Comparison of planar shear flow and planar elongational flow for systems of small molecules

We use nonequilibrium molecular dynamics to simulate steady state planar sh ear flow and planar elongational flow of fluids of small molecules at const ant volume and temperature. The systems studied are Lennard-Jones diatomic molecules (chlorine), and a series of linear Lennard-Jones molecules with o ne, two, and four sites. In our simulations of planar elongational flow, we employ Kraynik-Reinelt periodic boundary conditions, which allow us to obt ain precise values of the steady state planar elongational viscosity. We va lidate our application of Kraynik-Reinelt periodic boundary conditions by c omparing the zero strain rate shear and elongational viscosities. The resul ts show that the elongational viscosity is proportional to the shear viscos ity in the zero strain rate limit, as expected. The viscosity, pressure, an d internal energy of the atomic Lennard-Jones fluid show exactly the same b ehavior for the two types of flow when both sets of results are plotted aga inst the second scalar invariant of the strain rate tensor. The results for the diatomic and four-site molecules show differences in the pressure, ene rgy, and viscosity outside the Newtonian regime when plotted against the se cond scalar invariant of the strain rate tensor. The differences in the pro perties in the nonlinear regime increase with both strain rate and molecula r length. (C) 2000 American Institute of Physics. [S0021-9606(00)50144-5].