Effect of heterogeneous particles and surfactants on the stability and rheology of fluid interfaces

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Fluid-fluid interfaces, such as air-water interface in a soap bubble, are commonplace in a variety of products such as in household cleaning, cosmetics, and pharmaceuticals. Oftentimes, very small particles and some specific molecules (called surfactants) are used to stabilize such interfaces (to prevent them from popping). Therefore, the stability and strength of these interfaces is dictated by the type and strength of interactions between the small particles and surfactants. The goal of this project is to investigate the combined effect of particles and surfactants on the stability and flow behavior of interfaces. State-of-the-art laboratory experiments and computer simulations at the molecular level will be used in this research. This will lead to scientific advances in a wide range of technological applications from targeted delivery of drugs to recovery of oil trapped in underground reservoirs. The advanced experimental and computational techniques described in this project will also generate new knowledge to be used in educating students at both undergraduate and graduate level through the elective courses that will be offered by the investigators. The project will also support K-12 outreach activities that will be organized to demonstrate interfacial properties in soft materials to pre-college students in order to attract them to STEM disciplines.The proposed project will generate new fundamental knowledge on the interaction of heterogeneous colloidal particles and surfactant molecules in presence of interfaces. Tuning the assembly process using heterogeneous colloidal building blocks can advance the design of innovative materials and responsive structures. Additionally, the open nature of fluid interfaces makes them a promising platform for applications in which dynamic transport is crucial, such as bi-phasic catalytic reactions. Therefore, a critical knowledge gap exists in complex interfacial systems comprised of both heterogenous colloidal particles and surfactant molecules, especially in elucidating the rich physical mechanisms that affect the synergism of heterogeneous particles and surfactants at interfaces. Furthermore, fluid interfaces are not static and they are constantly subject to external disturbances; thus, there is a need to study the stability of these interfacial systems in response to applied stresses in order to develop a better understanding of their dynamic behavior and rheology in conditions relevant to real applications. Micro- and nanoparticles with several types of heterogeneities ? particularly shape and surface chemical heterogeneity ? will be fabricated and experimental analysis of particle-laden interfaces will be conducted in the absence of surfactants. Computations using coarse-grained techniques will be employed to understand the molecular interactions driving the experiments. The synergy between the experimental and computational tools will then be employed in more complex systems that involve both heterogeneous particles and surfactant molecules in order to address phenomena across multiple length and times scales (molecular scale interactions between micro- and nanoparticles and multifluid systems). The anticipated outcomes, rooted in fundamental understanding, include design rules for heterogeneous colloidal particles/surfactant systems with tailored interfacial behavior and rheology at fluid/fluid interfaces.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.