Measurement of extensional viscosity by stretching large liquid bridges in microgravity

Abstract

A liquid bridge stretching technique for measuring the extensional viscosity of polymer solutions is presented which provides the boundary conditions necessary for pure uniaxial elongation with constant extension rate. The experiments were carried out in the drop-tower facility of ZARM at the University of Bremen which provides microgravity conditions of about 10 −5 g o for 4.7 s. The liquid bridge is generated under microgravity and held between two plates by surface tension. The initial diameter and length of the cylindrical liquid bridge are 50 mm. The fluid bridge is elongated by increasing the plate distance exponentially to a maximum length of 200 mm providing a maximum Hencky strain of about ϵ = 1.4. At the same time the plate diameter can be reduced exponentially from 50 to 25 mm in order to preserve the cylindrical contour of the fluid bridge. The extensional viscosity growth function is determined from the axial force which is measured as a function of time. One Newtonian and different viscoelastic samples were investigated. All samples show initially a linear viscoelastic region where the Trouton ratio T r rises to a value of 3. Beyond this region the Trouton ratio remains in the range of 3 for the Newtonian sample and increases for the viscoelastic samples. The measured extensional viscosity's did not reach a constant value during the experiments. The reproducibility of the measured data is very good and it could be shown that the extensional viscosity growth function of the investigated polymer solutions increase with the polymer concentration and the extension rate. Nevertheless, the increasing values are mainly determined by the total strain.