Yield stress of structured fluids measured by squeeze flow

Various structured fluids were placed between the parallel circular plates of a squeeze-flow rheometer and squeezed by a force F until the fluid thickness h was stationary. Fluid thickness down to a few microns could be measured. Most fluids showed two kinds of dependence of F on h according to an experimentally-determined thickness h*. If h > h* then F varied in proportion to h(-1) as predicted by Scott (1931) for a fluid with a shear yield stress. tau(o) The magnitude of tau(o) from squeeze-flow data in this region was compared with the yield stress measured by the vane method. For some fluids tau(o) measured by squeeze flow was less than the vane yield stress, suggesting that the yield stress of fluid in contact with the plates was less than the bulk yield stress. If h < h* then F varied approximately as h(-5/2) and the squeeze-flow data in this region analysed with Scott's relationship gave a yield stress which increased as the fluid thickness decreased. This previously unreported effect may result from unconnected regions of large yield stress in the fluid of size similar to h* which are not sensed by the vane and which become effective in squeeze flow only when h < h*.