Effects of the absence of friction in coarse-grained molecular dynamics simulations of clay

Abstract

Coarse-grained molecular dynamics (CGMD) simulations can advance understanding of clay behaviour. In CGMD simulations the interactions between clay platelets are modelled and the data generated can be used to quantitatively link clay fabric to the overall material behaviour and examine its sensitivity to changes in the pore-fluid chemistry. A key element of a CGMD model is the potential function employed for particle interactions. One approach is to use Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to calibrate the contact models; however, DLVO theory does not account for a frictional component in the interaction. This contribution shows that omitting a frictional force results in an unexpected overall system response. The conclusion is developed by considering CGMD data generated during one-dimensional compression tests of assemblies of 10,000 kaolinite particles modelled as flat ellipsoids. When the Gay-Berne potential function, calibrated against DLVO predictions, is used to simulate the interactions and inter-particle friction is not explicitly modelled, the resulting coefficient of earth pressure at rest 𝐾0 = 𝜎ℎ′ 𝜎𝑣′⁄ is equal to 1. Furthermore, the packing density obtained in the CGMD is lower than that observed experimentally. Additional data generated using DEM simulations on assemblies of spherical particles demonstrate the sensitivity of 𝐾0 and packing density to the interparticle friction coefficient. Data presented here clearly support the need to explicitly consider a frictional-type component in particle interactions when simulating systems of clay platelets.