ANISOTROPIC AND ENHANCED SELF-DIFFUSION OF A MACROMOLECULAR CHAIN UNDER SIMPLE SHEAR-FLOW AS REVEALED BY MONTE-CARLO SIMULATION ON LATTICES

Two-dimensional simple shear flow of a self-avoiding macromolecular ch ain is simulated by a lattice Monte Carlo (MC) method with a pseudo-po tential describing the flow field. The simulated velocity profile sati sfies the requirements of simple shear flow unless the shear rate is u nreasonably high. Some diffusion problems for a free-draining bead-spr ing chain with excluded volume interaction are then investigated at lo w and relatively high shear rates. Three diffusion coefficients are de fined and examined in this paper: the conventional self-diffusivity in zero field, D-self, the apparent self-diffusivity in flow field, D-ap p,,, and the flow diffusivity in simulation, D-flow,, reflecting actua lly the transport coefficient. It is found that these three diffusivit ies for a flexible chain are different from each other. What is more i mportant is that self-diffusion exhibits a high anisotropy in the flow field. The apparent self-diffusion along the flow direction is enhanc ed to a large extent. It is increased monotonically with the increase of shear time or shear strain, whereas the chain configuration can ach ieve a stationary anisotropic distribution following an interesting ov ershoot of the coil shape and size. Besides a single self-avoiding cha in, an isolated Brownian bead and a group of self-avoiding beads with a quasi-Gaussian spatial distribution are also simulated. According to the comparison, the effects of the connectivity of the chain on the d iffusion behavior are revealed. Some scaling relations of D-app versus t are consistent with the theoretical analyses in the pertinent liter ature.