  The possible transport of fibers by fluid flow in fractures is investigated
experimentally in transparent models using flexible polyester thread (mean
diameter $280 \mu\mathrm{m}$) and Newtonian and shear thinning fluids. In the
case of smooth parallel walls, fibers of finite length $\ell = 20-150
\mathrm{mm}$ move at a constant velocity of the order of the maximum fluid
velocity in the aperture. In contrast, for fibers lying initially at the inlet
side of the model and dragged by the flow inside it, the velocity increases
with the depth of penetration (this results from the lower velocity - and drag
- in the inlet part). In both cases, the friction of the fiber with the smooth
walls is weak. 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. Time dependent
distorsions of the fiber are also often observed. When water is replaced by a
shear thinning polymer solution, the fibers move faster and continuously in
high aperture regions and their friction 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 no global aperture gradient but
with rough walls and a channelization parallel to the mean flow, fiber
transport was only possible in shear-thinning flows and pinning and
entanglement effects were studied.
