Non-Equilibrium Molecular Dynamics Studies of Complex Fluids

Significant Achievements, Anticipated Outcomes and Future Work

We have recently completed a study of the concentration dependence of the viscosity of a model short-chain polymer solution in which the solvent is modelled explicitly as an atomic fluid. By calculating the number of Kuhn segments in the polymer chain, we found that our model molecule was roughly equivalent to polyethylene with a molar mass of approximately 1800 g/mol. Many of the theories of polymer solutions apply only in the long-chain limit, and we found that they could not adequately describe our results. However, we still observed many of the well-known polymer solution phenomena that occur as the concentration is increased; the approach to random coil chain conformations, the decrease in hydrodynamic interactions and an increase in the viscosity and normal stress differences. These results are discussed in detail in our recent publications. Current work is aimed at obtaining the full set of transport coefficients for this system, and comparing the results of using non-equilibrium thermodynamics to model flow of a polymer solution though a narrow pore with direct molecular dynamics simulation. This will show where standard computational fluid dynamics breaks down for small pore sizes. We expect these computations to be completed during the next year.

A major redevelopment of our non-equilibrium molecular dynamics computer program has recently been completed. The new program uses Fortran 90 modules, derived data types and dynamically allocated storage to achieve a vastly improved level of flexibility and efficient memory usage. The flexible data structure also allows us to study more complex polymer-solvent-colloid mixtures, including polymer nanocomposite materials. Preliminary computations of the rheology of these systems have just begun.

Our work on the thermodynamics of shearing viscoelastic fluids is continuing. Computations are currently in progress to test the agreement between a Maxwell relation and a direct computation of the thermodynamic quantity related to the strain rate dependence of the free energy of a shearing viscoelastic fluid. The role of normal stresses in the thermodynamics of shearing fluids is also being studied.

Computational Techniques Used

We use our own non-equilibrium molecular dynamics computer program written in Fortran 90 for our simulations. Although we previously used three different programs for the polymer, colloid and wall-driven shear simulations, we have currently redeveloped the code into a complete suite of Fortran 90 modules that will allow more flexibility and portability for new program development. Several important innovations are present in the new code, but more work remains to be done to eliminate some computational bottlenecks and add important functionality. A fully mature version of the code will be completed this year, and we plan to publish the details of the new algorithms that it contains in 2005.

Publications, Awards and External Funding

External Funding and Awards

P. Daivis was recently awarded an RMIT University Research Infrastructure Fund grant with Prof S. Bhattacharya and A/Prof A. Mouritz to develop new polymer nanocomposite materials and predict their processing properties using molecular simulation.

Publications

I. Snook, B. O’Malley, M. McPhie and P. Daivis, The approach to the Brownian limit in particulate dispersions, J. Mol. Liq. 103-104C, 2003. 405.
M. L. Matin, P. J. Daivis and B. D. Todd, Modifications to the cell neighbour list construction method for molecular dynamics simulation of planar elongational flow, Computer Physics Communications 151, 2003, 35.
P. J. Daivis, M. L. Matin and B. D. Todd, Nonlinear shear and elongational rheology of polymer melts by non-equilibrium molecular dynamics, J. Non-Newtonian Fluid Mech. 111, 2003, 1.
P. J. Daivis and M. L. Matin, Steady-state thermodynamics of shearing viscoelastic fluids, J. Chem. Phys. 118, 2003 11111.
Z. Zhou, B. D. Todd and P. J. Daivis, Parallelisation of nonequilibrium molecular dynamics code for polymer melts using OpenMP, Computational Science - ICCS 2003, p275-285, Springer-Verlag (Berlin 2003).
T. Kairn, P. J. Daivis, M. L. Matin and I. K. Snook, Concentration dependence of the rheology of short chain polymer solutions, Polymer 45, 2004, 2453.
T. Kairn, P. J. Daivis, M. L. Matin and I. K. Snook, Effects of concentration on steady-state viscometric properties of short-chain polymer solutions over the entire concentration range, accepted for publication, Int. J. Thermophys. (2004).