Effect of flow instability (and of blood anti-instability properties) on viscosity of blood measured in rotational viscometer

Abstract

Flow instability has been observed in the plasma and in the blood of low haematocrit studied in the rotational rhombospheroid viscometer. As this observation is relevant, and fundamental, to all cone-in-cone and cone-and-plate viscometers, a detailed experimental series was carried out using a range of standard oils of different viscosities as well as blood of different haematocrits. The main attention was given to a shear rate of 180 sec⁻¹, corresponding to 250 r.p.m. at angular gap of 5° (although a 10° gap has been also used). Linear regressions have been established for flow instability (given as percentage increase of the apparent viscosity). The linear regression for Newtonian oils was: y = −0.1258x + 1.962 ± 0.133, (r = −0.981); and for blood: y = −0.354x + 2.002 ± 0.116, (r = −0.983), in which y represents log of percentage increase of viscosity due to instability, and x is viscosity in cP or mPas.s. Flow instability occurs at higher shear rates in blood than in Newtonian oils; this is attributed to the pseudoplasticity (or shear-thinning or thixotropy) of blood. Both regressions merge at viscosity of under 1 cP, and spread apart as viscosities increase.