Origins of normal stress in capillary jets of newtonian and viscoelastic liquids

Abstract

The individual contributions to the observed normal tensile stress in capillary jets of Newtonian and viscoelastic liquids, measured by means of expansion-contraction experiments described in earlier publications of the authors, is discussed. The surface tension contribution is calculated. The contribution from the relaxing velocity profile is determined empirically for the Newtonian jet; it is estimated for the viscoelastic jet. The internal normal stress, developed in the capillary, and the external normal stress, developed beyond the capillary nozzle exit, are described, but not in a quantitative manner. Because of the complexity of the equations describing the viscoelastic jet, these stresses are not yet able to be separated in the general case. At high ejection velocities, however, the profile relaxation and the external normal stress should be smaller than the internal normal stress if the liquid is highly viscoelastic. One should, however, be fully aware of the approximate nature of the results when expansion-contraction experiments are used to obtain a measure of the normal stress developed during flow in a capillary. Such experiments, unfortunately, are not as useful as one would like them to be.