A binary fluid mixture contains two components with a variable local composition. Temperature, pressure and interactions determine whether a uniform mixture is stable or separates into phases with different compositions. Composition is a conserved quantity in a closed sample even while individual molecules move.
A compositional order parameter can be the local concentration difference. A free energy with two competing minima favors phase separation, while interfacial tension penalizes the boundaries between domains. Conserved order-parameter dynamics describes diffusive redistribution; fluid motion additionally requires a hydrodynamic description such as Model H dynamics. Watching domain growth after a temperature change therefore probes both equilibrium driving forces and the transport processes that move material.
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