Simulations of concentrated suspensions of rigid fibers:Relationship between short-time diffusivities and the long-time rotational diffusion

摘要

Brownian dynamics simulations of the behavior of suspensions of fibers demonstrate that the scaling of the rotational diffusivity with respect to the number density(nL~3)is a sensitive function of the thickness and the parameter L~2D_R /D_(T_0),where D_R is the rotational diffusivity at infinite dilution,D_(T_0)is the average center-of-mass diffusivity at infinite dilution,and L is the fiber length.Existing theories for the long-time rotational diffusivities of rigid fibers in the semidilute and concentrated regimes fail to accurately account for the relationship with the dilute values of the rotational and translational diffusivities of the various physical models used to simulate the fibers.The concentration regime studied in this work ranges from a number density of nL~3approx= 0-150,which is below the transition from an isotropic to nematic state.The effect of the fiber thickness was studied by performing simulations of rods with aspect ratios(fiber length over diameter)of 25,50,and 500,as well as performing projections for infinitely thin fibers.The excluded volume of the rods was enforced through the use of short-range potentials.For a rod with an aspect ratio of 50 with a parameter of L~2D_R_0 /D_T_0 = 9,which corresponds to a slender-body model of the individual fibers,the rotational diffusivity(D_R)scales as D_R/D_R_0 ~(nL~3)~(-1.9)in the concentration regime of 70 <=nL~3 <= 150.Similarly with a parameter of L2D_R /D_T_0 =4,corresponding to a rigid-dumbbell model,the rotational diffusivity scales as D_RID~_R_0 ~(nL~3)~(-1.1)over the same range of concentrations.For rods with aspect ratios of 25,it is observed that a difference in the scaling is seen for L~2D_R_0/D_T_0<= 8,with higher values of this ratio exhibiting essentially the same scaling.Additional values of the ratio L~2D_R_0/D_T_0 were investigated to determine the overall behavior of the suspension dynamics with respect to this parameter.These findings resolve discrepancies between simulation results for rotational diffusivities reported by previous investigators and provide new insights for the development of an accurate theory for the diffusivity of rigid rods suspended in solution.