ABSTRACT
This chapter describes two-dimensional models in which a pair of pins moves, or alternatively a single pin moves, whilst other pins remain fixed. These models demonstrate changes of film pattern that are analogous to first-order transitions, as demonstrated by liquid-vapour and solid-liquid changes, and second-order phase transitions, as typified by the order-disorder phase transition in β-brass. Two-dimensional models are useful for finding the actual variation of soap-film energy as we alter the relative positions of the pins. The chapter considers a model consisting of four pins, A, B, C and D, arranged at the corners of a rectangle. In the 3-pin soap-film model, when the movable pin C moves from inside to outside the arc ADB, there is no overall change in the total symmetry of the complete soap-film system. Just as the soap film attempts to minimize its energy, a thermodynamic system tends towards that state where its free energy is minimum.
