Liquid-Liquid Demonstrations: Spinodal Decomposition

Under most conditions, a liquid mixture does not have long-range structural order and may be considered to be a homogeneous collection of molecules in random motion. However, if a single-phase critical mixture of a partially miscible liquid-liquid system is subjected to a small temperature jump into the two-phase region, then structural coherence across macroscopic distances can form in the liquid. The process is called spinodal decomposition and results because the temperature jump has made the entire sample thermodynamically unstable. This process is fundamentally different from the more typical phase-separation mechanism of nucleation and growth, which involves localized droplet formation. Here, a benchtop setup is described for showing students a critical isobutyric acid + water mixture undergoing spinodal decomposition using only ambient cooling and a low-power laser. When structural order within the phase-separating sample reaches the micron scale, students see a diffraction ring pattern formed by the transmitted laser. The pattern persists for up to 30 s, and students can use Bragg's law to estimate the size of structural order within the liquid mixture. The critical temperature of the isobutyric acid + water sample is just above room temperature, so the experiment is easily repeated. Preparation of a four-phase immiscible liquid mixture is also described; when this mixture is vigorously shaken, the resulting emulsion separates in a manner visually analogous to spinodal decomposition.