Blood plasma not so simple after all

Blood cells make up about 46% of human blood; the rest is protein-rich, aqueous plasma. Collectively, the liquid behaves as a non-Newtonian fluid: Unlike, say, water, whose viscosity is independent of flow rate, blood becomes less viscous the faster it flows. That behavior is crucial to understanding the flow instabilities that arise near aneurysms and vasoconstrictions, and it’s generally attributed entirely to the interactions between blood cells; the plasma itself is thought to be Newtonian. Although conventional shear measurements seem to confirm that view, new results obtained with a technique known as capillary breakup extensional rheometry suggest a more complicated picture. Researchers led by Christian Wagner (Saarland University, Saarbrücken, Germany) and Paulo Arratia (University of Pennsylvania) watched as capillary forces caused a liquid bridge of plasma to stretch, narrow, and eventually break. A Newtonian fluid would have broken up while the bridge was still relatively thick and would have left behind a lone satellite droplet. But as shown in this time series of images, the plasma formed a long, thin filament whose width decayed exponentially with time. And when the filament did break, it left behind a necklace-like string of droplets, barely visible in the final frame. The implication—that the fluid is not Newtonian but viscoelastic—suggests that plasma could itself be a factor in certain blood-flow instabilities. (M. Brust et al., Phys. Rev. Lett., in press.)