PERSPECTIVES PHASE TRANSITIONS: Y's and Ends

Coexisting phases of matter are ubiquitous all around us. For example, we are all familiar with the simultaneous presence of water and steam in a tea kettle or of ice and water in the polar regions of Earth. Multiphase systems do not have to consist of a single molecular species; for example, oil and water coexist but do not mix. In all these cases, depending on external constraints such as temperature, pressure, or magnetic field, the two phases may be induced to undergo a phase transition to a single homogeneous phase. In a multicomponent system, this is a more or less intimate mixture of the components. One example for such a mixture is milk, which is a dispersion of small solid particles (milk solids) suspended in water. The phase transition between the homogeneous phase and the two coexisting phases occurs at the thermodynamic "critical point. " The transition is generally the result of a competition between entropy, which favors the homogeneous phase because of the strong disorder inherent in mixing, and short-range attractive forces ("like attracts like") between similar objects, which favor phase separation. Attractive dispersion forces between milk solids lead to phase separation (creaming) in precisely this manner. Vital defects. (Left) Swollen end caps (arrows) of threadlike micelles imaged i