Using bidirectional shear protocols to determine microstructural changes responsible for thickening and dethickening in colloidal suspensions

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Shear-thickening fluids have viscosities that increase during vigorous flows. This property enables many fascinating natural and industrial phenomena ranging from people walking across pools filled with cornstarch in water mixtures, to the processing properties of concrete. Recent studies have started to define the mechanism of shear thickening in particle suspensions. These studies have shown that formation of contacts between the suspended particles acts to build a force-supporting network of interacting particles that aligns with the flows. Previous work has shown that when jostled perpendicularly to the flow direction these networks can in some cases be broken apart allowing for complete dethickening of the suspension. This project will determine if this dethickening approach depends on the density of the suspension. New experiments, simulations, and theory will be used to determine the equations governing this phenomenon. Such equations will predict suspension behavior in arbitrary flow geometries and enable new strategies for controlling the viscosity. Because controlling the thickening of colloidal suspensions is important for processing products such as tooth-pastes, paints, and cement, this work will have very broad impact with the potential to change practices in both the scientific and industrial communities to the benefit of the US economy and society. Understanding and controlling the non-Newtonian behavior of shear-thickening fluids is crucial for increasing the efficiency of many industrial phenomena ranging from controlling the flow of drilling muds to the processing of concrete. Recent studies have shown that shear thickening in dense colloidal suspensions arises from the formation of contacts between the suspended particles, which act to build a force supporting network that aligns with the flows. Previous work on continuous shear thickening has shown that when perturbed orthogonally to the flow direction these networks can be broken apart allowing for complete dethickening of the suspension. This award will support experiments, simulations, and theory aimed at addressing knowledge gaps that would aid development of more efficient and broadly applicable dethickening strategies by tackling three outstanding questions in the area of shear thickening. First, it is proposed to use experiments and simulations to determine whether the orthogonal shear technique can be extended to the discontinuous shear thickening and shear jamming regimes. Second, it is proposed that new shear protocols be developed to determine the tensorial nature of the viscosity. These measurements along with the simulations will be used to construct a tensorial model of the viscosity. Such a theory would aid in determining the contact network in other flow geometries. Third, a state of the art imaging confocal rheology technique will be used to image the contact network giving rise to thickening. These data will enable testing of theoretical predictions for the contact network. Collectively, these projects will yield a deeper understanding of shear thickening aimed at improving the efficiency of applying orthogonal shear and extending its range of application to other flows. This award will promote the progress of science and ultimately benefit the US economy and society. Additionally, a number of educational activities are proposed ranging from outreach focused around thickening fluids to continued development of a science communication course for graduate students and postdocs.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.