(163f) Dependence of Relaxations and Mechanical Properties on Molecule Shape in Dissipative Particle Dynamics

Authors: 
Greenfield, M. L., University of Rhode Island
Lemarchand, C. A., Universite Paris Sud
Hansen, J. S., Roskilde University
Dissipative Particle Dynamics (DPD) has been established as an effective way to reach long time scales in molecular simulations. Strong forces at short separations are neglected in the method, and consequently each DPD particle represents a local collection of neighboring atoms and molecules. Our long-term interest is simulating amorphous mixtures of molecules that relax over a range of time scales; DPD is of interest for averaging over sufficient numbers of relaxations. To quantify relaxation dynamics in DPD, we constructed systems that share common interparticle interactions yet differ in bonding geometry. We chose systems of spheres in which some were bonded into simple geometries -- dimers, trimers, triangles, squares -- to focus on how physical shape affects relaxations and mechanics. At fixed density, the pressure varied by about ± 3% as a consequence of bonding between the extremes of 0 and 100% unbonded spheres. Larger bonded entities diffused more slowly; at low temperatures they showed an anomalous region (r^2 ~ tn, n<1) over moderate time scales prior to reaching a diffusive regime. Diffusion coefficient and rotational relaxation times, in dimensionless simulation units, changed less than predicted by an Arrhenius dependence over temperatures of kBT/a = 0.15 to 1.2, where a is the parameter that quantifies conservative particle-particle repulsive forces. The difference from Arrhenius (i.e. "strong") or fragile liquid molecular-scale behavior indicates that no single set of mass, length, and time units can interrelate DPD and molecular results for these liquids. Comparing stress contributions to modulus and viscosity indicates that kinetic energy, random force, and dissipative force contributions are independent of particle-particle bonding. Only the conservative-conservative and bond forces lead to viscosities that increase with decreasing temperature and with the presence of bonds. Near kBT/a ≈ 1, viscosity rises with temperature, as in a gas. Relaxations of correlation functions that are derived form generalized hydrodynamics also support gas-like mechanical behavior near this temperature.