VELOCITY AND ELONGATION RATE DISTRIBUTIONS IN STRETCHED POLYMERIC AND NEWTONIAN LIQUID BRIDGES

In a Plateau simulation (neutral buoyancy tank) large cylindrical liquid bridges (initial length and diameter of 50 mm), which are placed between two circular plane disks and fixed by surface/interface tension, have been stretched with constant disk velocities. Next to the shape transitions of the bridge contour the velocity field within the stretched liquid bridge was measured by tracing particles visible in a light sheet. The observed contour transitions as well as the flow pattern could be correlated with the Capillary number. From the measured velocity field the local elongation and shear rate distributions were determined. Only in liquid bridges that show a cylindrical shape during stretching are the local elongation rates homogeneous within the bridge. Contracted or necked bridges show inhomogeneous elongation rate distributions and considerable shearing. The experiments show that the average and the actual local maximal elongation rates can be approximated from the changing contour data of the bridge. Conclusions are drawn for elongational viscosity measurements using stretched liquid bridges.