(328b) Less Is More: "Simple" Complex Fluids Reveal Rheological Fingerprints in Environmental Flows

Climate change has exacerbated the frequency of landslide events causing huge loss to human life and infrastructure. The main challenge in predicting such events is the absence of constitutive models to explain the flow and failure properties of natural heterogeneous dense suspension mixtures. From a fluid mechanics perspective, the onset of a landslide is dictated by the rearrangement dynamics of the constituent materials and the magnitude of external forcing. Here, we use a minimum ingredient complex fluid mixture - a suspension of silica sand and kaolin clay suspended in deionized water - to probe the effect of material properties on the yielding phenomena in dense suspensions. Silica sand particles (size ~ 100 μm) primarily interacts through frictional contacts, while the kaolin clay particles (size ~ 10μm) are attractive and forms system-spanning percolated structures. We systematically vary the “clay ratio” (clay to total solids) and the suspension volume fraction to generate steady shear rheological flow curves. By re-scaling the curves using yield stress and microscale rearrangement times (for stress and shear rate, respectively), we obtain universal yielding curves that only depend on the clay ratio parameter. Using the stress overshoot, estimated from stress-strain curves at various clay ratios, we show that sand-rich suspensions exhibit more “brittle-like” yielding behavior compared to the clay-rich suspensions. We propose the following microstructural mechanism for the observed behavior: the addition of sand to pure clay mixtures anneal the suspension by shifting the yielding envelope towards higher shear rates, and in doing so, the suspension fails more like a brittle material. Here, we developed a preliminary rheological framework to explore the effects of frictional interactions in model mud mixtures using a minimum ingredient complex fluid. We believe that our work reconciles previously contradictory observations in landslide rheology and can help modify the existing models that assess the hazard potentials of extreme environmental flows in the future.