Linear viscoelastic characterization from filament stretching rheometry

Traditionally, linear viscoelasticity is measured using small amplitude oscillatory shear flow. Due to experimental difficulties, shear flows are predominately confined to the linear and mildly nonlinear regime. On the other hand, extensional flows have proven more practical in measuring viscoelasticity well into the nonlinear regime. Therefore at present, complete rheological characterization of a material requires two apparatuses: a shear and an extensional rheometer. This work is focused on developing a linear viscoelastic protocol for the filament stretching rheometer (FSR) in order to measure both linear and nonlinear dynamics on a single apparatus. With a software modification to the FSR motor control, we show that linear viscoelasticity can be measured via small amplitude squeeze flow (SASF). Squeeze flow is a combination of both shear and extensional flow applied by axially oscillating parallel plates. During deformation, the fluid response is measured on the bottom plate mounted on a weight cell. Amplitude sweeps are conducted to identify the operating window. In order to determine optimal sample dimensions, varying aspect ratios are investigated. Results of the squeeze flow measurements are validated by comparing G’ and G” as a function of frequency for SASF and SAOS. We show that in the low frequency range the two techniques agree very well. At higher frequencies, SASF still needs optimization. Never the less, deviation stays below 50 % for all experiments.