Structural Viscosity of Blood at Very Low Rate of Shear

Abstract

A sensitive rotational viscometer with concentric double cylinders has been constructed, which makes it possible to measure the viscosity of blood at very low shear rates below 1sec-1. An outer cylinder is rotated by a d.c. motor coupled with a reduction gear. The speed of rotation of the cylinder is detected by a tachometer and is led to an X axis of an XY recorder which indicates the shear rate in the liquid. The torque due to the viscosity of liquid is detected by the deflection of a mirror attached to the suspended inner cylinder. The voltage induced in the phototube is amplified and the resulting d.c. current is introduced to the coil, placed in a magnetic field and attached to the inner cylinder, in order to balance the defection of the inner cylinder to null. The output current from the d.c. amplifier is led to a Y axis in the XY recorder, which indicates the shear stress in the liquid. The flow curve of the liquid is thus recorded in a few seconds with gradual increase or decrease of shear rate. The limit of measurable torque was approximately 0.1 dyne-cm.With ascending and descending shear rate in a range below 1sec-1, a hysteresis curve of blood with hematocrit 79% was observed as illustrated in Fig. 2. The descending curve indicated a steady flow curve and yield value was observed to exist at zero shear rate. If the rotation of liquid was raised in about a second up to a very low shear rate below 1sec-1 and sustained at that shear rate, the torque gradually increased to that indicating the shear stress in the steady flow condition, after passing through a maximum during the transient period. The gradual increase of the shear stress has been probably caused by the occurrence of viscous flow following the initial elastic deformation of liquid, which takes place under a stress below the yield value. The transient maximum of shear stress may be produced by the nonuniform packing of blood cells subjected to shear stress.When the disturbing vibration was given to the outer cylinder without affecting the sensitivity of torque measurement, both yield value and steady flow viscosity of blood decreased. The coagulative structure of blood cells must have been disrupted by the vibration.No hysteresis was observed in Newtonian liquids such as water, oil and glycerin. A marked hysteresis and a transient variation of shear stress were observed in aqueous dispersions of cocoa. It is considered that this kind of phenomenon is commonly observed in coagulative dispersions.