Collaborative Research: Multiscale Mechanics of Adsorption-Deformation Coupling in Soft Nanoporous Materials

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This grant supports research to pursue a fundamental understanding of adsorption-deformation coupling in soft nanoporous materials. The research will develop corresponding mechanical theories, aiming to better predict hygroscopic movements in complex nanoporous media and control sorption-induced actuation by design where sorption refers to the binding of ions to charged surfaces. Soft nanoporous materials having characteristic pore sizes below 100 nm are ubiquitous in nature (e.g., cellulose, protein) and in engineering applications (e.g., cement, gel, nanocomposites). These materials often exhibit significant swelling/shrinkage upon adsorption/desorption of fluids/gases due to nanoconfinement effects resulting from their network topology and interfacial interactions. Nature uses such stimuli-responsive features of cellulose nanofibers to facilitate the dispersal of plant seeds upon humidity change. Bio-inspired soft nanoporous materials have been recently developed for fast and reliable actuators, sensors, and artificial muscles driven by sorption of solvent molecules. This project will establish and validate a multiscale mechanics framework informed by pore-scale thermodynamics and molecular simulations for predicting the sorption-induced straining of nanoporous materials. The project will also pursue an educational initiative involving new course development on multiscale poromechanics and pre-college outreach by harnessing the excitement surrounding nano-engineered materials and leveraging it with the exceptional infrastructure for innovation and education at the participating institutes. This research is driven by the hypothesis that the complex coupling between sorption and deformation in nanoporous media can be predicted by focusing on two key pore-scale attributions, namely the disjoining pressure and surface tension induced by solid-adsorbate interactions. To test this hypothesis, the study will first establish a continuum theory guided by the thermodynamics of mixtures, i.e., by viewing material as a superposition of the solid, fluid and surface phases, through which the smeared pore-scale forces appear as macroscale adsorption stresses acting on the porous skeleton. Expressions of pore-scale forces will be then sought via molecular dynamics (MD) simulations and surrogate pore models. Specifically, simplified pore models will be developed based on Gibbs’ excess treatment of nanoconfined fluid films to link pore-scale forces induced by sorption with experimentally measurable quantities (i.e., adsorption isotherm). The pore model will be validated by MD simulations of nanopores subjected to fluid adsorption. These microscale forces will then be upscaled via statistical homogenization to complete the poromechanics framework. Finally, the theory will be applied to model the sorption-deformation behavior of amorphous cellulose interacting with water vapor. The prediction will be validated against experimental data and MD simulation results obtained from the same material system. The research will challenge the current paradigm of poromechanics where short-range interactions and surface forces within individual pores have been routinely neglected. If successful, the research will greatly expand our fundamental understanding on mechanics of active and soft porous materials.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.