Effective particle size from molecular dynamics simulations in fluids

Abstract

We report molecular dynamics simulations designed to investigate the effective size of colloidal particles suspended in a fluid in the vicinity of a rigid wall where all interactions are defined by smooth atomic potential functions. These simulations are used to assess how the behavior of this system at the atomistic length scale compares to continuum mechanics models. In order to determine the effective size of the particles, we calculate the solvent forces on spherical particles of different radii as a function of different positions near and overlapping with the atomistically defined wall and compare them to continuum models. This procedure also then determines the effective position of the wall. Our analysis is based solely on forces that the particles sense, ensuring self-consistency of the method. The simulations were carried out using both Weeks-ChandlerAndersen and modified Lennard-Jones (LJ) potentials to identify the different contributions of simple repulsion and van der Waals attractive forces. Upon correction for behavior arising the discreteness of the atomic system, the underlying continuum physics analysis appeared to be correct down to much less than the particle radius. For both particle types, the effective radius was found to be ∼ 0.75σ , where σ defines the length scale of the force interaction (the LJ diameter). The effective "hydrodynamic" radii determined by this means are distinct from commonly assumed values of 0.5σ and 1.0σ , but agree with a value developed from the atomistic analysis of the viscosity of such systems.

Molecular understanding of the impacts of structural characteristics on ethanol adsorption performance for adsorption heat pumps

“…1a. Because the pore sizes of most experimentally synthesized MOFs are in the range of 5-35 Å, 24,27 our pseudo nanoporous structures were built using different numbers of LJ spheres (n = 4, 7,10, 13,16,19,22,25,28,31,34,37,40) to create different predefined crystal cell lengths (l = 5, 12.5, 15, 17.5, 20, 22.5, 25, 27.5, 30, 32.5, 35 Å). The representative structures are presented in Fig.…”

“…36) with a nitrogen probe radius of 1.82 Å, in which the LJ parameter (σ) was used as van der Waals diameter of the framework atoms. [37][38][39] It was found that the accessible pore width for ethanol molecules should be greater than the dynamic size of ethanol molecules (3.95 Å) based on the TraPPE force field parameters of ethanol. GCMC simulations were carried out using RASPA 1.0 (ref.…”

Hindered Diffusion near Fluid–Solid Interfaces: Comparison of Molecular Dynamics to Continuum Hydrodynamics

“…Because of the repulsive interactions between the fluid particles and the colloidal particle, the radius of the colloidal particle is slightly larger than the packing radius. There have been discussions in the literature on determining the radius of a particle in MD simulations and the boundary conditions chosen for the calculation in eq . ,,,,− In our study, we carried out bulk simulations with one colloidal particle in a fluid with PBCs in all three directions (no walls) to obtain the radial distribution function g ( r ) between the fluid particles and the COM of the colloidal particle as shown in Figure . The radius of the colloidal particle R was defined to be the largest distance without any fluid particles in the g ( r ).…”