Hydrodynamic Fluctuations in Liquids Observed by NMR-Modulated Gradient Spin-Echo Method

The modulated gradient spin echo is an NMR method that provides direct insight into the low-frequency part of the molecular velocity auto-correlation spectra in fluids. Because the method gives a spectrum that is time-averaged over the trajectory elapsed, the spins are able to observe local inhomogeneities in the initial interval after spin excitation. In fluid measurements, it manifests as an initial non-exponential decay of the spin-echo signals, which we attribute to the spatial heterogeneity of molecular self-diffusion due to the molecular motion in microvortices of hydrodynamic oscillations. The hydrodynamic fluctuations occur in water, ethanol, toluene, and a mixture of water with lower glycerol content, while they disappeared with increasing glycerol content.