AbstractIt is shown, in confirmation of earlier work, that when a viscoelastic liquid flows into a capillary tube, a definite fraction of the inlet pressure drop is required to overcome effects of elastic origin. This pressure drop, ΔPE, is proportional to the same power of shear rate as the steady‐state first normal stress difference measured by means of the jet thrust method. The values of ΔPEare some 1.6–8.6 times higher than the values of the first normal stress difference, the amount depending on the fluid in use but not on the elastic power‐law exponentm. The importance of the extensional mode of deformation in entry region flow is discussed. It is shown by examining dilute polymer solutions after varying periods of degradation that changes in ΔPEare closely related to changes in the turbulence suppression characteristics of the solutions. The extensional mode of deformation is presumably important in both types of experiment. Bothn‐pentanol and a mineral oil, tested in the same small‐diameter capillary tubes as the polymeric solutions, give well‐defined values of ΔPE, which are compared with previously published normal stress figures. The mineral oil is shown to develop particularly high stresses in elongational flow, though the viscous stresses were also higher than for the polymeric solutions used. Previous evidence of the existence of shear elasticity in organic liquids is listed and the potential importance of the case of mineral oi
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