MEASUREMENT OF EXTENSIONAL VISCOSITY BY STRETCHING LARGE LIQUID BRIDGES IN MICROGRAVITY

A liquid bridge stretching technique for measuring the extensional vis cosity of polymer solutions is presented which provides the boundary c onditions necessary for pure uniaxial elongation with constant extensi on rate. The experiments were carried out in the drop-tower facility o f ZARM at the University of Bremen which provides microgravity conditi ons of about 10(-5) g0 for 4.7 s. The liquid bridge is generated under microgravity and held between two plates by surface tension. The init ial diameter and length of the cylindrical liquid bridge are 50 mm. Th e fluid bridge is elongated by increasing the plate distance exponenti ally to a maximum length of 200 mm providing a maximum Hencky strain o f about is-an-element-of = 1.4. At the same time the plate diameter ca n be reduced exponentially from 50 to 25 mm in order to preserve the c ylindrical contour of the fluid bridge. The extensional viscosity grow th function is determined from the axial force which is measured as a function of time. One Newtonian and different viscoelastic samples wer e investigated. All samples show initially a linear viscoelastic regio n where the Trouton ratio T(r) rises to a value of 3. Beyond this regi on the Trouton ratio remains in the range of 3 for the Newtonian sampl e and increases for the viscoelastic samples. The measured extensional viscosity's did not reach a constant value during the experiments. Th e reproductibility of the measured data is very good and it could be s hown that the extensional viscosity growth function of the investigate d polymer solutions increase with the polymer concentration and the ex tension rate. Nevertheless, the increasing values are mainly determine d by the total strain.