Effect of flow kinematics on the extensional viscosity of dilute polymer solutions

Abstract We investigate the effects of flow kinematics on the extensional viscosities of dilute polymer solutions by conducting dissipative particle dynamics simulations under uniaxial, planar, and biaxial extensional flows. At high extension rates, dilute polymer solutions exhibit strain hardening under these flows, whereas the quantitative behavior depends on the flow type. To elucidate the physical origin of this flow-kinematics dependence, we relate the extensional viscosity to the polymer conformation using an analytical expression derived from a single-chain model. The resulting relation allows us to separate the contribution of flow-induced polymer conformational changes and the purely kinematic contribution determined by the structure of the velocity gradient tensor. When polymers remain almost unperturbed by extensional flows, extensional viscosity differences are governed primarily by purely kinematic effects. In contrast, as the polymers are stretched, the gyration radius in the extensional direction becomes the dominant factor, and differences in the degree of stretching in this direction lead to corresponding variations in the extensional viscosity.