Appendix D: Impacts of Gravity and Surfactants on Drainage Flow and Rheology of Wet Foams - Z.Li

The objective of the proposed research is to investigate the dynamic and morphological evolution of wet liquid foams affected by the variations of surface tension and gravity. Foams comprised of polyhedral bubbles separated by thin films are widely used in many important industrial and commercial applications including cosmetics, dermal drug delivery, food industry, sealing and firefighting products. Liquid foams are intrinsically unstable because of their large interfacial energy, especially when the films connecting bubbles become thinner under the influence of the Earth’s gravity. Because most of the liquid in a foam is confined to the edges of polyhedral bubbles and connecting thin films, both the surface tension and the gravity play an import role in the formation of foam and its subsequent evolution or aging. Although the evolution of wet foams in microgravity has been recently studied by a FOAM-C experiment in the International Space Station (ISS), critical knowledge on multiscale features of foams is missing that would allow a full understanding of the foam evolution and stability, where the major challenge exists in how to best connect the multiple length scales coupling nanoscale thin films, mesoscale polyhedral structures, and bulk rheology of foams. We hypothesize that the evolution and aging of foams are induced by the drainage flow of confined liquid in the foam, which can be resolved by stabilizing the liquid films and reducing the fluid diffusion across the films between bubbles. We aim to use wet foams made of closely packed gas bubbles in a liquid, typically water with surfactant additives, to quantify the impacts of gravity and surfactants on drainage flow and foam rheology.

A better understanding of the relationships between nanoscale fluid dynamics of thin films and the evolution of mesoscale polyhedral structures would have a crucial impact on connecting nano-to-meso scale features in wet foams, and this research thrust (RT) is listed as RT-1. Another scientific challenge for wet foams is to build the connection between the mesoscale polyhedral structures and macroscale rheology of foams, which is listed as RT-2. Based on the PI’s preliminary innovation work on multiscale modeling of complex fluids and mesoscopic numerical methods, the PI will adopt multi-phase dissipative particle dynamics (DPD) models with different level of resolutions (different fidelity) to characterize the multiscale features of flow physics in wet foams. Specifically, high-resolution DPD models will be employed for RT-1 to investigate the impacts of surfactants and gravity on the rupture of thin films and aging of foams induced by drainage and gas diffusion. Low-resolution DPD models and a data-driven neural network approach will be used for RT-2 to study how foams’ microstructure changes the bulk rheology properties of wet foams, where computed DPD results will be taken as training data for a deep operator network (DeepONet) model to learn the effective viscoelastic equations of wet foams.

The proposed multi-fidelity framework can naturally incorporate the select-scale fluid physics into the multiphase models with consideration of intrinsic thermal fluctuations. Using this framework that tightly couples multiscale physics with state-of-the-art computational techniques, end-users of this modeling framework will be able to efficiently bridge the gap between nanoscale physics in thin films and bulk foam rheology, which will improve our fundamental understanding of the multiscale features in wet foams and help us create better foam microstructures for improved stability and lifetime of foam products. The multi-fidelity computational models for wet foams developed in this project will be compatible for integration into NASA’s ImMAC software design suite and will lead to new complex fluids applications to fulfill NASA missions.

This is a mature technology (TRL 7+) — the realistic path in is usually NASA's Technology Transfer Program: licensing an existing NASA patent, or a Space Act Agreement to use NASA facilities/expertise directly. NASA also runs a startup licensing program with no upfront fee for companies formed to commercialize a specific NASA technology.

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