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

International audienceThe transport of fibers by a fluid flow is investigated in transparent channels modelling rock fractures: the experiments use flexible polyester thread (mean diameter $280 \mu\mathrm{m}$) and water or a water-polymer solution. For a channel with smooth parallel walls and a mean aperture $\bar{a} = 0.65\,\mathrm{mm}$, both fiber segments of length $\ell = 20-150\, \mathrm{mm}$ and ``continuous'' fibers longer than the channel length have been used: in both 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 ($\bar{a} \gtrsim 1.1 \mathrm{mm}$) 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 $\bar{a}=0.65 \mathrm{mm}$, 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