Sphere Drag at Small Reynolds Number in a Herschel-Bulkley Fluid

When a sphere is pulled through a liquid with a yield stress, a fluidized region near the sphere is formed. To determine the size and shape of the fluidized region, we pulled steel spheres through tubes of various diameters filled with a Herschel-Bulkley fluid. By measuring the drag force on the sphere as a function of container size we indirectly determined the extent of the fluidized region surrounding the moving sphere. We found that the fluidized region increases in size with velocity. To directly study the shape of the fluidized region around the moving sphere, poppy seeds were scattered throughout the non-Newtonian liquid. Using particle tracking techniques and particle image velocimetry we visualized the fluidized region as a steel sphere moved through the fluid.