TRANSLATIONAL AND ROTATIONAL DIFFUSION OF DILUTE SOLID AMORPHOUS SPHERICAL NANOCOLLOIDS BY MOLECULAR-DYNAMICS SIMULATION

Following on from our previous study (Heyes, D. M., Nuevo, M. J, and M orales, J. J., 1996, Molec. Phys., 88, 1503), molecular dynamics simul ations have been carried out of translational and rotational diffusion of atomistically rough near-spherical solid Lennard-Jones (LJ) cluste rs immersed in a Weeks-Chandler-Andersen liquid solvent. A single clus ter consisting of up to about 100 LJ particles as part of an 8000 atom fluid system was considered in each case. The translational and rotat ional diffusion coefficients decrease with increasing cluster size and solvent density (roughly in proportion to the molar volume of the sol vent). The simulations reveal that for clusters in excess of about 30 LJ atoms there is a clear separation of timescales between angular vel ocity and orientation relaxation which adhere well to the small-step d iffusion model encapsulated in Hubbard's relationship. For 100 atom cl usters both the Stokes-Einstein (translation) and Stokes-Einstein-Deby e (rotation) equations apply approximately. The small departures from these reference solutions indicate that the translational relaxation e xperiences a local viscosity in excess of the bulk value (typically by similar to 30%), whereas rotational relaxation experiences a smaller viscosity than the bulk (typically by similar to 30%) reasonably in ac cord with the Gierer-Wirtz model. Both of these observations are consi stent with an observed layering of the liquid molecules next to the cl uster observed in our previous study.