Molecular Dynamics Simulations of Nanoparticle Interactions with a Planar Wall: Does Shape Matter?

Abstract

We investigate the hydrodynamic interactions of spherical colloidal nano particles and nano tetrahedra near a planar wall by means of molecular dynamics (MD) simulations of rigid particles within an all-atom solvent. For both spherical and nano-tetrahedral particles, we find that the parallel and perpendicular components of the local diffusion coefficient and viscosity, show good agreement with hydrodynamic theory of Faxén and Brenner. This provides further evidence that low perturbations from sphericality of a nanoparticle’s shape has little influence on its local diffusive behaviour, and that for this particular case, the continuum theory fluid dynamics is valid even down to molecular scales.

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

“…One might suspect the near-wall diffusion constants to be oscillatory because of the oscillatory solvation force reported by Challa and van Swol . However, such effects did not appear in their simulations …”

“…The near-wall dynamics of colloidal particles have been studied in different experiments, such as evanescent wave dynamic scattering, − video microscopy, − and total internal reflection microscopy (TIRM), − to test the hydrodynamic results for λ ⊥ and λ ∥ (or similarly, the corresponding D ⊥ and D ∥ ). − On the simulation side, molecular dynamics (MD) simulations have been the primary tool to study hindered particle diffusion near the wall on the molecular scale, − which is not readily probed in the experiment or in typical hydrodynamics calculations. In an MD simulation, one can precisely track the positions and velocities of the particles as a function of time and readily calculate equilibrium and dynamic quantities based on the particle trajectories.…”

“…When the colloidal particle is not trapped at a specific z , there are at least three common methods to handle the data so that a comparison can be made with the hydrodynamic results. The first method divides the perpendicular distance from the wall into equally spaced intervals, and the statistics are averaged over the intervals (see Figure with Δ z = constant). ,, The second method is similar to the first, but with a variable interval width that tends to be smaller near the wall where the drag force changes more rapidly (see Figure with a variable Δ z ). ,, The third common method does not include binning the distance z , but D or λ is averaged over the probability distribution of the distance from the wall P ( z ) measured by the experiment. , …”

“…In the previous studies, either the drag force exerted on the colloidal particle or the calculated diffusion constant of the colloidal particle from molecular dynamics (MD) simulations was compared with the hydrodynamic results by Brenner and Faxén. − To date, the agreement between the hydrodynamics results − and MD simulations is not entirely satisfactory. For D ⊥ , Vergeles et al studied systems of smooth and rough particles moving toward the wall in a Lennard-Jones (LJ) fluid to investigate the problem of the unphysical near-wall divergence in Brenner’s λ ⊥ result, in which the derivation assumed no-slip boundary conditions at particle and wall surfaces with perfectly smooth surfaces and a constant fluid density anywhere (no depletion of the fluid near the interfaces). , In Vergeles’ simulation, a heavy mass was given to the colloidal particle that was then pulled at a constant velocity U toward the wall.…”

Interaction of hot carriers with optical phonons in Selenium

“…Calculations of the electron-phonon matrix elements were performed in the framework of density functional theory (DFT) implemented in the ABINIT package [23,24] using planewave method and Troullier-Martins pseudopotential [25]. The full structural optimization was performed using two approximations for the exchange correlation functional: local (LDA) and generalized density (GGA) approximations.…”