Abstract
We identify a distinct two-phase flow invasion pattern in a mixed-wet porous medium. Time-resolved high-resolution synchrotron X-ray imaging is used to study the invasion of water through a small rock sample filled with oil, characterized by a wide non-uniform distribution of local contact angles both above and below 90°. The water advances in a connected front, but throats are not invaded in decreasing order of size, as predicted by invasion percolation theory for uniformly hydrophobic systems. Instead, we observe pinning of the three-phase contact between the fluids and the solid, manifested as contact angle hysteresis, which prevents snap-off and interface retraction. In the absence of viscous dissipation, we use an energy balance to find an effective, thermodynamic, contact angle for displacement and show that this angle increases during the displacement. Displacement occurs when the local contact angles overcome the advancing contact angles at a pinned interface: it is wettability which controls the filling sequence. The product of the principal interfacial curvatures, the Gaussian curvature, is negative, implying well-connected phases which is consistent with pinning at the contact line while providing a topological explanation for the high displacement efficiencies in mixed-wet media.
Highlights
If the angle that the interface between two fluids forms with a solid surface, is both lower and higher than 90◦, meaning that there is a mix2020 The Authors
Inside the 3D mixed-wet medium studied, the filling order is controlled by the thermodynamic contact angle [41], which represents the energetic threshold to be overcome for depinning of the interface for displacement
We have studied two-phase flow invasion patterns in a mixed-wet porous medium, using dynamic high-resolution X-ray synchrotron imaging
Summary
Of hydrophobic and hydrophilic regions, a material is defined as being mixed-wet [1]. Flow in mixed-wet porous media is omnipresent in nature [2]. Lotus and rice leaves [3,4], butterfly wings and gecko feet [5,6], and human skin [7] are natural porous systems which are not wetted by water and show different grades of mixed wettability (or hydrophobicity). Mixed wettability is studied for several applications: in the fabric industry, the wettability of textiles is altered to form anti-fogging, self-cleaning materials [8]; in the medical and cosmetic sectors, wettability controls skin friction and lubrication [9], while in earth science the wettability of the subsurface governs secure storage of CO2 [10,11], as well as oil recovery [12,13]
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