Multi-scale Study of Coupled Reaction and Wetting in Droplet Spreading

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TECHNICAL SUMMARYThis award supports theoretical and computational research and educational activities related to coupled reaction and wetting dynamics in droplet spreading. Special attention is paid to examine the effect of intermetallic compound formation on drop wetting kinetics, contact line advancement, and the evolution of the solid-liquid interface. Using a multi-scale model that integrates the hybrid phase-field and arbitrary Lagrangian-Eulerian approach at the macroscopic scale and molecular dynamics simulations at the atomistic level, this project seeks to address several fundamental issues in the understanding and prediction of reactive wetting: (i) What are the dominating driving forces and dissipation mechanisms in reactive wetting? The PI aims to understand whether the relevant driving forces and dissipation mechanisms can be separated into distinct regimes or whether they are strongly coupled in one or multiple regimes. Using inputs from molecular dynamics simulations, the PI will integrate the intermetallic compound formation and growth into the macroscopic moving boundary problem to examine the coupling mechanisms among flow, species transport, and chemical reaction.(ii) How is material delivered to the contact line during new interface formation? The PI will examine the effect of reaction and surface alloying on mechanisms that deliver new materials to the contact line region. The effects of intermetallic compound formation and growth in the contact line region on drop wetting kinetics will be quantified.(iii) How does the solid-liquid interface evolve during coupled reaction and wetting? The flow, temperature, and concentration fields will be computed to explain how they interact to result in a certain shape of the solid-liquid interface and types of intermetallic compounds. The PI will also quantify the intermetallic compound growth rate and relate it with the rate of substrate dissolution. The possibility of solid to liquid to solid phase transition will be explored and results will be compared with experimental observations from the literature.Fundamental understanding of coupled reaction and wetting dynamics during reactive wetting is crucial in creating stronger bonds in materials joining, better adhesion for thin film coating, novel composites for bio-implants, and new routes for surface modification with tunable functionalities. This work will have impact on areas ranging from materials processing, MEMS fabrication, electronics packaging, to energy conversion and storage and surface chemistry. This project will also advance a largely uncharted area of research that is concerned with multi-component, multi-phase systems with flow, heat/mass transfer, phase change, and chemical reactions. Graduate and undergraduate students will be trained working in an interdisciplinary research area at the intersection of physics, materials science, engineering, and chemistry. Collaborations with McMaster University and Sandia National Laboratories will enable the PI and her students to interact with leading research groups on performing atomistic modeling of phase transformation. The proposed research will also enable new course materials for two graduate-level courses and support undergraduate researchers via Drexel's Hess Honors program and the six-month intensive research co-op program. Outreach will extend to pre-college students and those from underrepresented groups through PI's mentoring of RET teachers and Girl Scouts of Eastern Pennsylvania from School District of Philadelphia.NONTECHNICAL SUMMARYThis award supports theoretical and computational research and educational activities related to how a droplet makes contact with a surface and spreads. The PI will also include the reaction of the liquid with the solid substrate on which it is spreading. This plays an important role in many technological applications including materials processing and joining, thin film coating, printable electronics fabrication, heterogeneous catalysis, droplet actuation and manipulation, and surface modification. Recent advances in materials that are structured down to a scale some ten thousand times smaller than a human hair require greater precision in the control of this reactive wetting processes. Despite its importance, reactive wetting remains a poorly understood process. The PI will perform computer simulations aimed to address fundamental issues in understanding reactive wetting and the ability to use computers to simulate the process. Model predictions will be validated against experimental observations. This project is important in materials processing and joining, thin film coating, and droplet manipulation processes. The process involves at least three interfaces that are typically away from the balanced state of equilibrium and plays a significant role in synthesis and processing of advanced materials for energy conversion and storage, bio-implants, micro-electro-mechanical devices, and electronics packaging. The complex multi-phase systems studied by the PI occur in numerous process industries, as well as in nature and in living organisms. Thus, this work will help to establish research methodology that can be used in a broad array of applications.The interdisciplinary nature of the project, at the intersection of physics, materials science, engineering, and chemistry, makes it a great educational opportunity for students. Graduate students and undergraduate students will be trained to utilize interdisciplinary tools in fluid dynamics, heat/mass transfer, and materials science to study transport phenomena in materials processing across length scales. The proposed research will also enable new course materials for two graduate-level courses. Research products will be disseminated in part through open-source computer code packages. The community outreach programs will extend to inner-city K-12 teachers and students in Philadelphia with approximately 80% minority students.