Combined length scales in dissipative particle dynamics.

When a particle model simulates fluid behavior, the calculation of all particle interactions causes long computation times. Especially in mesoscale simulations, the bulk areas can be computationally demanding. To reduce the time spent on such regions, we propose a model that combines different length scales in one system. This is a particle analog to mesh refinement in, for instance, finite-element methods. To this end, we define particles of a coarse-grained scale within the framework of dissipative particle dynamics. These particles have a lower number density, but the same mass density, pressure, temperature, and viscosity as the original description. Furthermore, the coarse-grained particles can directly interact with the "normal" particles. The two length scales are combined in one system, coupled by an overlap region. At the edges of this region, particles transform into the other scale, through local refining or coarse graining. The resulting combined system adequately reproduces the properties and flow behavior of a normal system. When half the system is coarse grained, the computation time reduces by a factor of two. Thus, computational efficiency can be greatly increased for a variety of mesoscale applications.