Influence of miscible viscous fingering with negative log-mobility ratio on spreading of adsorbed analytes

Viscous fingering (VF) between miscible fluids of different viscosities can affect the dispersion of localized samples in porous media. This is the case when a given fluid displaces a finite width sample consisting in a solvent of different viscosity and containing a dissolved analyte undergoing adsorption on the porous matrix. We investigate here numerically the influence of VF between the carrier fluid and the sample solvent on the spreading of a retained analyte concentration peak in a three-component system (displacing fluid, sample solvent, analyte). We compare the cases where the sample solvent is, respectively, more or less viscous than the displacing fluid or eluent by varying the log-mobility ratio R=ln(μ2/μ1)R=ln(μ2/μ1) where μ1μ1 and μ2μ2 are the viscosities of the eluent and sample solvent, respectively. We show that disentanglement of the analyte concentration peak from the fingering zone occurs earlier for less viscous samples, i.e. for R<0R<0 than for R>0R>0. Qualitative agreement with experimental evidences obtained in reversed phase liquid chromatography are shown.