Stokes-Einstein diffusion of critical fluctuations in a fluid

The diffusion coefficient associated with the order-parameter fluctuations near the critical point of fluids is expected to vanish asymptotically as $D=\frac{\mathrm{RkT}}{6\ensuremath{\pi}\ensuremath{\eta}\ensuremath{\xi}}$, where $\ensuremath{\eta}$ is the viscosity, $\ensuremath{\xi}$ the correlation length, and $R$ a universal constant. Our experiments, using 3-methylpentane-nitroethane, yield $R=1.02\ifmmode\pm\else\textpm\fi{}0.06$, in agreement with the mode-coupling theory of critical fluctuations, but in disagreement with the value $\frac{R}{6\ensuremath{\pi}}=\frac{1.2}{6\ensuremath{\pi}}\ensuremath{\simeq}\frac{1}{5\ensuremath{\pi}}$, recently suggested by several investigators.