広い速度勾配範囲に用いる回転粘度計

Abstract

For the purpose of studying the molecular conditions in aqueous solutions of biologically important macromolecules, we frequently measure the viscosity of their dilute solutions. Since the mean shear rate in an Ostwald type viscometer is usually over 1000sec-1, the results cannot be treated with Simha's equation. Therefore a viscometer with a long capillary tube was designed to reduce the shear rate down to about 100sec-1.Recently we designed a coaxial cylinder type (Couette type) viscometer to measure viscosity at lower shear rates for studying large molecules having small rotary diffusion coefficients.The outer and inner cylinders are made of“Carbate”. The inner cylinder is hollow so as to make the apparent density slighty smaller than water, and is supported by small pivot bearings at the top and the bottom. The pivot is made of iridosmine alloy to avoid any corrosion due to the solution to be measured. The cup bearing was made of synthetic saphire. The outer cylinder is supported by precision preloaded ball bearings, and immersed in oil, the temperature of which is kept constant. The outer cylinder has a pulley which is made to rotate through a rubber belt by the motor. The motor is a shunt type controlled by thyratrons. When the voltage of the power supply was kept constant, the speed of the motor is constant within 0.2% at any speed between 30 to 3000 r.p.m.The gap between the inner and the outer cylinders is 1mm. The bottom part is designed as a cone-and-plate type so as to make the shear rate uniform all over the gap between both cylinders. The total volume of the gap is about 4ml. For measurements, however, 7ml. of solution is used in order to minimize the interaction between the upper surface of the inner cylinder and the free surface of the solution.An aluminum disk is on the axis of the inner cylinder and rotates with it. The edge of the disk is in the gap of a permanent magnet, and receives braking torque when the inner cylinder rotates. The strength of the torque is proportional to the rotational speed of the disk, and the value may be calibrated by placing liquid of known viscosity in the gap.In order to measure the viscosity of a solution, the solution is placed in the gap, and while rotating the outer cylinder at various speeds, the rotational speed of the inner cylinder and the difference of the rotational speed of the inner and the outer cylinders are measured. In this way viscositty of the solution may be measured at various points over a wide range of shear rate.We have measured non-Newtonian viscosity of myosin-B, tobacco mosaic virus, nucleic acid, collagen, polyacrylic acid, etc. over the shear rate range of 1 to 100sec-1. The upper limit for the shear rate is due to the method of measuring the rotational speed. If the rotational speed should be measured by a suitable electronic method instead of the present naked eye and a stopwatch method, the upper limit would be raised considerably. In order to lower the lower limit, we are trying to reduce the bearing friction.