Elastic Properties of Confined Fluids and their Role for Wave Propagation in Nanoporous Media

许多天然岩层是多孔的，当孔隙被水、石油和其他流体饱和时，就会产生具有独特性能的复合材料。地球物理学家依靠机械（声波）波来探索这些复合材料的特性。波在流体饱和岩石中的传播速度取决于岩石和孔隙中流体的弹性特性（可压缩性）。与其他岩石不同的是，在过去十年中引起越来越多关注的页岩地层，其孔隙大小在纳米范围内，每个孔隙可容纳数百个流体分子。这类受限流体的许多物理和化学性质与体积流体不同，本项目将研究纳米孔中碳氢化合物流体的弹性性质如何偏离体积。该项目将有助于推进对约束对流体影响的基本理解，并将为石油和水资源勘探以及温室气体封存产生新的计算方法。通过STEM劳动力的发展将实现额外的社会影响——NJIT拥有全国最多样化的学生群体之一，为团队提供了与STEM中代表性不足的背景的研究学生接触的机会。此外，该项目将通过在研究团队成员教授的课程中纳入与提案相关的主题，为改善STEM教育做出贡献。在夏季的几个月里，研究小组将与社区大学和高中生一起工作，包括为经济困难的高中生提供的ACS SEED项目的参与者。本研究计划的目标是发展一种分子理论，能够量化限制对流体弹性特性的影响，并利用该理论预测波在流体饱和纳米多孔介质中的传播。波在这种介质中的传播由固体和流体组分的弹性模量决定。然而，当流体被限制在纳米孔中时，它们的许多物理化学性质与体积相比发生了变化，例如密度、冰点、扩散率等。最近的超声波实验表明，超声波在饱和了液态氩气、氮气、正己烷和水的纳米多孔玻璃中的传播速度与大孔介质中的传播速度不同，这表明纳米孔中这些流体的弹性特性（体积模量或压缩率）也偏离了体积值。研究小组假设，当孔隙大小与分子大小相当时，约束将改变所有流体的弹性特性，这种变化的程度取决于孔隙/分子大小比和固液相互作用的强度。为了验证这一假设，研究团队将探索以下问题：1。约束如何影响化学性质相似但分子大小不同的流体的可压缩性？2. 孔隙表面的原子细节如何影响承压流体的可压缩性？3. 纳米约束能诱导流体的有限剪切模量吗？4. 波在多孔介质中的传播理论如何修改为纳米多孔介质？为了回答这些问题，分子尺度的建模工作将集中在限制在不同大小和表面性质的孔隙中的短链烷烃上。实验将集中在多孔玻璃和碳干凝胶中的限制烷烃，它们具有明确的1- 10nm孔径和不同的表面化学性质作为模型多孔介质。超声波在这些介质中传播的测量将提供弹性特性，可用于直接验证分子模型预测。模型和实验结果将有助于修正波在流体饱和多孔介质中的传播理论，以解释纳米孔隙。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。

Many natural rock formations are porous, and when the pores are saturated with water, petroleum, and other fluids, composite materials with unique properties result. Geophysicists rely on mechanical (sound) waves to explore the properties of these composite materials. Speed of wave propagation in fluid-saturated rocks depends on the elastic properties (compressibility) of both the rock and the fluid in the pores. Unlike other rocks, shale formations, which have attracted increasing attention in the past decade, have pores with sizes in the nanometer range, each fitting as few as hundreds of fluid molecules. Many physical and chemical properties of such confined fluids are different from those of their bulk counterparts and this project will study how elastic properties of hydrocarbon fluids in nanopores deviate from bulk. The project will help advance fundamental understanding of effects of confinement on fluids and will produce new computational methods for petroleum and water resource exploration and greenhouse gas sequestration. Additional societal impacts will be achieved through STEM workforce development - NJIT has one of the most diverse student populations in the country, providing the opportunity for the team to engage in research students with backgrounds that are underrepresented in STEM. Additionally, the project will contribute to improved STEM education through inclusion of proposal-related topics in the courses taught by the research team members. During summer months, the research team will work with community college and high-school students, including the participants of the ACS SEED program for economically disadvantaged high-school students.The objective of this research program is to develop a molecular theory capable of quantifying the effects of confinement on elastic properties of fluids and use this theory to predict wave propagation in fluid-saturated nanoporous media. Wave propagation in such media is determined from the elastic moduli of both solid and fluid constituents. However, when fluids are confined in nanopores, many of their physio-chemical properties change as compared to bulk, e.g., density, freezing point, diffusivity, etc. Recent ultrasonic experiments showed that the speed of ultrasound propagation in nanoporous glass saturated with liquid argon, nitrogen, n-hexane, and water differs from what is expected for a macroporous media, suggesting that the elastic properties of those fluids in nanopores (bulk modulus or compressibility) also deviate from the bulk values. The research team hypothesizes that confinement will change the elastic properties of all fluids when the pore size is comparable to the molecular size, and the extent of this change is determined by the pore / molecule size ratio and strength of the solid-fluid interactions. To test this hypothesis, the research team will explore the following questions: 1. How does confinement affect the compressibility of fluids with similar chemistry but different molecular sizes? 2. How do atomistic details of the pore surface affect the compressibility of the confined fluid? 3. Can nanoconfinement induce finite shear modulus for the fluid? 4. How does a theory of wave propagation in porous media have to be modified for nanoporous media? Molecular-scale modeling efforts to answer these questions will focus on short alkanes confined in pores of various sizes and surface properties. Experiments will focus on confined alkanes in porous glasses and carbon xerogels with well-defined 1-10 nm pore sizes and different surface chemistries as model porous media. The measurements of ultrasonic wave propagation in these media will provide elastic properties, which can be used to directly verify the molecular modeling predictions. Modeling and experimental results together will help to modify the theory of wave propagation in fluid-saturated porous media to account for the nanoporosity.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.