A simple technique is described for investigating the elasticity of polymeric liquids. The method is based on the extensional flow through a typical converging channel , i.e., a conical channel. It is shown that, by measuring flow rate and pressure drop for the laminar flow of a viscoelastic liquid through a conical channel of known shape, it is possible to detect its elasticity level. As the first step in using this method, it is necessary to obtain the Cp-Re data (i. e., the pressure drop coefficient vs Reynolds number data) for several Newtonian fluids. As expected from dimensional analysis, for Newtonian fluids, the Cp-Re curve is found to be unique with the property that at sufficiently low Reynolds numbers, Cp data are inversely proportional to the Re. The unique curve obtained this way can then be used as the baseline (or reference line) representing all inelastic liquids. In the next step, tests are performed with the solution of interest (for example, a polymer solution). Any probable deviation is between their Cp-Re data from that of the inelastic baseline, is attributed to the elasticity of the material, and so the fluid is labeled as elastic. To investigate the applicability of the method, tests were conducted with two different polymer solutions (i, e., the 0.2% w/w
PAA solution and the Ml fluid) both having known elastic behavior in “steady
shear flow”. Based on the data obtained using the conical channel, and as judged by the extent of the deviation from the inelastic baseline, it is concluded that, the Ml fluids is more elastic than the PAA solution in both steady shear flow and extensional flow. The technique thus seems to be capable of investigating the elastic behavior of polymeric liquids quite successfully.
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