A large fluid-bridge device to measure the deformation of drops in uniaxial extensional flow fields

摘要

Uniaxial extensional flow appears in transport and processing and is able to destroy desired rheological properties of emulsions by tearing drops. During the last few decades, many theoretical approaches used to describe the drop-deformationbehaviour of emulsions have involved the assumption that uniaxial extensional flow fields are involved. Recent experiments investigating drop deformation in planar extensional flow fields were restricted to single droplets in a surrounding fluid. Thepresent device considerably improves this situation by allowing one to study the drop-matrix interaction for several drops in uniaxial extensional flow. To determine the deformation of a few drops, a uniaxial flow field with a constant rate of extensionwith respect to space and time is necessary. This is realized by stretching a large cylindrical liquid bridge under microgravity. The bridge should remain cylindrical throughout the stretching. Therefore, a special mechanical adaptation is used tosuppress necking at the ends of the filament. However, bridge deformations caused by viscous stress and by dynamic and capillary pressure gradients still remain. The strain and shear rate distributions in the liquid bridge are evaluated in order todetermine the most suitable position for drop-deformation experiments. Examples of several drop-deformation experiments with elliptical and pointed-ends shapes of the stretched bridge are presented.