Interfacial boundary conditions and residual trapping: A pore-scale investigation of the effects of wetting phase flow rate and viscosity using micro-particle image velocimetry

We introduce a new two-phase and two-fields-of-view micro-Particle-Image-Velocimetry (μPIV) experimental apparatus that is used to conduct systematic studies of pore fluid occupancy and velocity fields under two-phase flow conditions in different micromodels. The apparatus allows simultaneous study of flow fields at the pore- and micromodel-scales. This system is utilized to develop a deeper insight into the distribution of fluids and shear stress at the interface of invading and defending fluids in two Polydimethylsiloxane (PDMS) micromodels: a pore-doublet configuration and a two-dimensional replica of Bentheimer sandstone. We investigate the effect of changes in invading wetting phase flow rate and viscosity and injection of a non-wetting droplet on pore fluid configuration and residual trapping. We discuss how the local perturbations of velocity fields impact displacement of non-wetting phase, the residual trapping, and distributions of the trapped non-wetting phase globules. Furthermore, we map the rotational velocity of the non-wetting fluid within a selected group of trapped globules and discuss how the flow rate of the passing wetting fluid and the momentum transfer across the fluid/fluid interface impacts the stability of the local pore occupancy of the non-wetting phase. The experimental observations provide direct evidence of slip boundary condition at the fluid/fluid interfaces. They show the mechanisms of momentum transfer across the interfaces and their impact on pore-scale displacements and fluid occupancy.