Paired waltz of red blood cells - Shear induced RBC collective behavior in dilute suspension and the rheological effects

In simple shear flow, we observed red blood cells (RBC) form synchronized rotating "waltzing" pairs in the slit confined geometry.  We modeled the RBC dynamics with fully coupled hydrodynamic interactions (HI) and without inter-particle adhesion.  We found that shear-induced HI between the disc-shaped particles induce the formation of synchronized rotating doublets at low capillary number (Ca), low Reynolds number (Re), and dilute to moderate RBC volume fractions of φ=3 to 10 %.  In addition, the HI between the RBC and the wall boundaries induce migration towards the slit centerline, resulting in crystalline-like order between the doublets.  At sufficiently high Ca, the doublets dissolve into singlets but continue to flow in evenly-spaced layers.  This collective behavior further effect non-monotonic dependence of the suspension intrinsic viscosity on the flow shear rate and the suspension volume fraction.