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.

中文翻译：

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混合湿多孔介质中流体驱替动力学


我们在混合湿多孔介质中识别出独特的两相流侵入模式。时间分辨高分辨率同步加速器 X 射线成像用于研究水通过充满油的小岩石样本的侵入，其特征是局部接触角在 90° 以上和以下的广泛不均匀分布。水在相连的前沿前进，但喉咙并没有按照大小递减的顺序入侵，正如均匀疏水系统的入侵渗透理论所预测的那样。相反，我们观察到流体和固体之间三相接触的钉扎，表现为接触角滞后，这可以防止折断和界面收缩。在没有粘性耗散的情况下，我们使用能量平衡来找到位移的有效热力学接触角，并表明该角度在位移过程中增加。当局部接触角克服固定界面处的前进接触角时，就会发生位移：润湿性控制着填充顺序。主界面曲率的乘积（高斯曲率）为负，这意味着连接良好的相与接触线处的钉扎一致，同时为混合湿介质中的高驱替效率提供了拓扑解释。