New Optical Techniques For Particle Studies In The Bottom Boundary Layer

The effect of shape on the fall velocity of silt-size mineral particles was investigated by recording sequential transmission holograms of settling particles. Particles with similar densities and settling speeds varied in their sphere-equivalent radii by a factor of 5. The large particles were edgewise-settling thin flakes while the smaller particles were much more compact. Variations of the Stokes settling equation for thin disks accurately described the settling data for large thin flakes. The fall velocities for small, compact, fast-settling particles were much greater than for volume-equivalent spheres and were adequately described by the Stokes equation for elliptical cylinders. An unresolved problem remains in that the theoretical drag on thin disks is about four times that on flat elliptical cylinders of the same surface area.