Amorphous silica between confining walls and under shear: a computer simulation study

TL;DR

This study uses molecular dynamics simulations to explore the behavior of amorphous silica confined between walls under shear, revealing layering, orientational ordering, and shear-dependent structural rearrangements near the walls.

Contribution

It provides detailed insights into the microscopic structure and flow behavior of silica melts confined and sheared between walls, highlighting wall effects and shear-induced structural changes.

Findings

Pronounced layering and orientational ordering near walls.

Shear does not alter bulk structure but causes rearrangements at walls.

Velocity profiles match hydrodynamic predictions, and shear viscosity agrees with bulk measurements.

Abstract

Molecular dynamics computer simulations are used to investigate a silica melt confined between walls at equilibrium and in a steady-state Poisseuille flow. The walls consist of point particles forming a rigid face-centered cubic lattice and the interaction of the walls with the melt atoms is modelled such that the wall particles have only a weak bonding to those in the melt, i.e. much weaker than the covalent bonding of a Si-O unit. We observe a pronounced layering of the melt near the walls. This layering, as seen in the total density profile, has a very irregular character which can be attributed to a preferred orientational ordering of SiO4 tetrahedra near the wall. On intermediate length scales, the structure of the melt at the walls can be well distinguished from that of the bulk by means of the ring size distribution. Whereas essentially no structural changes occur in the bulk under the influence of the shear fields considered, strong structural rearrangements in the ring size distribution are present at the walls as far as there is a slip motion. For the sheared system, parabolic velocity profiles are found in the bulk region as expected from hydrodynamics and the values for the shear viscosity as extracted from those profiles are in good agreement with those obtained in pure bulk simulations from the appropriate Green-Kubo formula.