Dynamics of granular raft on a moving interface: assembly, structure, and property

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The “Cheerio’s effect” refers to the spontaneous aggregation of floating particles on the fluid-fluid interface through capillary interactions. The particles floating on the interface can form stable particle rafts with unique properties that have inspired technological innovations such as liquid marbles and bicontinuous gels. In addition, aggregates of large granular particles on water surfaces are commonly seen in the open environment, ranging from the assembly of mosquito eggs, shade balls to prevent evaporation of reservoirs, to sand particles deployed to clean oil spills. Particle rafts of environmental importance are naturally subject to perturbations due to flow, which can affect the collective dynamics and stability of the raft. Yet, the connections between the dynamics of fluid-fluid interfaces and particle assembly remains unclear. This project aims to address the issue by combing tabletop experiments with a theoretical investigation on the dynamics of granular rafts on moving interface. The results of the research will provide not only a fundamental understanding of the behavior of particle rafts in natural contexts, but also a new engineering tool to design and tailor macroscopic properties of particle assembly. The project will include an educational plan that emphasizes engagement of undergraduate students. The students and post-docs involved in the project will have outreach and networking opportunities with industrial partners though the industrial program at University of Minnesota. In addition, the visual nature of the research provides a good opportunity for integrating commonplace surface tension phenomena, such as “Cheerio’s effects”, in undergraduate fluid mechanics courses. The overall objective of this research is to gain fundamental understanding of particle rafts on a moving fluid-fluid interface, ranging from the assembly to macroscopic properties of the rafts. A particle raft is a monolayer of particles trapped at the interface between two fluids. As unique low-dimensional materials comprising discrete elements, particle rafts have a wide range of technical applications. However, the focus of the state of the art is on the particles themselves, while the fluid-fluid interface serves mostly as a passive site with a globally fixed geometry onto which particles can self-organize. Only limited studies have considered the dynamics of the rafts made of large granular particles, where gravity plays a dominant role. By combining experiments and theory, this project will directly address the role of the moving fluid-fluid interface in the assembly process and the mechanical properties of granular rafts. Experimentally, the PIs will investigate the influence of the moving interface on the structure of granular rafts and characterize the mechanical properties of resulting rafts via linear and nonlinear compression tests. In parallel to experiments, the PIs will develop a continuum model incorporating the intrinsic gravity-driven dynamics missing in the current theory of colloidal rafts. Together, the theoretical model and experimental measurements will provide a comprehensive physical understanding on the formation and the response of granular rafts under different interfacial perturbations.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.