Competition between Red Blood Cells aggregation and break-up: Depletion force due to filamentous viruses vs shear flow

Human blood is a shear thinning fluid with complex response that strongly depends on the Red Blood Cells (RBC) ability to form aggregates, called rouleaux.
Both, depletion and bridging have long been believed to play a role in rouleaux formation, mediated by the presence of macromolecules such as fibrinogen in the plasma. However, despite numerous investigations, microscopic understanding of the formation and break up of RBC aggregates has not been fully elucidated.
In order to distinguish the mechanisms behind RBCs aggregation we employ a depletant agent with very long ranged interaction force, the filamentous fd bacteriophage. These colloidal rod-like particles have high length-to-diameter ratio and carry the same charge as the RBCs. Fine-tuning of the depletion force is achieved through mixtures of RBCs in PBS at different concentrations of rods. We study the breakup of aggregates during shear flow to quantify the interaction between the cells, combining a home-build counter-rotating cone-plate shear cell with an ultra-fast confocal microscope enabling visualization of the structures while shearing.
We present a non-equilibrium phase diagram of shear rates versus depletant concentrations, showing regions for different flow response and stages of separation of the RBCs doublets.