simulation group

2023

1. Analytic elastic coefficients in molecular calculations: Finite strain, nonaffine displacements, and many-body interatomic potentials

   Jan Grießer, Lucas Frérot, Jonas A. Oldenstaedt, Martin H. Müser, and Lars Pastewka

   Phys. Rev. Mater. 7, 073603 (2023)

   Abstractdoi PDF

   Elastic moduli are among the most fundamental and important properties of solid materials, which is why they are routinely characterized in both experiments and simulations. While conceptually simple, the treatment of elastic coefficients is complicated by two factors not yet having been concurrently discussed: finite-strain and nonaffine, internal displacements. Here, we revisit the theory behind zero-temperature, finite-strain elastic moduli and extend it to explicitly consider nonaffine displacements. We further present analytical expressions for second-order derivatives of the potential energy for two-body and generic many-body interatomic potentials, such as cluster and empirical bond-order potentials. Specifically, we revisit the elastic constants of silicon, silicon carbide, and silicon dioxide under hydrostatic compression and dilatation. Based on existing and recent results, we outline the effect of multiaxial stress states as opposed to volumetric deformation on the limits of stability of their crystalline lattices.

2. Confinement-induced diffusive sound transport in nanoscale fluidic channels

   Hannes Holey, Peter Gumbsch, and Lars Pastewka

   Phys. Rev. Lett. 131, 084001 (2023)

   Abstractdoi PDF

   Molecular dynamics (MD) simulations have been widely used to study flow at molecular scales. Most of this work is devoted to study the departure from continuum fluid mechanics as the confining dimension decreases. Here, we present MD results under conditions where hydrodynamic descriptions typically apply, but focus on the influence of in-plane wavelengths. Probing the long wavelength limit in thermodynamic equilibrium, we observed anomalous relaxation of the density and longitudinal momentum fluctuations. The limiting behavior can be described by an effective continuum theory that describes a transition to overdamped sound relaxation for compressible fluids.

3. Entropic stress of grafted polymer chains in shear flow

   Jan Mees, Thomas C O’Connor, and Lars Pastewka

   J. Chem. Phys. 159 (2023)

   Abstractdoi PDF

   We analyze the shear response of grafted polymer chains in shear flow via coarse-grained molecular dynamics simulations with an explicit solvent. We find that the solvent flow penetrates into almost the whole brush for “mushroom”-type brushes but only a few bond distances for dense brushes. In all cases, the external stress on the wall equals the entropic stress associated with the distorted polymer conformations. We find that the external stress increases linearly with shear rate at low rates and sublinearly at high rates. The transition from linear to sublinear scaling occurs where chains react to flow by reorienting. Sublinear scaling with shear rate disappears if the shear rate is nondimensionalized with the effective relaxation time of chain subsegments located in the outer part of the brush that experiences flow.

4. Molecular simulations of sliding on SDS surfactant films

   Johannes L Hörmann, Chenxu Liu, Yonggang Meng, and Lars Pastewka

   J. Chem. Phys. 158 (2023)

   Abstractdoi PDF

   We use molecular dynamics simulations to study the frictional response of monolayers of the anionic surfactant sodium dodecyl sulfate and hemicylindrical aggregates physisorbed on gold. Our simulations of a sliding spherical asperity reveal the following two friction regimes: at low loads, the films show Amonton’s friction with a friction force that rises linearly with normal load, and at high loads, the friction force is independent of the load as long as no direct solid-solid contact occurs. The transition between these two regimes happens when a single molecular layer is confined in the gap between the sliding bodies. The friction force at high loads on a monolayer rises monotonically with film density and drops slightly with the transition to hemicylindrical aggregates. This monotonous increase of friction force is compatible with a traditional plowing model of sliding friction. At low loads, the friction coefficient reaches a minimum at the intermediate surface concentrations. We attribute this behavior to a competition between adhesive forces, repulsion of the compressed film, and the onset of plowing.

5. Yielding under compression and the polyamorphic transition in silicon

   Jan Grießer, Gianpietro Moras, and Lars Pastewka

   Phys. Rev. Mater. 7, 055601 (2023)

   Abstractdoi PDF

   We investigate the behavior of amorphous silicon under hydrostatic compression using molecular simulations. During compression, amorphous silicon undergoes a discontinuous nonequilibrium transition from a low-density to a high-density structure at a pressure of around 13-16 GPa. Ensemble-averaged density and elastic constants change discontinuously across the transition. Densification of individual glassy samples occurs through a series of discrete plastic events, each of which is accompanied by a vanishing shear modulus. This is the signature of a series of elastic instabilities, similar to shear transformation zones observed during shear yielding of glasses. We compare the structure obtained during compression with a near-equilibrium form of amorphous silicon obtained by quenching a melt at constant pressure. This gives structures identical to nonequilibrium compression at low and high pressure, but the transition between them occurs gradually rather than discontinuously. Our observations indicate that the polyamorphic transition is of a nonequilibrium nature, and it has the characteristics of a yield transition that occurs under compression instead of shear.