Apparatus for Extensional Viscosity Measurements

Abstract

Although most analyses in tribology deal with the behavior of fluids in shear, many fluids, such as greases or printing inks, can develop significant forces when subjected to pure extension. These forces can impact performance, especially in the exit region of tribological interfaces. 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 viscosity). 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 data in terms of extensional viscosity. Both techniques described have the advantage of a dimensionally small measurement element. The first technique involves the use of a vapor deposited surface pressure transducer. This transducer is a thin strip of ytterbium. The electrical resistance of ytterbium is pressure sensitive. Small changes in resistance can be related to extensional stress. The extensional viscometer apparatus 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 discussed herein involves the use of a small-beam transducer in conjunction 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 exit of the nip. Extensional stresses of several hundred thousand Pa have been measured.