Highlighting the need for high-speed imaging in capillary breakup extensional rheometry

The capillary breakup extensional rheometer is commonly used to determine material properties of complex fluids. This is achieved by tracking the diameter evolution of a liquid bridge undergoing capillary thinning and breakup in a uniaxial extensional flow. Typically, the filament diameter evolution is tracked at the mid-plane between the two end-plates using a laser micrometer. We show using high-speed imaging that while this arrangement is satisfactory in flows where the filament is long (relative to its initial diameter), slender and approximately cylindrical, errors can be significant when the filaments are short (and with a non-negligible curvature) such as encountered when using the so-called slow-retraction-method and 'Dripping-onto-Substrate' rheometry. We will further highlight the need for high-speed imaging in CaBER experiments by considering errors induced when the laser micrometer is misaligned with the location of filament breakup. This latter source of error will be particularly relevant for capillary breakup experiments where the location of filament breakup is not typically known a priori, such as the case for many so-called 'yield-stress' fluids.