Wetting and particle adsorption in nanoflows

Abstract

Molecular dynamics simulations are used to study the behavior of closely-fitting spherical and ellipsoidal particles moving through a fluid-filled cylinder at nanometer scales. The particle, the cylinder wall and the fluid solvent are all treated as atomic systems, and special attention is given to the effects of varying the wetting properties of the fluid. Although the modification of the solid-fluid interaction leads to significant changes in the microstructure of the fluid, its transport properties are found to be the same as in bulk. Independently of the shape and relative size of the particle, we find two distinct regimes as a function of the degree of wetting, with a sharp transition between them. In the case of a highly-wetting suspending fluid, the particle moves through the cylinder with an average axial velocity in agreement with that obtained from the solution of the continuum Stokes equations. In contrast, in the case of less-wetting fluids, only the early-time motion of the particle is consistent with continuum dynamics. At later times, the particle is eventually adsorbed onto the wall and subsequently executes an intermittent stick-slip motion. We show that van der Walls forces are the dominant contribution to the particle adsorption phenomenon and that depletion forces are weak enough to allow, in the highly-wetting situation, an initially adsorbed particle to spontaneously desorb.

The von Kármán Sodium experiment: Turbulent dynamical dynamos

“…[24][25][26][27][28][29][30][31] The topology of the time-averaged velocity field has received much attention because it is similar to the s 2 t 2 flow considered in a sphere by Dudley and James. 32 This topology generates a stationary kinematic dynamo [33][34][35] whose main component is a dipole perpendicular to the rotation axis ͑an "m = 1" mode in cylindrical coordinates͒. The threshold, in magnetic Reynolds number, for kinematic dynamo generation is very sensitive to details of the mean flow such as the poloidal to toroidal velocity ratio, [33][34][35] the electrical boundary conditions, [34][35][36][37][38] or the details of the velocity profile near the impellers.…”

“…32 This topology generates a stationary kinematic dynamo [33][34][35] whose main component is a dipole perpendicular to the rotation axis ͑an "m = 1" mode in cylindrical coordinates͒. The threshold, in magnetic Reynolds number, for kinematic dynamo generation is very sensitive to details of the mean flow such as the poloidal to toroidal velocity ratio, [33][34][35] the electrical boundary conditions, [34][35][36][37][38] or the details of the velocity profile near the impellers. 38 Specific choices in the actual flow configuration have been based on a careful optimization of the kinematic dynamo capacity of the VKS mean flow.…”

Diffusion of a Janus nanoparticle in an explicit solvent: A molecular dynamics simulation study

“…In general, it was shown that the slip length is a tensor quantity for flows over anisotropic textured surfaces [9][10][11][12]. Transport properties of chemically homogeneous nanoparticles were recently studied in cylindrical pores filled with a wetting fluid [13] and in an atomistic solvent confined between flat walls [14]. However, the diffusion process of individual Janus particles even in the absence of flow or confinement remains not fully understood.…”

Fluid enhancement of particle transport in nanochannels

“…In fact, in previous work, MD simulations have provided valuable insight into the transport of suspended solid particles under geometric confinement at nanometer scales. [12][13][14] In the present work, our attention is focused on the effect that fluid density has on the mobility of nanometer particles in nanochannels. Specifically, we examine the mobility of a Lennard-Jones type nanoparticle, moving under the action of a uniform external force, inside a platinum nanochannel filled with nitrogen fluid molecules.…”