Frequency-dependent critical diffusion in a classical fluid

The frequency dependence of diffusion in a classical fluid is a consequence of the frequency dependence of the viscosity and leads to a small but observable deviation from a pure exponential decay of the order-parameter fluctuations. Our calculation of the deviation at the critical point gives a good fit to the data of Burstyn, Chang, and Sengers. The theory also agrees with the temperature dependence that they found.