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

这笔赠款支持对软纳米多孔材料中的吸附-变形耦合进行基本了解的研究。这项研究将发展相应的力学理论，旨在更好地预测复杂纳米孔介质中的吸湿运动，并通过设计控制吸附诱导的致动，其中吸附是指离子与带电表面的结合。具有低于100 nm特征孔径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)中普遍存在。由于其网络拓扑和界面相互作用产生的纳米限制效应，这些材料在流体/气体的吸附/解吸过程中经常表现出显著的膨胀/收缩。自然界利用纤维素纳米纤维的这种刺激反应特性来促进植物种子在湿度变化时的传播。生物激发的软纳米多孔材料最近被开发出来，用于快速可靠的致动器、传感器和通过吸附溶剂分子驱动的人造肌肉。这个项目将建立和验证一个多尺度力学框架，该框架由孔尺度热力学和分子模拟提供信息，用于预测纳米多孔材料的吸附诱导应变。该项目还将通过利用围绕纳米工程材料的兴奋，并将其与参与机构的卓越创新和教育基础设施相结合，实施一项教育倡议，涉及多尺度孔力学和大学前推广的新课程开发。这项研究是基于这样一个假设，即纳米多孔介质中的吸附和变形之间的复杂耦合可以通过关注两个关键的孔尺度属性来预测，即固体-吸附相互作用引起的分离压力和表面张力。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，即通过将材料视为固体、流体和表面相的叠加，通过该相涂抹的孔尺度力表现为作用于多孔骨架的宏观尺度吸附应力。然后将通过分子动力学(MD)模拟和替代孔模型来寻找孔标度力的表达式。具体来说，简化的孔隙模型将基于Gibbs对纳米受限流体薄膜的过度处理，以将吸附诱导的孔隙尺度力与实验可测量的量(即吸附等温线)联系起来。孔模型将通过流体吸附纳米孔的分子动力学模拟来验证。然后，这些微尺度的力将通过统计均化来放大，以完成孔隙力学框架。最后，将该理论应用于无定形纤维素与水蒸气相互作用的吸附-变形行为的模拟。用实验数据和同一材料体系的分子动力学模拟结果对预测结果进行了验证。这项研究将挑战目前的孔隙力学范式，在这种范式中，单个孔隙内的短程相互作用和表面力通常被忽视。如果成功，这项研究将极大地扩展我们对活性和柔软多孔材料力学的基本理解。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.