A method for assessing the time reversibility of molecular displacements in fluids is presented. The method utilizes pulsed field gradient NMR experiments, in which the flow driving force is inverted during the magnetization lifetime in each measurement cycle. The method is suitable for opaque three-dimensional systems and short displacements, and provides inherent separation between thermal diffusion and displacements driven by externally controlled forces. This approach was applied to study the time reversibility of an electric-field-driven flow of water in natural sand samples, over time scales of up to 0.4 s and displacement scales of the order of one particle diameter. It is demonstrated that the intensity loss of the NMR signal, caused by flow-induced phase dispersion, is fully refocused upon inversion of the polarity of the applied electric field, resulting in flow echoes.
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