Abstract: The pressure-driven flow of water and long-chain hydrocarbons in nanosized pores is important in energy, environmental, biological, and pharmaceutical applications. There is evidence of significantly enhanced fluid flow for water flowing in carbon nanotubes (CNTs) in comparison to the Haagen-Poiseuille flow. In this talk we present experimental and molecular dynamics simulation results to show that fluid-solid interactions govern the extent of deviation (positive or negative) in confined flow in comparison to the continuum.

We first examine the flow of hexane, heptane, and decane in carbon nanotubes (CNTs) of pore diameters 1–8 nm using molecular dynamic simulations. We used the OPLS-AA force field to simulate the hydrocarbons and the CNTs. Our simulations predicted the bulk densities of the hydrocarbons to be within 3% of the literature values. We observed moderate flow enhancements for all the hydrocarbons (1–100) flowing through small-sized CNTs. For very small CNTs the larger hydrocarbons were forced to flow in a corkscrew fashion. As a result of this flow orientation, the larger molecules flowed as effectively (similar enhancements) as the smaller hydrocarbons.

Water, hexane, and methanol flow behaviors were also simulated in silica nanotubes for pore diameters ranging from 1-8 nm. Water and methanol molecules were completely stabilized in the 1 nm pore. Hexane molecules could not enter the 1 nm pore, due to geometric considerations. For the 2 – 8 nm pores, all fluids showed reduced flow rate compared to the Hagen–Poiseuille flow, due to an interfacial molecular layer stabilized at the pore surface. Water flow showed almost constant stick length for all the pore sizes. Methanol flow had the largest stick length. Hexane flow was reduced because of overcrowding of molecules at the pore surface. The effect of confinement faded away with an increase in pore diameter.