Swirling Propulsion in Complex Fluids and Micro-Swimming Rheometry

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The movement of micro-organisms is often complicated by the complexity of the fluids in which they reside. Many of the fluids (e.g., mucous) in which these organisms “swim” contain large macromolecules such as proteins that limit successful swimming. It is more difficult to swim through these “sticky” fluids. It has recently been suggested theoretically that a type of microbial swimming called “swirl” can create propulsion in complex liquids that would not be possible in “simple,” less sticky fluids. Swirl is characterized by parts of the body spinning around the axis of an axisymmetric body. Swirl is a key feature of the swim stroke of many micro-organisms. This award will develop a novel micro-robot that demonstrates the characteristics of swirl propulsion in complex fluids and uses that propulsion to measure the properties of the surrounding fluid. Thus, it is a dynamic sensor of complex fluid properties or a “swimming rheometer”. Standard rheometers are desktop devices where fluid is brought to the device and the native environment of the fluid application is reproduced in the device. The state of stress in the shear flow of complex fluids requires the measurement of at least three material properties as a function of the shear rate in the fluid. The goal in the present award is that through design, miniaturization, and optimization of a newly developed “swirling” robot, rheometry will be transformed to a remote, in situ sensing science. Thus, one will bring the “rheometer to the fluid application” rather than bringing a fluid sample to a fixed rheometer. A tennis ball-size prototype has already been created and the primary concepts are successfully demonstrated. The robot was designed and will continue to be optimized via large scale computer simulation. The miniaturized robot will have immediate medical and biological applications including measuring the complex rheological properties of synovial fluid as a direct mapping to several disease conditions. Moreover, multiple miniaturized robots will be used to examine the collective dynamics of these micro-swirlers.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.