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Elastohydrodynamic Interaction of Flowing Particles with Surfaces
Summary
This dissertation investigated how elasticity affects the movement of particles near surfaces in slow-flowing viscous fluids, with relevance to soft microplastic filtration and sedimentation. Researchers found unexpected behaviors including wall attraction at low flow speeds and persistently unsteady particle motion that are typically attributed to inertial forces. Very soft spheres exhibited additional nonlinear dynamics, indicating an elastohydrodynamic memory effect that could influence how soft microplastics behave during filtration and transport processes.
The influence of elasticity on the dynamics of nonheavy particles in creeping flows near rigid and elastic walls is investigated in this dissertation. These dynamics are central to processes like filtration and sedimentation of soft microparticles, including bioparticles and soft microplastics. To isolate elastic effects from microscale phenomena such as diffusion and electrostatics, centimeter-scale model experiments were performed using rigid and elastic spheres and rods in a highly viscous Newtonian fluid. The sedimentation of elastic spheres in the center of a large rectangular tank - initially similar to that of rigid ones - diverged beyond a characteristic distance from rest, where softer spheres experienced a second acceleration and reached terminal velocities up to 9 % higher. Near-wall experiments revealed even more unexpected phenomena: inertial wall attraction during the transient mass acceleration phase at low Reynolds numbers (\(Re_ ext{P}\lessapprox 0.1\)) and persistently unsteady, nonlinear kinematics even in the creeping-flow regime, where \(Re_ ext{P}\approx O(10^{-2})\). These behaviors are typically attributed to inertial forces in form of so-called memory forces. In the case of very soft spheres, the kinematics were superimposed by further nonlinearities, which indicates the presence of an elastohydrodynamic memory effect. Rods with small aspect ratios \(l/r\approx 5\) exhibited additional complexity. While axial sedimentation of rigid rods sedimenting at \(Re_ ext{P}\approx O(10^{-1})\) was in accordance with theoretical predictions, rods at lower \(Re_ ext{P}\) showed kinematic instabilities and transitions from flipping to drifting motion near walls as deformability increased - reminiscent of the tank-treading motion of red blood cells in shear flows. Complementary CFD simulations visualized the transient flow fields and confirmed the importance of container boundaries and fluid inertia, even at low \(Re_ ext{P}\). The findings of this thesis challenge the widespread assumption that particle dynamics at \(Re_ ext{P}< 0.1\) are purely linear. Instead, they highlight the crucial role of fluid inertia and memory effects in the dynamics of soft particles near walls. A deeper understanding and accurate modeling of these overarching effects are essential for advancing the theory of elastohydrodynamics in confined, low-Reynolds-number flows.