Transition to irreversibility for the dispersion of a tracer in porous media

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Abstract
The transition from reversibility to irreversibility for tracer dispersion in an unconsolidated porous medium made of 2 and 4 mm diam glass beads has been studied; a low dead volume electrochemical technique has been used to produce and detect the tracer. It is shown that the transition occurs over a small number of layers (ten) at the inlet of the sample; this distance depends only on the ratio of the penetration distance and the bead diameter and varies very little with the Peclet number. For large penetration distances, the dispersion length values obtained in classical transmission dispersion measurements are recovered (dispersion has become fully irreversible). At very small distances, dispersion is largely reversible and dispersion coefficient values 20 or 80 times smaller than the asymptotic value have been measured.

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