Showing papers by "Billy D. Todd published in 2017"

Nonequilibrium Molecular Dynamics

Abstract: Written by two specialists with over twenty-five years of experience in the field, this valuable text presents a wide range of topics within the growing field of nonequilibrium molecular dynamics (NEMD). It introduces theories which are fundamental to the field - namely, nonequilibrium statistical mechanics and nonequilibrium thermodynamics - and provides state-of-the-art algorithms and advice for designing reliable NEMD code, as well as examining applications for both atomic and molecular fluids. It discusses homogenous and inhomogenous flows and pays considerable attention to highly confined fluids, such as nanofluidics. In addition to statistical mechanics and thermodynamics, the book covers the themes of temperature and thermodynamic fluxes and their computation, the theory and algorithms for homogenous shear and elongational flows, response theory and its applications, heat and mass transport algorithms, applications in molecular rheology, highly confined fluids (nanofluidics), the phenomenon of slip and how to compute it from basic microscopic principles, and generalized hydrodynamics.

Modeling slip and flow enhancement of water in carbon nanotubes

Abstract: Transport properties of fluids in nanopores are of both fundamental as well as practical interest. Water flow in carbon nanotubes (CNTs) has received significant attention since the early 2000s for technological applications of CNTs. In this article, we provide a brief overview of modeling the slip and flow enhancement of water in CNTs. A number of experimental and computational studies have found water to flow very fast in CNTs, but the measured flow rates, which are high compared to classical hydrodynamics predictions, are scattered over 2–5 orders of magnitude. Slip lengths of 1 to 500,000 nm, resulting in almost zero to 500,000 flow enhancement, are reported for water in CNTs with diameters of 0.8 to 10 nm. We highlight some challenges in modeling fluid flow in nanopores and outline a few research directions that may resolve the order of slip and flow enhancement of water in CNTs in computational studies.

Electropumping of Water in Functionalized Carbon Nanotubes Using Rotating Electric Fields

Abstract: The development of new methods for the effective pumping of water at the nanoscale is an important building block in the development of nanotechnologies due to the limitations of current approaches, such as electro-osmotic flow. In this paper we present the first nonequilbrium molecular dynamics results that demonstrate an effective net positive flow of water molecules within a functionalized carbon nanotube without the use of an effective pressure gradient. We have applied a spatially uniform rotating electric field that couples with the permanent dipole moment of the water molecules. Breaking the symmetry of the nanotube walls by asymmetrically functionalizing the carbon nanotube with carboxyl groups, we then take advantage of the coupling of spin angular momentum of the water molecules to their linear momentum to induce a positive net flow. Surprisingly our results show that a net flow can be achieved with only a small amount of functionalization. The net flow resulting from only a small amount of func...

Deviations From Classical Hydrodynamic Theory in Highly Confined Planar Poiseuille Flow of a Polymer Solution

Abstract: The behaviour of polymer solutions in highly confined geometries remains a subject of interest in rheology and fluid dynamics. In this paper, we investigate how well the classical hydrodynamic description based on the Navier-Stokes equations, Fourier's Law and Fick's Law describes the flow of a highly confined polymer solution. In particular, we examine the effects of depletion of polymer concentration at the wall-fluid interface and strain rate coupling to the heat flux. We present data from molecular dynamics simulations of a model polymer solution in explicit solvent undergoing planar Poiseuille flow for channel widths ranging from around 10 solvent atomic diameters to around 80 solvent atomic diameters. We find that the classical continuum approach works very well for channels wider than 20 solvent atomic diameters. For narrower channels, we observe deviations in the velocity, temperature and concentration profiles due to density oscillations near the walls, the polymer depletion effect, and possible weak strain rate coupling. For the narrowest channel, the wall effects extend to the centre of the channel but the underlying profiles are quite well described by the classical continuum picture. By allowing very long times of order 104 reduced time units for relaxation to the steady state and averaging over very long runs of order 105 reduced time units and 16 independent ensemble members, we are able to conclude that previously reported deviations from the classical continuum predictions (I.K. Snook, P.J. Daivis, T. Kairn, J. Physics-Condensed Matter 20, 404211 (2008)) were probably the result of insufficient equilibration time. Our results are also sufficiently accurate and precise to verify the expected quartic temperature profile predicted by classical hydrodynamic theory, with only a very small deviation which we can attribute to nonlinear coupling of the heat flux vector to the strain rate.