  Compressing a porous, fluid-filled material will drive the interstitial fluid
out of the pore space, as when squeezing water out of a kitchen sponge.
Inversely, injecting fluid into a porous material can deform the solid
structure, as when fracturing a shale for natural gas recovery. These
poromechanical interactions play an important role in geological and biological
systems across a wide range of scales, from the propagation of magma through
the Earth's mantle to the transport of fluid through living cells and tissues.
The theory of poroelasticity has been largely successful in modeling
poromechanical behavior in relatively simple systems, but this continuum theory
is fundamentally limited by our understanding of the pore-scale interactions
between the fluid and the solid, and these problems are notoriously difficult
to study in a laboratory setting. Here, we present a high-resolution
measurement of injection-driven poromechanical deformation in a system with
granular microsctructure: We inject fluid into a dense, confined monolayer of
soft particles and use particle tracking to reveal the dynamics of the
multi-scale deformation field. We find that a continuum model based on
poroelasticity theory captures certain macroscopic features of the deformation,
but the particle-scale deformation field exhibits dramatic departures from
smooth, continuum behavior. We observe particle-scale rearrangement and
hysteresis, as well as petal-like mesoscale structures that are connected to
material failure through spiral shear banding.
