Flexible fiber transport by a fluid flow in fractures with smooth and rough walls

The transport of flexible fibers by a flowing fluid has been studied experimentally in transparent model fractures. Both finite length segments (20mm ≤ l ≤ 150 mm) and continuous fibers penetrating freely into the model were used; their motion is monitored by means of a digital camera and of an image thresholding technique and is induced by the flow of water or of a polymer solution at a mean velocity U (50 ≤ U ≤ 400 mm.s-1). In a model with plane smooth parallel walls, the influence of the friction with the walls is small: fiber segments reach quickly a constant velocity as their distance xm to the inlet increases, the velocity of continuous fibers increases slower with distance before reaching a constant value. The second model fracture has two complementary rough self-affine walls with a relative lateral shift; it displays, in addition, a gradient of the aperture in the direction transverse to the mean flow. For this model, the transport of the fibers by flowing water is only possible in the region of largest aperture and is of a 'stop and go' type at low velocities. If water is replaced by the shear thinning polymer solution, the fibers move faster and continuously in high aperture regions; fiber transport also becomes possible in narrower regions.