The growing demand for environmentally friendly and healthier products will require from the process industry the use of predictive models of formulation performance for rapid and effective screening of new products. In addition, an increasing regulatory pressure means that in the coming decade many products will need to be re-formulated, at a huge cost for the industry, because by and large, product development is still done by trial-and-error. One of the bottlenecks is the characterization and understanding of complex fluid behavior in realistic operating conditions: many industrial flows apply deformations to complex fluids at high frequency, typically in excess of 1,000 Hz, often beyond the capabilities of commercial instruments. On the other hand, predictions based on properties measured at lower frequency are not sufficiently accurate for materials or process design. The ability to explore this dynamic regime of deformation of complex fluids will not only provide crucial data for the design of new formulations, but can also unveil unexpected emergent phenomena that have so far remained inaccessible to experimental observation. In our lab we have developed an acoustofluidics approach to deform and probe complex fluids at high frequency (10,000 Hz). This approach enables us to reproduce processing flow conditions on a microscope stage and directly observe dynamical phenomena on the microscale. Using this method, we are gaining new insights into the complex behavior of yield-stress fluids, which are widely used in foods and construction materials owing to their elastic-solid behavior below a threshold in applied stress. We have investigated whether deformations induced by embedded bubbles activated by acoustic field can cause local yielding of the material and promote bubble removal, which is desirable in many applications. We developed a theoretical model describing acoustic bubble dynamics in yield-stress fluids, to estimate the critical acoustic pressure required to initiate yielding, and conducted experiments in a Carbopol microgel. Combining precision dynamical measurements with direct visualization of the microstructure of complex fluids under high-frequency deformation will pave the way to new manipulation strategies for advanced materials and processes.

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