Molecular dynamics studies on inter-nanoparticles forces

Abstract

The continuum approach to describing interparticle forces, such as Hamaker approach for van der Waals (vdW) attraction force and Born repulsion force, Hertz model for mechanical force due to elastic deformation, does not generally or definitely apply to nanoparticles due to the neglect of their atomic discrete structures and its surface effect or its unique mechanical properties at the nanoscale. Much effort has been devoted to direct measurements of interparticle force by experiments, such as atomic force microscopy, surface force apparatus and colloidal particle scattering, etc, these techniques, however, often are hampered by the surface roughness of the sample and the issues such as surface contamination, ambient vibration and limited distance resolution. Molecular dynamics (MD) simulation can play an important and efficient role in determining inter-nanoparticles forces. In this work, first the vdW attraction and Born repulsion between silica nanospheres were studied by MD simulations, the results show that inter-nanoparticle forces are much larger than those predicted by the Hamaker approach, and one modified expression was proposed for vdW attraction and Born repulsion force, respectively, based on the Hamaker approach and MD simulation data. Second, the normal contact forces between silica nanospheres were examined and the results show that Hertz model still hold to describe the elastic force at low compression at the nanoscale, but the contact area is larger than that predicted by continuum models such as Hertz, JKR models by a factor of almost 2, mainly due to roughness arisen from the atomic discrete structure. Moreover, the effect of different atomic discrete structure on interparticle forces was probed based on three more nanomaterials, crystalline silicon, diamond and amorphous carbon nanospheres. Also interparticle forces between silicon and diamond dissimilar nanospheres were considered. The modified expressions for interparticle vdW attraction and Born repulsion were compared between the four materials. The results show that for interparticle vdW attraction force, the modified expressions vary with nanomaterials, but for a given particle size, the normalized ratios as a function of surface separation are independent of nanomaterials; in contrast, for interparticle Born repulsion force, the corresponding modified expressions as a function of surface separation are weakly dependent on nanomaterials. The Hertz model still holds for the mechanical forces between identical or dissimilar particles. Lastly, effect of particle shape on interparticle force was investigated in terms of silicon nano-ellipsoids. Both non-contact forces including vdW attraction and Born repulsion, and normal contact force were discussed. The results show that the modified Hertz model originally used for macro-ellipsoids is still applicable for nano-ellipsoids. But for vdW attraction and Born repulsion forces, the modified expressions will be much more complicated than spherical particles as they are not only dependent on particle size, but also on relative orientations, aspect ratios, etc. Much more effort is still needed in this research direction.