Experimental study of miscible viscous fingering with different effective interfacial tension

Viscous fingering (VF) occurs when a more viscous fluid is displaced by a less viscous one in porous media or in Hele-Shaw cells. Generally, VF can be divided into two types: immiscible VF and miscible VF. The typical immiscible finger is wider than the equivalent miscible finger because of interfacial tension. Recently, it has been pointed out that an effective interfacial tension (EIT) is present even in miscible systems when there is a steep concentration gradient of chemical species at the interface. The effects of EIT on miscible VF have so far mainly been studied numerically showing that the fingers become wider owing to EIT. Here, we perform an experimental investigation of the effects of EIT on miscible VF by establishing two solution systems that have different concentration differences but the same viscosity contrast. One is a glycerol solution with a concentration of 99 wt. % and water and has a higher water concentration difference of ∆Cw = 99 wt. %. The other is a polymer solution with a concentration of 8.5 wt. % and water and has a lower water concentration difference of ∆Cw = 8.5 wt. %. We show by direct measurement with a spinning drop tensiometer that the glycerol–water system exhibits greater EIT, and we demonstrate experimentally that typical fingers with high EIT become wider than those with low EIT. We suggest that under the experimental condition employed, the VF in the glycerol–water system with high EIT exhibits a characteristic property of immiscible VF, although it has generally been regarded as a typical representative of classical miscible VF.