Porous Flow, Gravity, and Soil Creep, on a Chip
IPGP - Îlot Cuvier
Séminaires Dynamique des fluides géologiques
Under very energetic fluid flow, behavior of hard frictional particles ensembles, which would otherwise be a jammed static sediment, can be turned into a gas one. In this seminar we will talk about the intermediate scenario, where particles can be made slightly mobile due to a weak porous flow stresses combined with the gravity field. As the system remains dense, gas dynamics modeling, governed by fast collisions, becomes irrelevant. Although physically complex, this situation corresponds to most of the sediment volume, on continents (soils) and undersea (benthic and sea floor sediments), and is important to understand for enhancing civil engineering and natural hazards predictions.?We present a novel microfluidic design to visualize the dynamics of a 2D, gravity-settled bed of fully submerged, 400 um polymeric particles, which is subject to an upward flow. The design allows for a precise, metered, interstitial flow, and a high accuracy measurement of the individual particle small displacements and the bulk deformation of the bed due to the flow.?In a first set of experiments, a flat and horizontal sediment bed is prepared, and the porous flow intensity is varied. Compaction and dilation are observed consecutively as the porous flow discharge increases, and as the system crosses the criterion for the hydromechanical instability to develop, and a channel to grow. In a second set, we vary both porous flow and the system slope, under the critical slope of avalanche. ?Over time, the sediment bed slope decays logarithmically, and the log rate increases with both the system tilt and porous flow conditions. We identify a new dimensionless parameter, $P^*$, taking into account, multiplicatively, porous flow and sediment layer slope effect on particles, able to reconcile all the creep deformation results on a single curve. The latter presents two very distinct creep regimes whose natures remain not fully understood. Yet, concomitant observations of the voids sizes distribution also show a systematic change of the microstructure between the two regimes; this rather supports a transition from sliding-dominated plastic events to rolling-dominated ones.