A reciprocal theorem for the prediction of the normal force induced on a particle translating parallel to an elastic membrane

TL;DR

This paper derives analytical expressions for the lift force on a particle translating near an elastic membrane, revealing how membrane shear and bending influence attractive and repulsive interactions respectively, with implications for soft matter physics.

Contribution

It introduces a reciprocal theorem-based analytical framework to quantify the lift force on a particle near an elastic membrane, accounting for shear and bending effects.

Findings

Shear-related force causes attraction decreasing quadratically with system size.

Bending causes a repulsive force that diverges logarithmically for large membranes.

Regularization with a cut-off length makes the bending-induced force finite.

Abstract

When an elastic object is dragged through a viscous fluid tangent to a rigid boundary, it experiences a lift force perpendicular to its direction of motion. An analogous lift mechanism occurs when a rigid symmetric object translates parallel to an elastic interface or a soft substrate. The induced lift force is attributed to an elastohydrodynamic coupling that arises from the breaking of the flow reversal symmetry induced by the elastic deformation of the translating object or the interface. Here we derive explicit analytical expressions for the quasi-steady state lift force exerted on a rigid spherical particle translating parallel to a finite-sized membrane exhibiting a resistance toward both shear and bending. Our analytical approach proceeds through the application of the Lorentz reciprocal theorem so as to obtain the solution of the flow problem using a perturbation technique for small deformations of the membrane. We find that the shear-related contribution to the normal force leads to an attractive interaction between the particle and the membrane. This emerging attractive force decreases quadratically with the system size to eventually vanish in the limit of an infinitely-extended membrane. In contrast, membrane bending leads to a repulsive interaction whose effect becomes more pronounced upon increasing the system size, where the lift force is found to diverge logarithmically for an infinitely-large membrane. The unphysical divergence of the bending-induced lift force can be rendered finite by regularizing the solution with a cut-off length beyond which the bending forces become subdominant to an external body force.