Collaborative Research: Active and Nonlinear Microrheology

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PROPOSAL NO.: 0730292/0730270PRINCIPAL INVESTIGATOR: Furst, Eric M./Squires, Todd M. INSTITUTION: University of Delaware/University of California-Santa Barbara COLLABORATIVE RESEARCH: ACTIVE AND NONLINEAR MICRORHEOLOGY The past decade has seen the development of techniques in passive microrheology, where the Brownian motion of colloidal tracer particles is related to the linear viscoelastic properties of the surrounding material. Significantly benefits from microrheological measurements include: requiring mere microliters of sample, providing an extended range of frequencies, and having the ability to probe spatial rheological variation. For this reason, industry has shown significant interest in adapting and adopting microrheological approaches. It has long been appreciated that most industrial flows for processing complex fluids involve significant departures from linear rheology. However, passive microrheology is by nature incapable of measuring nonlinear rheological properties. Here, the goal is to develop the first microrheological techniques to measure nonlinear material properties. The collaborative effort will develop both experimental techniques (the use of laser tweezers and high speed confocal microscopy to measure the force and non-equilibrium structure as a colloidal probe is driven through a material) as well as the theoretical basis to understand and interpret the results. Currently, all theory and experiments have focused exclusively on direct probe-bath interactions, which play no role in macro-rheology and appear as artifacts in passive microrheology. To establish nonlinear microrheology as a general technique for material characterization, these issues must be understood and addressed, and well-founded methods for interpreting the results must be developed and validated. Having established the theoretical and experimental framework for these issues, the Principle Investigators anticipate using more sophisticated techniques (e.g. involving multiple and anisotropic probes) for more faithful measurements of nonlinear bulk rheology as well as normal stress measurements. The intellectual merit is in proposing to develop and refine the first techniques for nonlinear microrheology, thus establishing an entirely new area of rheology. Broader impact is the development of microrheology promises to measure nonlinear properties before scaling up production to industrial scale. Interactions with Procter and Gamble will speed transfer of these methods, both through collaboration and PhD industrial internships. Additionally, the PIs will leverage existing NSF-funded outreach programs to incorporate undergraduates, under-represented minorities and high-school students and teachers into their research efforts, providing educational opportunities in this economically and technically important field.