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 thick ness h was stationary. Fluid thickness down to a few microns could be measu red. Most fluids showed two kinds of dependence of F on h according to an e xperimentally-determined thickness h*. If h > h* then F varied in proportio n to h(-1) as predicted by Scott (1931) for a fluid with a shear yield stre ss. tau(o) The magnitude of tau(o) from squeeze-flow data in this region wa s compared with the yield stress measured by the vane method. For some flui ds tau(o) measured by squeeze flow was less than the vane yield stress, sug gesting 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 relationsh ip gave a yield stress which increased as the fluid thickness decreased. Th is previously unreported effect may result from unconnected regions of larg e yield stress in the fluid of size similar to h* which are not sensed by t he vane and which become effective in squeeze flow only when h < h*.