Single Fiber Transport in a Fracture Slit: Influence of the Wall Roughness and of the Fiber Flexibility

The transport of fibers by a fluid flow is investigated in transparent channels modeling rock fractures: the experiments use flexible polyester thread (mean diameter 280 μm) and water or a water–polymer solution. For a channel with smooth parallel walls and a mean aperture ā = 0.65 mm, both fiber segments of length ℓ = 20–150 mm and “continuous” fibers longer than the channel length have been used: in both the cases, the velocity of the fibers and its variation with distance could be accounted for while neglecting friction with the walls. For rough self-affine walls and a continuous gradient of the local mean aperture transverse to the flow, transport of the fibers by a water flow is only possible in the region of larger aperture (ā ≲ 1.1 mm) and is of “stop and go” type at low velocities. With the polymer solution, the fibers move faster and more continuously in high aperture regions and their interaction with the walls is reduced; fiber transport becomes also possible in narrower regions where irreversible pinning occurred for water. In a third rough model with parallel walls and a low mean aperture ā = 0.65 mm, fiber transport is only possible with the water–polymer solution. The dynamics of fiber deformations and entanglement during pinning–depinning events and permanent pinning is analyzed.