How does confinement affect the dynamics of viscous vesicles and red blood cells?

TL;DR

This study explores how confinement influences the transition between steady and unsteady motion in viscous vesicles, modeling red blood cells, revealing that confinement alone can trigger this transition.

Contribution

It demonstrates that reducing confinement can induce the tank-treading to tumbling transition without changing viscosity contrast, highlighting the role of confinement in vesicle dynamics.

Findings

Transition can be triggered solely by confinement reduction.

Viscous, pressure, and lubrication torques determine dynamics.

Implications for microfluidic device design and diagnostics.

Abstract

Despite its significance in microfluidics, the effect of confinement on the transition from the tank-treading (steady motion) to the tumbling (unsteady motion) dynamical state of deformable micro-particles has not been studied in detail. In this paper, we investigate the dynamics of a single viscous vesicle under confining shear as a general model system for red blood cells, capsules, or viscous droplets. The transition from tank-treading to tumbling motion can be triggered by the ratio between internal and external fluid viscosities. Here, we show that the transition can be induced solely by reducing the confinement, keeping the viscosity contrast constant. The observed dynamics results from the variation of the relative importance of viscous-, pressure-, and lubrication-induced torques exerted upon the vesicle. Our findings are of interest for designing future experiments or microfluidic devices: the possibility to trigger the tumbling-to-tank-treading transition either by geometry or viscosity contrast alone opens attractive possibilities for microrheological measurements as well as the detection and diagnosis of diseased red blood cells in confined flow.