Swinging of red blood cells under shear flow

We reveal that under moderate shear stress (eta gamma approximate to 0.1 Pa) red blood cells present an oscillation of their inclination (swinging) superimposed to the long-observed steady tank treading (TT) motion. A model based on a fluid ellipsoid surrounded by a viscoelastic membrane initially unstrained (shape memory) predicts all observed features of the motion: an increase of both swinging amplitude and period (1/2 the TT period) upon decreasing eta gamma, a eta gamma-triggered transition toward a narrow eta gamma range intermittent regime of successive swinging and tumbling, and a pure tumbling at low eta gamma values.