Effects of shear rate and suspending medium viscosity on elongation of red cells tank-treading in shear flow.

Elongation measurements of red cells subjected to simple shear flow are usually performed using a single suspending medium (viscosity η(0) ) and varying the mean shear rate (y). Such data are often plotted versus the shear stress (tau = no(y) suggesting that the elongation scales with τ. In this work, normal blood samples were tested in a rheoscope varying both η(0) and(y.). The ranges of (y.) were chosen to restrict the elongation of the red cells to low values where the behavior is dominated by their intrinsic properties. It was found that the elongation scales with η(0)(s)γ with s decreasing from two at η(0) = 20 mPas to unity at η(0) = 70 mPas. Above η(0) = 70 mPas, the elongation is therefore essentially determined by the membrane elasticity alone. A side observation was a large variation of the elongation both intraindividually and interindividually.