Rheology of water in small nanotubes

Taylor-Aris Dispersion in Nanotubes: Analytical Solution, Effects of Slip and Surface Potential Landscape, and Measurement of the Slip Length

Taylor-Aris (T-A) dispersion of a solute in a flowing solvent is a fundamental phenomenon in most mass-transfer processes. Despite its significance and numerous applications in microreactors, colloidal transport in confined media, chromatographic separation, and transport in biological tissues, the effect of the slip length and the topology of surface potential landscapes on T-A dispersion in nanostructured channels has not been studied in detail. We propose a novel methodology for molecular dynamics (MD) simulation of T-A dispersion in such systems, derive an analytical expression for the dispersion coefficient in them, and report on the results of extensive MD simulations of the phenomenon in carbon nanotubes and hexagonal carbon nanochannels. By broadening the topology of the surface energy landscape, we vary the slip lengths, making it possible to distinguish between the effects of confinement, the topology of the energy landscape, and the slip length on the T-A dispersion coefficient. It is demonstrated that measuring the T-A dispersion coefficient in laminar flow is a straightforward and reliable approach for estimating the slip length in nanotubes and other nanostructured materials.

Equilibrium distribution functions: connection with microscopic dynamics

Standard textbook derivations of the equilibrium distribution function rely on assumptions that may not satisfy all readers. Here, we present a straightforward approach to derive the equilibrium distribution function from the microscopic dynamics, and review how it can be used to obtain the expected expressions. In molecular dynamics simulations the equations of motion are often modified to simulate different ensembles or phenomena. We show that in some cases these equations will sample an equilibrium ensemble whereas in other cases they will not. For example, we find that for charged particles driven by a field, an equilibrium distribution is only possible when the system is confined. Furthermore, the approach correctly predicts that neither SLLOD shear flow dynamics nor constant temperature dynamics with a Berendsen thermostat sample any time-independent phase space distributions.