APPARATUS FOR EXTENSIONAL VISCOSITY MEASUREMENTS

Although most analyses in tribology deal with the behavior of fluids i n shear, many fluids, such as greases or printing inks, can develop si gnificant forces when subjected to pure extension. These forces can im pact performance, especially in the exit region of tribological interf aces. The resistance of a fluid to an imposed shear rate is a measure of the fluid's shear viscosity (usually just referred to as its viscos ity). The resistance of a fluid to an imposed extensional strain rate is a measure of the fluid's extensional viscosity. In this paper, two techniques for the measurement of extensional forces are discussed. A subsequent companion paper will discuss interpretation of the force da ta in terms of extensional viscosity. Both techniques described have t he advantage of a dimensionally small measurement element. The first t echnique involves the use of a vapor deposited surface pressure transd ucer. This transducer is a thin strip of ytterbium. The electrical res istance of ytterbium is pressure sensitive. Small changes in resistanc e can be related to extensional stress. The extensional viscometer app aratus consists of two counter-rotating cylinders. As the fluid exits the nip between the cylinders, the extensional stress is detected by a transducer attached to one of the cylinders. The second technique dis cussed herein involves the use of a small-beam transducer in conjuncti on with the counter-rotating cylinder apparatus. The deflection of the beam due to the fluid's extensional force is detected and interpreted in terms of extensional stress as a function of strain rate at the ex it of the nip. Extensional stresses of several hundred thousand Pa hav e been measured.