior of the system at the transition remain unclear. Here, we use both experimental observations and numerical simulations to investigate the phase transition dynamics. It is found that domains of inside and outside are formed which appear connected to each other and that they nucleate as droplets despite the L_{1} phase being the majority phase. Although such behavior is significantly different from what is seen in ordinary phase separation, we successfully reproduced all typical behavior seen in the experiments with numerical simulations. For the formation of the connected domains, it is found that the correlation length associated with the concentration needs to be larger than that of the inside/outside patterns. This offers a means to control pattern formation in systems with a parity mechanism, such as chirality.