Molecular dynamics simulation of uniaxial extensional deformation of Newtonian and non-Newtonian liquid bridges

The extensional rheology of Newtonian and non-Newtonian fluids is investigated by molecular dynamics simulations of appropriate model liquids in an extending liquid bridge configuration. The fluids are modeled as molecular species and the polymers as chains of monomers bonded by nonlinear springs, both interacting with atomistic solid walls. In the simulations, a cylinder of liquid is placed between solid end-plates which separate at an exponentially increasing rate until the liquid breaks up. A quantitative description of the time evolution of the liquid filament profile and the forces exerted on the end-plates is obtained and can be directly compared to experiment. In addition, monitoring of the atomic displacements and forces during simulation yields information on the internal dynamics of the fluid---the velocity and stress fields and the molecular configurations. The simulations are in good agreement with laboratory data and with the results of macroscopic numerical simulations of the device based on appropriate rheological models. In particular, in the range of deformations up to Hencky strains of &egr; ≈ 0.7, the dynamic response of the model liquids is well described by a lubrication approximation of Newtonian "reverse squeeze flow". Alignment of polymer chains in the model solution leads to strain-hardening of the filament manifested in a very uniform deformation of the filament over most of the flow regime. The stress fields are both spatially and temporally nonhomogeneous with characteristic layers of high stresses.