Visualization experiments and pore network models on evaporation of capillary porous media are presented in this work. We show that in a simple model porous medium the pores occupied by gas can be refilled with liquid, snapping off a gas bubble, which then moves to a stable configuration. This phenomenon, referred to as capillary instability, is induced due to heterogeneity in wettability of the pore surfaces and has a much smaller time scale compared to the evaporation process. The capillary instability is explored based on the optical images obtained from visualization experiments. The residual liquid in pores can suppress the capillary valve effect, which is induced by the sudden geometrical expansion and can hinder the movement of the gas-liquid interface. For better understanding of the capillary instability induced gas-liquid displacement, a pore network model that accounts for capillary and viscous forces as well as the inertial effect is developed. The pore network simulation results agree well with experimental data. The ratio of the square of the average meniscus moving speed predicted by the pore network model with the inertial effect to the average meniscus moving speed obtained from the model without the inertial effect is a linear function of the Weber number. When the Weber number exceeds a critical value, more pores are invaded by the gas-liquid interface in the pore network model with the inertial effect than in the model neglecting the inertial effect. The pore network model developed here opens up a route to better understand the role of inertial effects in two-phase transport in porous media.

• Received 9 March 2020
• Accepted 15 September 2020
• Corrected 15 December 2020

DOI:https://doi.org/10.1103/PhysRevFluids.5.104305