Numerical-experimental observation of shape bistability of red blood cells flowing in a microchannel

TL;DR

This study combines experiments and simulations to map the shape bistability of red blood cells in microchannels, revealing how initial conditions and flow velocity influence their stable shapes, mainly croissants and slippers.

Contribution

It provides the first comprehensive shape phase diagram of red blood cells in microchannels, highlighting the role of initial conditions in shape bistability.

Findings

Red blood cells exhibit two main stable shapes: croissants and slippers.

Shape bistability occurs at both low and high flow velocities.

Initial conditions significantly influence the resulting cell shape.

Abstract

Red blood cells flowing through capillaries assume a wide variety of different shapes owing to their high deformability. Predicting the realized shapes is a complex field as they are determined by the intricate interplay between the flow conditions and the membrane mechanics. In this work we construct the shape phase diagram of a single red blood cell with a physiological viscosity ratio flowing in a microchannel. We use both experimental in-vitro measurements as well as 3D numerical simulations to complement the respective other one. Numerically, we have easy control over the initial starting configuration and natural access to the full 3D shape. With this information we obtain the phase diagram as a function of initial position, starting shape and cell velocity. Experimentally, we measure the occurrence frequency of the different shapes as a function of the cell velocity to construct the experimental diagram which is in good agreement with the numerical observations. Two different major shapes are found, namely croissants and slippers. Notably, both shapes show coexistence at low (<1 mm/s) and high velocities (>3 mm/s) while in-between only croissants are stable. This pronounced bistability indicates that RBC shapes are not only determined by system parameters such as flow velocity or channel size, but also strongly depend on the initial conditions.