Inertial forces and elastohydrodynamic interaction of spherical particles in wall-bounded sedimentation experiments...

Area of Science:

• Fluid dynamics
• Particle transport
• Rheology

Background:

• Sedimentation of particles near boundaries is crucial in various industrial and environmental processes.
• Classic models often assume creeping flow conditions, which may not hold true near walls.
• The interplay between fluid inertia, particle deformability, and boundary proximity is not fully understood.

Purpose of the Study:

• To experimentally investigate unsteady, wall-bounded sedimentation of elastic and rigid spheres at low particle Reynolds numbers (ReP ≲ 0.1).
• To identify and analyze novel phenomena arising from fluid inertia and elastohydrodynamic effects.
• To develop a conceptual framework for classifying near-wall sedimentation regimes.

Main Methods:

• Experimental measurement of the complete kinematics of elastic and rigid spheres sedimenting from rest near a rigid plane wall.
• Varying initial distances from the wall and particle Reynolds numbers (ReP ≈ O(10⁻¹) and ReP ≈ O(10⁻²)).
• Supported by computational fluid dynamics (CFD) simulations to analyze flow fields.

Main Results:

• Observed an inertial wall attraction during initial acceleration.
• Rigid spheres at ReP ≈ O(10⁻¹) showed classic wall-lift behavior.
• At ReP ≈ O(10⁻²), both rigid and elastic spheres exhibited unsteady sedimentation with deceleration.
• Identified an elastohydrodynamic memory effect due to coupled inertial forces and particle deformability.
• Demonstrated breakdown of classic assumptions for creeping flows near boundaries.

Conclusions:

• Classic assumptions for particle sedimentation in creeping flows fail near boundaries.
• Fluid inertia becomes more significant at lower particle Reynolds numbers in near-wall flows.
• A conceptual framework is proposed to classify near-wall sedimentation regimes based on ReP and boundary proximity.
• Findings are relevant for understanding deformable microplastic particle dynamics in environmental and wastewater flows.