Yield Value of Bentonite Suspensions

Abstract

I HAVE recently been measuring the yield values of thixotropic aqueous suspensions of bentonite by determining the force required to move a thin flat plate in its own plane, through the suspension^1–3. The results indicate that the yield value, as measured, cannot be regarded, as an intrinsic property of the suspension but depends to a very marked extent on the material of the plate. The plate (3 cm. × 2·5 cm. × 0·008 cm.), immersed in the suspension, is suspended from a horizontal arm attached rigidly to a torsion wire. The vessel containing the suspension is slowly lowered (0·18 cm./min.) and the stress on the plate measured by the deflexion, recorded photographically as a function of time, of the horizontal arm. The suspensions (pH c. 8·5) are aged for at least a month after mixing and are thoroughly dispersed by a high-speed stirrer immediately before each experiment. The time of repose, until the yield point is reached, is approximately constant (c. 4 min.).