Capillary thinning extensional rheometry for Newtonian fluids

The extensional viscosity of a fluid can be calculated from the rate at which a filament thins under capillary forces. Capillary thinning rheometry involves two plates with fluid between them, which are rapidly separated to a fixed distance to form a narrow fluid filament. This filament then thins under capillary forces, typically developing a narrow, cylindrical thread at its center, and a pair of rounded reservoir regions attached to either plate. Current models assume that thinning is dominated by forces within the central thread, but the effects of reservoir dynamics have never been thoroughly explored. Here, we present the results of experiments on well-characterized silicone oils that undergo capillary thinning in the Newtonian regime, investigating the influence of experimental parameters such as fluid volume, fluid viscosity, plate diameter, and total plate separation distance on thinning rates, reservoir dynamics, and apparent extensional viscosities. We find that for medium- to high-viscosity silicone oils (>1 Pa s), the choice of experimental parameters, particularly the initial fluid aspect ratio (defined as the plate separation distance over the plate diameter) systematically influences filament thinning rates. Greater aspect ratios are associated with a pronounced change in reservoir shape and volume during an experiment, leading to additional forces acting on the central thread, resulting in faster thinning and, thus, lower apparent viscosities. Our results demonstrate that all capillary thinning rheometry experiments should begin with an investigation into the effects of experimental parameters, particularly the initial aspect ratio, to determine the uncertainty and the reliable parameter space associated with each unique setup.