  Colloidal particles hold promise for mobilizing and removing trapped
immiscible fluids from porous media, with implications for key energy and water
applications. Most studies focus on accomplishing this goal using particles
that can localize at the immiscible fluid interface. Therefore, researchers
typically seek to optimize the surface activity of particles, as well as their
ability to freely move through a pore space with minimal deposition onto the
surrounding solid matrix. Here, we demonstrate that deposition can,
surprisingly, promote mobilization of a trapped fluid from a porous medium
without requiring any surface activity. Using confocal microscopy, we directly
visualize both colloidal particles and trapped immiscible fluid within a
transparent, three-dimensional (3D) porous medium. We find that as non-surface
active particles deposit on the solid matrix, increasing amounts of trapped
fluid become mobilized. We unravel the underlying physics by analyzing the
extent of deposition, as well as the geometry of trapped fluid droplets, at the
pore scale: deposition increases the viscous stresses on trapped droplets,
overcoming the influence of capillarity that keeps them trapped. Given an
initial distribution of trapped fluid, this analysis enables us to predict the
extent of fluid mobilized through colloidal deposition. Taken together, our
work reveals a new way by which colloids can be harnessed to mobilize trapped
fluid from a porous medium.
