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

该补助金支持研究，以追求在软纳米多孔材料的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致动，其中吸附是指离子与带电表面的结合。具有低于100 nm的特征孔径的软纳米多孔材料在自然界中普遍存在（例如，纤维素，蛋白质）和工程应用（例如，水泥、凝胶、纳米复合材料）。这些材料通常在流体/气体的吸附/解吸时表现出显著的溶胀/收缩，这是由于它们的网络拓扑结构和界面相互作用引起的纳米限制效应。大自然利用纤维素纳米纤维的这种刺激响应特征来促进植物种子在湿度变化时的传播。生物启发的软纳米多孔材料最近已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工肌肉。该项目将建立并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。该项目还将推行一项教育计划，涉及多尺度孔隙力学和大学预科外展的新课程开发，利用围绕纳米工程材料的兴奋，并利用参与机构的创新和教育的特殊基础设施。这项研究是由假设，吸附和变形之间的复杂耦合，在纳米多孔介质中可以预测通过专注于两个关键的孔隙尺度属性，即分离压力和表面张力引起的固体吸附相互作用。为了验证这一假设，本研究将首先建立一个由混合物热力学指导的连续介质理论，即，通过将材料视为固体、流体和表面相的叠加，通过该叠加，模糊的孔隙尺度力表现为作用在多孔骨架上的宏观尺度吸附应力。然后，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。具体而言，将基于吉布斯对纳米限制流体膜的过度处理来开发简化的孔隙模型，以将由吸附引起的孔隙尺度力与实验可测量的量（即，吸附等温线）。将通过对流体吸附的纳米孔的MD模拟来验证孔模型。这些微尺度的力量，然后将通过统计均匀化放大，以完成孔隙力学框架。最后，将该理论应用于模拟无定形纤维素与水蒸气相互作用的吸附变形行为。预测将验证对实验数据和MD模拟结果从相同的材料系统。这项研究将挑战当前的孔隙力学范式，其中单个孔隙内的短程相互作用和表面力通常被忽视。如果成功的话，这项研究将大大扩展我们对活性和软多孔材料力学的基本理解。这个奖项反映了NSF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.