(402a) Determination of Viscoelastic Properties of Complex Fluids Using Active Nanorheology Simulations

Traditionally viscoelastic properties of complex fluids are experimentally measured using a rheometer. In recent years, microrheology has been proposed as an alternative method for determining these rheological properties by using microscale probes. Such measurements require only a small quantity of the sample and furthermore, provide the ability to determine the rheological properties at the microscale. In this work, we extend the approach for the purposes of determination of viscoelastic properties of the complex materials at the nanoscale. The molecular scale structure in the system as well as the specific chemical interactions significantly influence the rheological properties at these length scales. Molecular simulations provide the unique ability to systematically account for these effects.

In previous work, we used molecular dynamics (MD) simulations to study the translational motion of a nanoparticle in simple fluids with an emphasis on elucidating the effects of the channel surface, velocity slip at the nanoparticle surface and the interparticle hydrodynamic interactions on the nanoparticle motion. In this work, we present an active nanorheology MD simulation technique involving oscillatory motion of a nanoparticle in a complex fluid. The specific system that is studied consists of a nanoparticle embedded in a matrix material consisting of either a polymer solution or a polymer melt. The diffusion and hydrodynamic interactions in the system are completely determined by the intermolecular interactions. The response of the complex material to the nanoparticle motion is characterized in terms of the local viscoelastic properties of the matrix. Simulation results are used to elucidate the effects of molecular structure and interactions on the observed rheological behavior.