The PI discovered two phenomena, persistent holes and negative flows, which together suggest that shear-thickening complex fluids possess a vibration-activated stress-bearing mechanism. A persistent hole can maintain a stable free surface oriented parallel to gravity. Negative flows are flows that are opposite to the mean applied stress. Laboratory experiments and modeling will reveal the origin of these phenomena which the PI believes have the same physical origin. The PI will study the two classes and develop a rheological model based on these results in collaboration with Ronald Larson and develop and validate a free surface model based the flow measurements and the rheological model in collaboration with Rich Kerswell. The experiments will be conducted on a vibrating test platform and in a custom built shear cell using Particle Image Velocimetry. The rheology will be characterized with a commercial stress-controlled rheometer. The fluid structure will be characterized using particle tracking, light scattering microscopy, and confocal microscopy. The flow field measurements combined with the rheological model will help build and validate a hydrodynamic model of persistent holes and negative flows. The structural characterization will reveal the link between the microscopic structure and the rheological behavior. The results of this study will be of interest broadly to complex fluids community and specifically to the shear banding, shear thickening, and jamming communities. It will also be of interest to the many industries - from plastics to cosmetics to pharmaceuticals - in which non-Newtonian fluids are commonly used. This study has the potential to reveal a fundamental mechanism in complex fluids which may be as significant as normal stress or extensional viscosity. The PI's educational program builds on his YouTube video of persistent holes, that has been viewed over 2.5 million times and on an exhibit in the final stages of development at the San Francisco Exploratorium. In addition, the PI will develop and teach a new laboratory course on nonlinear science in the interdisciplinary Center for the Study of Complex Systems. Undergraduates participating in summer internships in the PI's lab will benefit from an increased awareness of career opportunities in science and competitiveness for graduate school entry.
Non-Newtonian fluids is a catchall term for fluids that display behaviors that simple fluids like water or honey do not. Examples include blood, oil, and pyroclastic flows. The flow of non-Newtonian fluids is crucial in industries from oil extraction to chemical processing to food preparation, and can dramatically affect our environment and our health. We are in the infancy of discovering the potential and understanding the behavior of non-Newtonian fluids. The research program funded by this grant contributed to this effort. Specifically, we studied the formation of localized waves on the surface of material composed of liquid and small solid particles, a particulate suspension. Consider poking a hole in a fluid by inserting and then rapidly withdrawing a solid object. In a Newtonian fluid the hole is unstable because hydrostatic pressure and surface tension act to close it. Surprisingly, in at least two types of non-Newtonian fluids, namely particulate suspensions and worm-like micellar solutions, such a hole can be maintained indefinitely by the application of vibrations; the initial hole evolves into a stable cylindrically-symmetric cavity extending from the free-surface to nearly the bottom of the container. The goal of our research was to uncover the stabilizing mechanism of these persistent holes. From our experiments we deduced that persistent holes are stabilized by normal forces. When a Newtonian fluid is sheared between two plates it exerts no force perpendicular to the plates. However, when a non-Newtonian fluid is sheared it either pushes the plates apart or pulls them together. We found that the fluid around persistent holes forms a ring of convective flow that pushes against neighboring fluid elements; this flow prevents the ring from contracting to zero radius as it would need to for the fluid to enter the void.
