  We show that during evaporation of a pore network, liquid can refill the gas
occupied pores, snapping off a gas bubble, which then moves to a stable
configuration. This phenomenon is induced by the capillary instability due to
the wettability heterogeneity of the pore network and has a much smaller time
scale as compared to the evaporation process. The capillary instability induced
liquid refilling and bubble movement are explained in detail based on the
analysis of the images obtained from the visualization experiment. The
capillary valve effect, which hinders the movement of the gas-liquid interface
and is induced by the sudden geometrical expansion between small and large
pores, can be suppressed by the residual liquid in the large pore. For better
understanding of the capillary instability induced gas-liquid two-phase
transport during evaporation, a novel pore network model is developed, which
considers not only the capillary and viscous forces but also the inertial
forces that are seldom taken into account in the previous models. The pore
network modeling results are in good agreement with the experimental data,
demonstrating the effectiveness of the developed pore network model, which
opens up a new route for better understanding of the role of inertial forces in
two-phase transport in porous media.
