Time-resolved yield stress measurement of evolving materials using a creeping sphere

Physicochemical phenomena influenced by aging or reaction can result in rheological changes across several orders of magnitude, but the classical rheometry methods available for analysis of concentrated suspensions can face challenges in correctly measuring the yield stress of aging/reacting (evolving) materials and need some precautions to enable precise measurement of the evolution of the yield stress with time. Here, a creeping sphere method has been applied to measure time-resolved yield stress; the force required to pull a solid sphere at very low velocity is used to calculate yield stress using previous analytical solutions for local flow of a creeping sphere in yield stress materials. The measured yield stress values agree well with the data recorded using vane-in-cup geometry for time-independent measurements using Carbopol gel. The creeping sphere is less affected by shear history because of the constantly changing shear region and therefore measures yield stress changes in evolving materials such as cement for a long time period in a single run, without altering ongoing structural network bond formation.