The yielding of concentrated cohesive suspensions can be deformation rate dependent

The yielding of concentrated cohesive suspensions can be deformation-rate dependent. One consquence of this is that a single suspension can present in one several different ways, depending upon how it is tested, or more generally, how it is caused to flow. We have seen variously Herschel-Bulkley flow, highly non-monotonic flow curves and highly erratic or chaotic yield, all in one suspension. In controlled-rate testing one sees a systematic effect of deformation rate. In controlled stress testing, matters are more subtle. Whereas step-stress creep testing will elicit reproducible behaviour, any attempt to determine a flow curve by, e.g. stepping up or sweeping stress at an inappropriate rate can lead to highly irreproducible behaviour.