GENERIC PROPERTIES OF RHEOLOGICAL FLOW CURVES

The interpretation and comparison of viscosity data for fluids in plan ar Couette how are discussed using a generic rheological function, fam iliar from studies of polymeric liquids. The particular data are gener ated by non-equilibrium molecular dynamics (NEMD) simulations of model fluids, however the analysis has wider validity. The mathematical pro perties of the established flow curves are investigated and, inter ali a, a consistent definition of the critical shear rate is established. It is also shown that the use of inappropriate, but commonly used, ind ependent variables leads to curves whose qualitative behaviour is extr emely sensitive to the physical parameters. Such a range of qualitativ e behaviour can lead to erroneous quantitative conclusions when interp reting physical (or model) data. A particular example is the assumptio n of a linear dependence of the viscosity on the square-root of the sh ear rate, as often used in the NEMD literature. The developed framewor k also enables the systematic comparison of simulations of the same mo lecular fluid performed at different state points. This is illustrated for the case of three recently published and apparently conflicting N EMD results for n-butane. (C) 1997 Shell Research Ltd.