Strong confinement, thermal fluctuations, and mobility of a tightly fitting vesicle in a very narrow microcapillary tube

College of Engineering, Swansea University, Bay Campus - Fabian Way, Swansea, SA1 8EN, UK

Received: 7 August 2024
Accepted: 22 April 2025

Abstract

We investigate theoretically the critical role of thermal fluctuations in maintaining a narrow gap between a very narrow capillary tube and a highly confined enclosed membrane vesicle. We quantitatively find that the size of the slender gap between a tightly fitting incompressible vesicle and an enclosing cylindrical tube depends on a subtle interplay between membrane area dilation and vesicle fluctuations. This work is therefore likely to be of crucial importance for investigating the paradigmatic properties of highly confined membrane vesicles inside a very narrow capillary tube. Additionally, fluid flow can also occur in this gap, giving rise to a finite vesicle mobility along a narrow capillary tube. Typically, for most (small to moderate) fluid velocities, we find that (in the vesicle fluctuation dominated regime) the gap size remains essentially insensitive to fluid flow. However, for relatively large fluid velocities, it is approximately found (in the fluid flow dominated regime) that the gap size grows with increasing fluid velocity as a power law, and we are able to evaluate the extra hydrodynamic pressure drop due to the presence of the vesicle, as well as the vesicle's relative mobility. This work is thus also likely to be highly relevant for considerations of the stalling and dynamic arrest of tightly confined vesicles in narrow constrictions. Possible applications of this work might thus also include biological transport, microfluidics, and drug delivery.