A viscometer for measurements during thixotropic recovery; results with a compounded latex

Abstract

A viscometer is described for measuring the viscosity of a liquid immediately after an abrupt reduction of the shearing stress to a very low value. A hollow cylinder on a torsion suspension is immersed in the liquid contained in a narrow annular channel. The inner wall of the channel is rotatable with respect to the outer wall, and by this rotation the liquid is stirred or sheared. After the stirring is stopped, the viscosity is measured by photographically recording the rotation of the hollow cylinder, driven by a nearly constant torque applied through the torsion suspension. Measurements were made on a commercial compounded rubber latex containing 47.5% total solids. Immediately after stirring, the latex is a Newtonian liquid with a viscosity of .05 poises. Thereafter the viscosity increases at an initial rate and to an ultimate value both of which decrease as the shearing stress for measuring the viscosity is increased. The steady viscosity is attained approximately in 10 seconds. The observed extremes in steady viscosity were 11 and .5 poise, corresponding to the shearing stresses 1.3 and 10.7 dynes/cm. 2. The experimental data are fitted approximately by theoretical curves based on these assumptions: All aggregates of solid particles are broken up by the stirring. During subsequent thixotropic recovery, loose aggregates are formed which contain considerable trapped water. The rate of aggregation follows Smoluchowski's theory of slow coagulation, but both the coagulation rate constant and the per cent volume of water in the coagulates vary with the applied shearing stress. The relative viscosity of the suspension of aggregates follows an empirical law based on Eilers' data on aqueous suspensions of Bitumen spheres.