Optimal liquid bridges for extensional rheometry

The capillary thinning of liquid bridges is commonly exploited for measuring the extensional viscosity of Newtonian and complex fluids. The viscosity is extracted typically by comparing the prediction of the capillary thinning obtained with a mid-filament stress balance against the radial thinning data experimentally obtained with a test sample. The prediction of the stress balance is expected to be valid only close to pinch-off, and often cannot match the full thinning profile of the mid-filament radius.

Recent work has highlighted the significant influence that the geometric parameters of a liquid bridge can have on the early stages of capillary thinning in Newtonian, as well as, viscoelastic polymer solutions. We compared the effect of the sample volume and aspect ratio of the bridge on the agreement between mid-filament stress-balance predictions with results obtained with 1D simulations of liquid bridges. It appears that, for a Newtonian liquid bridge, these geometric parameters can be chosen carefully such that the stress-balance predictions agree quantitatively with the full-filament results over nearly the entire duration of the radial thinning. For such a "magic combination" of the volume and aspect ratio, nearly quantitative agreement is also obtained for viscoelastic liquid bridges of polymer solutions. The implications of this finding for capillary-breakup rheometry of Newtonian liquids and viscoelastic polymer solutions will be discussed.