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Mobility of particles embedded in a laterally bounded membrane
Published online by Cambridge University Press: 11 April 2025
Abstract
The hydrodynamic analysis of motion of small particles (e.g. proteins) within lipid bilayers appears to be naturally suitable for the framework of two-dimensional Stokes flow. Given the Stokes paradox, the problem in an unbounded domain is ill-posed. In his classical paper, Saffman (J. Fluid Mech., vol. 73, 1976, pp. 593–602) proposed several possible remedies, one of them based upon the finite extent of the membrane. Considering a circular boundary, that regularisation was briefly addressed by Saffman in the isotropic configuration, where the particle is concentrically positioned in the membrane. We investigate here the hydrodynamic problem in bounded membranes for the general case of eccentric particle position and a rectilinear motion in an arbitrary direction. Symmetry arguments provide a representation of the hydrodynamic drag in terms of ‘radial’ and ‘transverse’ coefficients, which depend upon two parameters: the ratio $\lambda$ of particle to membrane radii and the eccentricity
$\beta$. Using matched asymptotic expansions we obtain closed-form approximations for these coefficients in the limit where
$\lambda$ is small. In the isotropic case (
$\beta = 0$) we find that the drag coefficient is
$4\pi /(\ln ({1}/{\lambda })- {1})$, contradicting the value
$4\pi /(\ln ({1}/{\lambda })- {1}/{2})$ obtained by Saffman. We explain the oversight in Saffman’s argument.
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- © The Author(s), 2025. Published by Cambridge University Press