PTV and PIV study of the motion of viscous drops in yield stress material

We report a study of the slow viscous sedimentation of viscous Newtonian drops in a viscoplastic domain comprised of low concentration Carbopol aqueous solution. The study applies visualization methods. Direct photography is used to track the drop settling speed. In steady sedimentation PTV (Particle Tracking Velocimetry) is applied to determine the boundary of the yield stress region around the drop, while PIV (Particle Image Velocimetry) techniques are employed for the quantitative description of the flow field within this region. In infinite domains, the study addresses undisturbed sedimentation of slow and fast moving single drops and a tracking of the coalescence process of a train of two drops. Maps of the flow structure and intensity within the yielded regions around the drops in these cases are provided. When the sedimentation occurs in the vicinity of an adjacent vertical glass wall the drop settling speed may be either enhanced or retarded depending on the proximity to the wall. It is shown that the motion is retarded when the yielded region boundary remain intact but is deformed toward the drop on the wall side. Whenever the yielded region achieves complete contact with the wall the drop speed is augmented and an apparent slip is evident at the wall.