A slotted plate device for measuring static yield stress

A slotted-plate device was constructed with a balance and a linear-motion p latform to directly measure static yield stresses of suspensions by moving the plate in the suspension in a similar mode as is done in the well-known Withelmy-plate technique for measuring surface tension. Wall effects associ ated with the original plate yield-stress instrument [De Kee et al. (1980)] were minimized by opening a series of slots on the plates. Yield-stress ex periments were conducted on both high-concentration (40, 50, 60, and 70 wt % TiO2) and low-concentration (2, 3, and 5 wt % bentonite) aqueous suspensi ons. The new setup avoids the disadvantages of the vane instrument, possibl e secondary flow between the blades as well as a nonuniform stress distribu tion along a virtual cylindrical surface. Yield stress values of TiO2 suspe nsions were compared with the values obtained via a variety of other method s, including indirect extrapolation from steady-shear data, vane creep test ing. and vane stress-ramp measurements using an SR-5000 rheometer. Very sma ll yield stress (up to similar to 10(-4) Pa) measurements of low-concentrat ion bentonite suspensions (2, 3, and 5 wt %) could be determined only with the slotted-plate device. The vane method could not measure yield stress va lues of bentonite suspensions of less than 7 wt % concentration. Relaxation tests on high-concentration suspensions indicated that these suspensions m ay not be purely elastic below yield stress. (C) 2001 The Society of Rheolo gy.