Symmetric wetting heterogeneity suppresses fluid displacement hysteresis in granular piles

TL;DR

This study experimentally shows that symmetric wettability heterogeneity in granular media reduces fluid displacement hysteresis, contrasting with geometric heterogeneity, which does not have this effect, revealing new insights into porous media behavior.

Contribution

It demonstrates that symmetric wettability heterogeneity suppresses hysteresis in fluid invasion, a novel finding contrasting with the effects of geometric heterogeneity.

Findings

Symmetric wettability heterogeneity reduces capillary pressure hysteresis.

Geometric heterogeneity does not significantly affect hysteresis.

Qualitative explanation based on geometric arguments supports the findings.

Abstract

We investigate experimentally the impact of heterogeneity on the capillary pressure hysteresis in fluid invasion of model porous media. We focus on `symmetric' heterogeneity, where the contact angles the fluid interface makes with the oil-wet ($\theta_1$) and the water-wet ($\theta_2$) beads add up to $\pi$. While enhanced heterogeneity is usually known to increase hysteresis phenomena, we find that hysteresis is greatly reduced when heterogeneities in wettability are introduced. On the contrary, geometric heterogeneity (like bi-disperse particle size) does not lead to such effect. We provide a qualitative explanation of this surprising result, resting on rather general geometric arguments.