The coarse-graining of a simple all-atom 2D microscopic model of graphene, in terms of "blobs" described by center of mass variables, is presented. The equations of motion of the coarse-grained variables take the form of dissipative particle dynamics (DPD). The coarse-grained conservative forces and the friction of the DPD model are obtained via a bottom-up procedure from molecular dynamics (MD) simulations. The separation of timescales for blobs of 24 and 96 carbon atoms is sufficiently pronounced for the Markovian assumption, inherent to the DPD model, to provide satisfactory results. In particular, the MD velocity autocorrelation function of the blobs is well reproduced by the DPD model, provided that the effect of friction and noise is taken into account. However, DPD cross-correlations between neighbor blobs show appreciable discrepancies with respect to the MD results. Possible extensions to mend these discrepancies are briefly outlined.
A mesoscale dissipative particle dynamics model of single wall carbon nanotubes is designed and demonstrated. The coarse-grained model is produced by grouping together carbon atoms and by bonding the new lumped particles through pair and triplet forces. The mechanical properties of the simulated tube are determined by the bonding forces which are derived by virtual experiments. Through the introduction of van der Waals interactions, tube-tube interactions were studied. Owing to the reduced number of particles, this model allows the simulation of relatively large systems. The applicability of the presented scheme to model carbon nanotube based mechanical devices is discussed.
The development of a new simulation tool for micro powder injection molding (MicroPIM) needs experimental material data and verification experiments to describe the process correctly. A new and innovative approach is to use dissipative particle dynamics (DPD) to describe the form filling process with respect to the interactions on a mesoscopic scale (Hoogerbrugge and Koelman in Europhys Lett 19 (1): [155][156][157][158][159][160] 1992). The individual parameters that enter DPD modeling of this process have to be adjusted using specially designed experiments for the injection molding process. The material properties in the standard injection molding process are primarily determined by the bulk material. In micro dimensions surface effects begin to dominate because of the large surface to volume ratio. Therefore, the surface interactions between feedstock and mold insert were studied. Finally, first observations of the injection molding experiments are shown and qualitatively compared to DPD simulation results.
The calculation of the friction matrix in the coarse‐grained (CG) description of an atomistic system is a crucial issue, in order to properly account for the dissipative effects inherent to any reduced representation of the atomistic dynamics. Within the Mori‐Zwanzig projection operator approach to CG, there are several possibilities for the definition of the friction matrix, depending on the projector that is being used. In this paper, the connection of two of these projectors (Mori's and Zwanzig's) is discussed and the corresponding merits and limitations are analysed. Moreover, three different ways of computing the friction matrix in the Mori's framework are presented, along with a discussion of their mutual connections. By the example of CG centre of mass blob variables in the graphene lattice, it is shown that, even though the three approaches are equivalent from a theoretical viewpoint, they may differ considerably in terms of practical implementation for computer simulation purposes. In the given example, which is representative for atomic lattices, it turns out that a linear regression of the velocity–velocity correlation function, inspired by the Einstein–Helfand approach, is the least error‐prone against disturbances from optical modes. magnified image
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