Collaborative Research: Impact of a colloidal suspension droplet: Suspension flows at extreme shear rates

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Non-technical Abstract:An impacting liquid drop is not only an eye-catching phenomenon, it is also an ideal testbed for studying material properties. This is due to the high speed at which an impacting drop spreads out (up to 10 m/s); this spreading process has been carefully studied in ordinary fluids, but little work has been done to understand how this impact behavior is modified in a complex fluid. Complex fluids such as ketchup, paint, and oobleck (cornstarch-in-water) have the property that their viscosity (resistance to flow) depends on the applied stress. Thus, these fluids flow quite differently depending on the stress they experience. This research project explores how this stress-dependent viscosity alters drop-impact behavior. Complex fluids are ubiquitous, and a deeper understanding of their behavior in impact processes is essential to industries such as food and drug processing, additive manufacturing, and printing/coating. In addition to the research synergy between PI Driscoll and Co-PI Cheng, the collaboration allows a unique opportunity for expanding students’ training: a personnel exchange has been implemented, so that the students involved in the project spend two weeks each year at their non-home institution. This personnel exchange provides trainees opportunities to connect with a larger network of scientists, as well as the experience of another discipline and institution. Additionally, the research team leverages the stunning and eye-catching phenomena of drop impact to design demos to educate the broader public about the fascinating properties of complex fluids. Technical Abstract:Liquid drop impact is a classic problem in fluid mechanics and soft materials and has been studied extensively for many decades. In comparison, studies focusing on the impact of non-Newtonian fluid droplets, particularly suspension droplets, have been few and far between, even though such processes are essential in industries such as food and drug processing, additive manufacturing, and printing/coating. This project addresses this knowledge gap by conducting an extensive study of impacting colloidal suspension droplets, providing a full understanding of the dynamics of suspension-drop impact and drawing a direct comparison between Newtonian and non-Newtonian drop impact processes. This systematic study varies control parameters such as suspension volume fraction, particle shape, impact energy, and target size. Using both advanced imaging techniques, as well as new tools to measure impact forces and stress distributions, the team aims to fully characterize both the kinematics and dynamics of impacting suspension droplets. Suspension-drop impact provides an ideal model system to probe suspension dynamics under complex and extreme conditions: a spreading droplet generates shear rates at least an order of magnitude higher than those that are accessible via standard rheometry. In addition, the isolated air-fluid interface in drop impact imposes a unique open boundary rarely seen in other circumstances. Thus, this project provides crucial missing information on suspension dynamics under extreme conditions, complementary to the current understanding of suspension flows obtained in conventional bulk rheological tests.This Division of Materials Research (DMR) grant supports research to study impacting colloidal suspension droplets between Newtonian and non-Newtonian drop impact processes with funding from the Condensed Matter Physics (CMP) Program in DMR of the Mathematical and Physical Sciences (MPS) Directorate.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.