FORMATION, EVOLUTION, AND RHEOLOGY OF 2-DIMENSIONAL FOAMS IN SPREAD MONOLAYERS AT THE AIR-WATER-INTERFACE

We discuss the properties of two-dimensional (2-D) foams formed in the gas-liquid coexistence region of spread monolayers on the basis of observations on a surface-active fluorescent dye. Their appearance and general behavior are analogous to those of three-dimensional (3-D) foams. The line tension between the gas and liquid 2-D phase regions plays a role similar to that of the surface tension in conventional foams. It determines the thinning of the strips of liquid monolayer phase which separate the gas cells in the 2-D polygonal surface structure: a lower surface pressure in the corner regions causes the equivalent of the Plateau-border suction. The rate of strip thinning can be slowed down by several orders if a gradient in line tension opposes monolayer flow. Such a gradient presupposes the existence of the unidimensional equivalent of surface activity, for which there is so far only circumstantial evidence. The line tension also controls cell disproportionation, i.e., the growth of large cells at the expense of smaller ones, and "line activity" can also reduce the rate of this process. Finally, the rearrangement process, in which contact between four strips changes into two three-strip contacts, is based on the minimization of the total line energy. It was observed that this rearrangement actually occursbefore a small central cell has disappeared to cause the four-strip contact. This phenomenon was shown to be theoretically expected. © 1991 Academic Press, Inc. All rights reserved.