3D imaging studies of rigid-fiber sedimentation

Fibers are industrially important particles that experience coupling between rotational and translational motion during sedimentation. This leads to helical trajectories that have yet to be accurately predicted or measured. Sedimentation experiments and hydrodynamic analysis were performed on 11 copper “fibers” of average length 10.3 mm and diameter 0.20 mm. Each fiber contained three linear but non-coplanar segments. Fiber dimensions were measured by imaging their 2D projections on three planes. The fibers were sequentially released into silicone oil contained in a transparent cylinder of square cross section. Identical, synchronized cameras were mounted to a moveable platform and imaged the cylinder from orthogonal directions. The cameras were fixed in position during the time that a fiber remained in the field of view. Subsequently, the cameras were controllably moved to the next lower field of view. The trajectories of descending fibers were followed over distances up to 250 mm. Custom software was written to extract fiber orientation and trajectory from the 3D images. Fibers with similar terminal velocity often had significantly different terminal angular velocities. Both were well-predicted by theory. The radius of the helical trajectory was hard to predict when angular velocity was high, probably reflecting uncertainties in fiber shape, initial velocity, and fluid conditions associated with launch. Nevertheless, lateral excursion of fibers during sedimentation was reasonably predicted by fiber curl and asymmetry, suggesting the possibility of sorting fibers according to their shape.