Uncovering the microscopic origins of nonlinear rheology in glassy nanocolloidal suspensions

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Soft solids such as gels, pastes, and emulsions will change their shape and flow when subjected to sufficient forcing. Understanding and controlling this behavior is central to the processing of these materials in applications ranging from ceramics and pharmaceuticals to food and personal care products. At the microscopic level, this deformation and flow involves rearrangements of the constituents that are often difficult to identify and characterize. Furthermore, once flow stops, the force needed to maintain a soft solid's new shape can display a long, slow evolution with time that depends on details of the preceding flow. This phenomenon provides a potential mechanism for tailoring the properties of the soft solids; however, the microscopic motions within the material that underlie this slow evolution are poorly understood. This award will support a novel experimental investigation to identify the key microscopic motions associated with the onset and cessation of flow in soft solids and to connect these motions with the solids' macroscopic properties. A central feature is a technique that uses x-rays to provide unprecedented resolution of the motions within materials at lengths as small as a molecule. The development of these x-ray methods for investigating soft solids under deformation and flow will have impact far beyond the planned experiments. This new tool has potential applications that span physics, chemical engineering, and the biosciences. Graduate and undergraduate students who will be trained in advanced x-ray methods will further contribute to the talent pool needed to advance research and development in the United States. In addition, the award will provide Baltimore City public school students with opportunities for research internships in the Investigator's laboratory through partnership with a local magnet science high school.Soft disordered solids can possess characteristic mechanical properties, such as the emergence of a yield stress, shear thinning upon flow, and thixotropy. This deformation and flow behavior is central to the utility of these materials in a range of technologies and provides a point of contact with theoretical concepts such as jamming and soft glass rheology. At the root of the nonlinear rheology are shear-induced changes to the internal microstructure and microstructural dynamics of the soft solids. Understanding the connections between these microscopic structural dynamics and macroscopic rheological properties remains a central goal for the fields of mechanics and colloid science. This effort is made challenging by the strong spatial and temporal fluctuations that can characterize the microscopic response to stress and by the out-of-equilibrium nature of many soft disordered solids, which causes their properties to depend not only on thermodynamic conditions but also on their history. The proposed research will take advantage of the unique strengths of x-ray photon correlation spectroscopy (XPCS) for illuminating the salient nanometer-scale structural dynamics in soft materials to bring insightful perspectives to this problem. Specifically, the research will leverage and expand newly developed experimental capabilities that combine XPCS with rheometry in novel ways to reveal the particle-scale and mesoscale dynamics at the origins of macroscopic deformation and flow. Through three specific and inter-related projects focused on nanocolloidal suspensions that form soft-glass and concentrated-gel phases, the experiments will elucidate the microscopic processes leading to stress relaxation, yielding, and aging in ways that enable tests of prevailing theoretical ideas regarding the mechanical properties of soft disordered solids.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.