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

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.