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

许多天然岩层是多孔性的，当孔隙中充满水、石油和其他流体时，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性质(可压缩性)。与其他岩石不同，页岩地层在过去十年里越来越受到关注，它的孔隙大小在纳米级，每个孔隙可以容纳数百个流体分子。这种受限流体的许多物理和化学性质与它们的主体流体不同，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主体流体。该项目将有助于促进对限制对流体影响的基本了解，并将为石油和水资源勘探和温室气体封存产生新的计算方法。通过STEM劳动力发展将产生更多的社会影响-NJIT拥有全国最多元化的学生群体之一，为团队提供了与STEM中代表性不足的背景的研究型学生接触的机会。此外，该项目将通过在研究小组成员教授的课程中加入与提案有关的主题，促进改善STEM教育。在夏季的几个月里，研究小组将与社区大学和高中生合作，包括针对经济困难的高中生的ACS SEED计划的参与者。该研究计划的目标是开发一种能够量化限制对流体弹性性质的影响的分子理论，并使用该理论来预测波在流体饱和的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成分的弹性模数决定。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会发生变化，例如密度、冰点、扩散系数等。最近的超声波实验表明，超声在饱和了液体Ar、N、正己烷和水的纳米多孔玻璃中的传播速度与大孔介质的预期速度不同，这表明这些流体在纳米孔中的弹性特性(体积弹性系数或可压缩性)也与体积值不同。研究小组假设，当孔隙大小与分子大小相当时，限制会改变所有流体的弹性性质，这种变化的程度取决于孔隙/分子大小比和固体-流体相互作用的强度。为了验证这一假设，研究小组将探索以下问题：1.受限如何影响化学成分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受限流体的可压缩性？3.纳米受限是否会导致流体的有限剪切模数？4.如何对多孔介质中的波传播理论进行修正？为了回答这些问题，分子尺度的模拟工作将集中于限制在不同大小和表面性质的孔中的短烷烃。实验将集中在多孔玻璃中的受限烷烃和具有明确定义的1-10 nm孔径和不同表面化学成分的碳干凝胶作为模型多孔介质。超声波在这些介质中传播的测量将提供弹性性质，可用于直接验证分子模拟预测。建模和实验结果将有助于修改波在流体饱和的多孔介质中的传播理论，以解释纳米孔。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。

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.