Swinging of Red Blood Cells under Shear Flow

We reveal that under moderate shear stress (η ̇ γ≈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 η ̇ γ, a η ̇ γ-triggered transition toward a narrow η ̇ γ range intermittent regime of successive swinging and tumbling, and a pure tumbling at low η ̇ γ values.