Mechanisms of interface jumps, pinning and hysteresis during cyclic fluid displacements in an isolated pore

Quasi-static displacements of one immiscible fluid by another in a single pore can lead to interface jumps, pinning and capillary hysteresis, depending on the pore dimensions. It is expected that there is a critical pore configuration for which the interface becomes unstable and an interface jump is triggered. These processes are at the origin of hysteresis in porous media and control macroscopic two-phase fluid displacements.

We conduct quasi-static imbibition and drainage experiments and detailed numerical simulations in three and two-dimensional pores, represented by capillaries of different radii that are joined by a conical section (ink-bottle). A theoretical model for the interface is derived based on pressure balance that captures the full spectrum of possible interface behaviors.

Depending on the slope of the conical section, we observe a range of interfacial behaviors, including capillary jumps and interface pinning during both imbibition and drainage, which give rise to capillary hysteresis, that is, history dependence of the interface position. We identify a critical pore configuration for the occurrence of interface jumps and hysteresis, which depends on surface tension and contact angle.