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

该基金支持对软纳米多孔材料中吸附-变形耦合的基本理解。该研究将发展相应的力学理论，旨在更好地预测复杂纳米多孔介质中的吸湿运动，并通过设计控制吸附诱导的驱动，其中吸附是指离子与带电表面的结合。孔径小于100纳米的软纳米多孔材料在自然界（如纤维素、蛋白质）和工程应用（如水泥、凝胶、纳米复合材料）中无处不在。由于其网络拓扑结构和界面相互作用产生的纳米限制效应，这些材料在吸附/解吸流体/气体时通常表现出显著的膨胀/收缩。大自然利用纤维素纳米纤维的这种刺激响应特性来促进植物种子在湿度变化时的传播。仿生软纳米多孔材料最近被开发用于快速可靠的致动器、传感器和由溶剂分子吸附驱动的人造肌肉。该项目将建立并验证一个基于孔隙尺度热力学和分子模拟的多尺度力学框架，以预测纳米多孔材料的吸附诱导应变。该项目还将推行一项教育倡议，包括开发多尺度孔隙力学的新课程，并通过利用纳米工程材料的兴奋感，将其与参与机构的卓越创新和教育基础设施相结合，开展大学预科教育。该研究基于这样一个假设，即通过关注固体-吸附物相互作用引起的分离压力和表面张力这两个关键孔隙尺度属性，可以预测纳米多孔介质中吸附和变形之间的复杂耦合。为了验证这一假设，该研究将首先建立一个以混合物热力学为指导的连续统理论，即将材料视为固体、流体和表面相的叠加，通过这种叠加，涂抹的孔隙尺度力表现为作用在多孔骨架上的宏观吸附应力。然后将通过分子动力学（MD）模拟和替代孔隙模型寻求孔隙尺度力的表达式。具体来说，将基于Gibbs对纳米流体膜的过量处理，开发简化的孔隙模型，将吸附引起的孔隙尺度力与实验可测量的量（即吸附等温线）联系起来。孔隙模型将通过流体吸附纳米孔的MD模拟来验证。然后，这些微尺度的力将通过统计均匀化放大，以完成孔隙力学框架。最后，该理论将应用于模拟非晶态纤维素与水蒸气相互作用的吸附变形行为。该预测将通过实验数据和同一材料体系的MD模拟结果进行验证。该研究将挑战目前孔隙力学的范式，即单个孔隙内的短程相互作用和表面力通常被忽略。如果研究成功，将大大扩展我们对活性和软多孔材料力学的基本认识。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.