Nanoscale Fluid Transport

Nanoscale fluid transport is usually referred to the fluid flow through a channel with size along one or more directions below 100mm. At this small length scale, due to high surface area to volume ratio, the fluid-solid interaction at solid/fluid interface is one of the most dominant factors that governs fluid behaviour. Because of this unique feature many interesting transport phenomena occur on this length scale. In this chapter I provide the literature review on the novel fluid transport phenomena such as hydrodynamic slip boundary condition, fast fluid transport in carbon nanotubes and graphene nanopores, and gas transport in shale rocks. I also provide a short overview of the usage of molecular modeling techniques in studying fluid flow in nanochannels.

Molecular modeling techniques have been extensively used in studying fluid flow in nanochannels. In this Chapter I discuss the basic background of equilibrium and non-equilibrium molecular dynamics simulations. 

Using molecular dynamics simulation I investigate the correlation between interfacial water properties and the hydrodynamic boundary condition. The most important results presented demonstrate that water can slip on hydrophilic surface. The contact angle larger than 90° is not necessary to attain hydrodynamic slip. Instead, microscopic properties of water in the first hydration layer, which are controlled by the strength of water-solid interactions and surface morphology, are responsible for this surprising result. When favourable adsorption sites exist, but are separated from each other by well-defined sub-nanometer distances, no slip is observed. When favourable adsorption sites are present that are close to each other, liquid slip can occur, provided water-solid attractions are not too strong. My results shed a new light into the molecular-level understanding of hydrophobic effects and their macroscopic consequences. This understanding will benefit many applications including the design of hydrophilic nano-porous membranes with high permeation and self-cleaning hydrophilic surfaces.

It is well known that the electric double layer plays important roles in a variety of applications, ranging from biology to materials sciences. Many studied the electric double layer using a variety of techniques, and as a result our understanding is mature, although not complete. Based on detailed understanding, I expect that by manipulating the electric double layer we could advance tremendously applications in the water-energy nexus. This is particularly true for electric double layer capacitors and capacitive desalination devices. However, such manipulation is not straightforward because of a competition of phenomena that occur within the electric double layer itself, including solvation effects, excluded volume phenomena, and ion-ion correlations. Using molecular dynamics simulations, I designed a composite graphene-based electrode to manipulate structural and dynamical properties of the electric double layer. My design favours the formation of the compact Helmholtz layer. Inherent to my design is that the compact Helmholtz layer not only is atomically thick, but it is also highly mobile in the direction parallel to the charged surface. I suggest here how to exploit the properties of the engineered electric double layer towards developing a new continuous desalination process that combines the advantages of membrane and capacitive desalination processes, reducing their shortcomings. Insights on the molecular mechanisms relevant to the water-energy nexus are provided.

In the last decade, because of the shale gas revolution in the USA the fluid flow in shale nanopores have attracted great attention of scientists worldwide. In shale formation water and natural gas can co-exist within the narrow pores, leading to the possibility of two-phase flow. In this work, I designed the molecular simulation system, that include water and methane confined in slit-shape muscovite nanopore, to investigate the effect of the two-phase flow patterns on the fluids transport and on the pore structure. The results indicate that when the driving force, i.e., the pressure drop, increases above a pore-size dependent threshold the two-phase flow pattern is altered. As a result, the velocity of methane with respect to that of water changes. My results also illustrate the importance of the capillary force, due to the formation of water bridges across the clay pores, not only on the fluid flow, but also on the pore structure, in particular its width. When the water bridges are broken, perhaps because of fast fluid flow, the capillary force vanishes leading to significant pore expansion. Because muscovite is a model for illite, a clay mineral often found in shale rocks, these results advance our understanding regarding the mechanism of water and gas transport in tight shale gas formations.

In this thesis I focus on studying the interfacial fluid behaviours and how these behaviours govern nanoscale fluid transport. In particular, I employ equilibrium and non-equilibrium atomistic molecular dynamics simulations using LAMMPS and GROMACS to study the properties of water, aqueous electrolyte solutions, and methane in contact with metal oxide surfaces, clay minerals, and graphene. From the fundamental understanding of the structural and dynamical properties of the interfacial fluids I learn how to manipulate these properties to enhance the performance of practical applications including nano-fluidic devices, water desalination, energy storage, and shale gas exploration.