Rheology of dilute suspensions of vesicles and red blood cells

TL;DR

This study investigates how the viscosity ratio affects the flow behavior of dilute vesicle and red blood cell suspensions, revealing complex rheological responses that challenge existing models and suggest vesicles as better RBC mimics.

Contribution

It provides new experimental insights into the rheology of vesicles and RBCs, highlighting the importance of viscosity ratio and challenging traditional elastic capsule models.

Findings

Viscosity decreases with increasing viscosity ratio in the tank-treading regime.

Viscosity exhibits a minimum near the tank-treading to tumbling transition.

Vesicle models may better describe RBC rheology than elastic capsule theories.

Abstract

We present rheology experiments on dilute solutions of vesicles and red blood cells (RBC). Varying the viscosity ratio $\lambda$ between internal and external fluids, the microscopic dynamics of suspended objects can be qualitatively changed from tank-treading ($tt$) to tumbling ($tb$). We find that in the tt regime the viscosity $\eta$, decreases when $\lambda$ increases, in contrast with droplet emulsions and elastic capsule theories which are sometimes invoked to model RBC dynamics. At a critical $\lambda$ (close to the tt-tb transition) $\eta$ exhibits a minimum before it increases in the tb regime. This is consistent with a recent theory for vesicles. This points to the nontrivial fact that the cytoskeleton in RBC does not alter the qualitative evolution of $\eta$ and that, as far as rheology is concerned, vesicle models might be a better description.