Frequency Analysis of Electrical Fluctuations — A New Method to Study Flow Instabilities in Capillary Flow
Results of measurements of electrical thermal noise (fluctu. ations in the frequency range 0.01 — 1000 Hz) on elastic, shear thinning polymer solutions of polyethylene oxide, Polyacrylamide and maltose/polyacrylamide during capillary flow are reported. The flow of these solutions gave rise to a thermal noise exhibiting a 1/f-frequency distribution, increasing in intensity with the flow rate. At a critical flow rate, when flow instabilities were observed, the noise showed a pulsating character. The corresponding frequency spectra contained a number of sharp peaks, all being multiples of a fundamental frequency of the order of 1Hz, indicating a very regular nature of the flow instabilities.
Solutions with comparable shear thinning properties but lacking elasticity did not produce pulsations in the frequency range used.
The observed effects were compared with the results of frequency analysis of the pressure fluctuations observed during extrusion of polymer melts through dies (melt fracture and shark skin phenomena).