Excess thermal noise and low-frequency oscillations in capillary flow of non-Newtonian aqueous polymer solutions exhibiting elasticity

Abstract

Results of thermal noise measurements (frequency range 0.01–1000 Hz) on aqueous solutions of poly(ethylene oxide) and maltose/poly(acrylamide) during capillary flow are reported. The flow of these solutions, both being of the pseudoplastic (shear thinning) type, gave rise to an excess thermal noise exhibiting a 1/fα-frequency distribution. The excess noise level increased with the flow rate. At a critical value of this rate, 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 1 Hz. Increasing the flow rate still higher caused the pulsations to disappear, the excess noise regaining its 1/fα character. The pulsations appear to be associated with the elasticity of the solutions used. Solutions with comparable shear thinning properties but lacking elasticity did not produce such effects. No excess thermal noise was observed with Newtonian solutions of comparable viscosity.

Electrical fluctuations and entry flow instabilities during capillary flow of aqueous poly (ethylene oxide) solutions

“…The spectral distribution of the noise measured at varying values of shear rate exhibited, on the whole, features similar to those reported in [23][24][25]. At rest the noise was white, its level corresponding to the thermal noise value calculated according to Nyqvist, cf.…”

“…1.6), that depended to some extent on PEO concentration, temperature etc. In [23][24][25], the low frequency end of the spectrum was thought to be a white noise plateau, the breaking point between this and the 1/f ~ region shifting to higher frequencies with ?). Within certain y-ranges, depending on experimental conditions, a set of distinct peaks appeared in the 1/f~ region, the peaks being multiples of a fundamental frequencyf0.…”

“…The flow noise data reproduced in the earlier papers [23][24][25] were obtained with platinum electrodes. Using other electrodes materials, such as gold, did not affect the results.…”

“…The flow cell containing the capillary, and the electronic equipment for the measurement of voltage fluctuations between two electrodes placed on both sides of the capillary have been described in [23][24][25].…”

“…In a series of papers [23][24][25], the present authors have reported on electrical fluctuations generated without any external voltage during capillary flow of aqueous solutions of PEO, PAM and some other systems, including non-aqueous solutions. The fluctuations (noise) were characterized by a 1/f ~ type frequency spectrum with a plateau at its low frequency end.…”