Phase Behavior, Filling Dynamics, and Packing of Fluids inside Isolated Carbon Nanotubes

Abstract

Fluids behave differently inside nanoscale pores than they do in bulk solution. When confined inside so-called single digit nanopores – pores with diameters smaller than 10 nm – the atomic configuration, phase behavior, and dynamics of fluids vary markedly from their bulk behavior and depend sensitively on the confining diameter. Understanding fluid behavior at these scales is critical to the design of a wide variety of engineering systems, such as membranes for chemical separations and batteries for energy storage. The study of nanofluidics also informs our understanding of natural phenomena, including the single-file transport of water into biological cells and flow through nanoporous geologic media.

In this thesis, we develop experimental platforms to study confinement effects on fluid packing, filling, and phase behavior inside isolated, substrate-bound carbon nanotubes with diameters ranging from 0.8 nm to 3 nm. Carbon nanotubes are grown by chemical vapor deposition on marked silicon substrates and segmented by photolithography or use of a focused ion beam, producing multiple, identical segments of the same diameter and chirality carbon nanotube. By Raman spectroscopy, it is possible to determine on the micrometer length scale and second time scale whether an isolated carbon nanotube is empty, fluid-filled, or partially fluid-filled as a function of location, time, temperature, and nanotube diameter.

After building precision nanopore systems and developing techniques to characterize nanopore filling, we address several topics of interest to the field of nanofluidics, as explored in the chapters of this thesis. First, we explore knowledge gaps in nanofluidics, including gaps in our understanding of phase behavior and dynamics of fluids under conditions of extreme confinement. Second, we study the diameter dependence of fluid packing and filling inside carbon nanotubes, showing that the variation in the change of the Raman radial breathing mode upon fluid filling is indicative of configurational changes in water inside nanotubes of different sizes. Third, we develop continuum elastic shell theories to explain why double-walled nanotubes, but not single-walled nanotubes, can distinguish between interior fluid filling and exterior fluid adsorption by changes in radial vibrations alone. Fourth, we perform a thermodynamic analysis of water-filled, closed carbon nanotubes, calculating enthalpies of phase change from a Clausius-Clapeyron type expression for nanoconfined water. Fifth, we perform a thermodynamic analysis of water-filled carbon nanotubes in a thermodynamically open system, observing a phase change driven by variable laser heating and calculating enthalpies of adsorption by comparison to a Langmuir-type adsorption model. Sixth, we observe dynamic changes in filling state with time, calculating diffusion coefficients of vapor-like and liquid-like water inside carbon nanotubes as a function of diameter. Finally, as an aside, we perform a computational analysis of a heated mask.

Measurement of water inside carbon nanotubes, as explored in this thesis, expands our view of the thermodynamic and kinetic properties of fluids under confinement and addresses key knowledge gaps in nanofluidics. These measurements can inform new theories, force fields, and mechanisms for fluids in nanoconfined environments.