Dynamics of elastic, nonheavy spheres sedimenting in a rectangular duct

Understanding of sedimentation dynamics of particles in bounded fluids is of crucial importance for a wide variety of processes. While there is a profound knowledge base regarding the sedimentation of rigid solid particles, the fundamental principles of sedimentation dynamics of elastic spheres in bounded fluids are not well understood. This especially applies to nonheavy spheres, whose density is close to that of the surrounding medium and which therefore show extended inertial effects upon acceleration. Here, we present model experiments of the sedimentation dynamics of deformable, nonheavy spheres in the presence of walls. Despite the deformations of the particles being small, the particle dynamics of elastic spheres differed fundamentally from that of rigid spheres. Initially, the sedimentation of elastic spheres is comparable with the sedimentation of rigid spheres. From a characteristic onset position of about 10·R, deformability effects kick in and a second acceleration appears. Finally, the deformable spheres reach a terminal sedimentation velocity. The softer the spheres are (in terms of Young's elastic modulus), the higher the terminal velocity is. In the present setup, a terminal velocity up to 9% higher than the velocity for comparable rigid spheres was reached. By analyzing the obtained data, insights into the dynamics are given that could serve as basic approaches for modelling the dynamics of elastic spheres in bounded fluids.