The supply of oxygen and nutrients and the disposal of metabolic waste in the organs depend strongly on how blood, especially red blood cells, flow through the microvascular network. Macromolecular plasma proteins such as fibrinogen cause red blood cells to form large aggregates, called rouleaux, which are usually assumed to be disaggregated in the circulation due to the shear forces present in bulk flow. This leads to the assumption that rouleaux formation is only relevant in the venule network and in arterioles at low shear rates or stasis. Thanks to an excellent agreement between combined experimental and numerical approaches, we show that despite the large shear rates present in microcapillaries, the presence of either fibrinogen or the synthetic polymer dextran leads to an enhanced formation of robust clusters of red blood cells, even at haematocrits as low as 1%. Robust aggregates are shown to exist in microcapillaries even for fibrinogen concentrations within the healthy physiological range. These persistent aggregates should strongly affect cell distribution and blood perfusion in the microvasculature, with putative implications for blood disorders even within apparently asymptomatic subjects.

Vesicles under shear flow exhibit various dynamics: tank treading $\left(\mathit{TT}\right)$, tumbling $\left(\mathit{TB}\right)$, and vacillating breathing $\left(\mathit{VB}\right)$. The $\mathit{VB}$ mode consists in a motion where the long axis of the vesicle oscillates about the flow direction, while the shape undergoes a breathing dynamics. We extend here the original small deformation theory [C. Misbah, Phys. Rev. Lett. 96, 028104 (2006)] to the next order in a consistent manner. The consistent higher order theory reveals a direct bifurcation from $\mathit{TT}$ to $\mathit{TB}$ if $C_a\equiv \tau \dot{\gamma }$ is small enough—typically below 0.5, but this value is sensitive to the available excess area from a sphere ($\tau =\text{vesicle}$ relaxation time towards equilibrium shape, $\dot{\gamma }=\text{shear}$ rate). At larger $C_a$ the $\mathit{TB}$ is preceded by the $\mathit{VB}$ mode. For $C_a⪢1$ we recover the leading order original calculation, where the $\mathit{VB}$ mode coexists with $\mathit{TB}$. The consistent calculation reveals several quantitative discrepancies with recent works, and points to new features. We briefly analyze rheology and find that the effective viscosity exhibits a minimum in the vicinity of the $\mathit{TT}\text{−}\mathit{TB}$ and $\mathit{TT}\text{−}\mathit{VB}$ bifurcation points. At small $C_a$ the minimum corresponds to a cusp singularity and is at the $\mathit{TT}\text{−}\mathit{TB}$ threshold, while at high enough $C_a$ the cusp is smeared out, and is located in the vicinity of the $\mathit{VB}$ mode but in the $\mathit{TT}$ regime.

Les cellules eucaryotes et les matériaux biologiques sont présentés du point de vue de leur rhéologie. Les cellules individuelles possèdent des propriétés rhéologiques typiques, qui peuvent affecter le comportement cellulaire, en étroite relation avec leurs propriétés adhésives. Les caractéristiques individuelles des cellules sont aussi utiles pour décrire des systèmes multicellulaires, tels que des tissus biologiques. Les résultats d'expériences sont analysés et des modèles proposés tant à l'échelle cellulaire qu'à l'échelle macroscopique. Pour citer cet article : C. Verdier et al., C. R. Physique 10 (2009).