Excess thermal noise generated during Poiseuille flow of certain polymer solutions

Abstract

Abstract:The previously reported results concerning the generation of excess thermal noise induced by capillary flow of aqueous solutions of poly(ethylene oxide) (PEO) are supplemented by measurements on the following solutions: PEO/DMF, PEO/i-PrOH, PS/THF, PVAC/cyclohexanone, and poly(acrylamide)/ water. Similarly to the previous findings, a noise level increasing with the flow rate is recorded, the noise exhibiting a 1/f~-frequency spectrum. Within a certain flow range, distinct peaks are recorded in the spectrum (harmonics of a fundamental frequency, Jò). Thef0-values of the various solutions under varying flow conditions arrange themselves along a common f0-shear rate curve. They appear to be associated with transversal oscillations of the solution upstream the capillary entranee.Key words: Electrical noise, capillary flow, instability, polymer solution lntroductionIn a previous investigation, electrical noise induced by Poiseuille flow of aqueous solutions of poly(ethylene oxide) (PEO) was studied [1]. With PEO-solutions having a sufficiently high molecular weight the noise intensity increased with the flow rate. At a critical flow rate (capillary, typical diameter 0.2 mm, L / D ~ 20), a number of narrow peaks appeared in the frequency spectrum of the flow induced noise, the peak frequencies being harmonics of a fundamental frequency. At the onset of these oscillations, the overall noise intensity increased sharply with the flow rate.The purpose of the present investigation was to verify the generation of electrical noise above the thermal (Nyqvist) level in some other systems, including solutions of PEO in dimethylformamide (DMF) and isopropyl alcohol containing 9% water. Also studied were solutions of polystyrene (PS) in tetrahydrofuran (THF), poly(vinyl alcohol) in cyclohexanone, and of poly(acrylamide) in water. In the latter case, the rheological properties o f the solution were varied by adding a substantial quantity of maltose. For PEO, also the influence of the molecular weight was investigated. 910The excess noise generated during capillary flow of polymer solutions appears to represent a general phenomenon; the same is true of the oscillations appearing in the frequency spectra of such noise. Results to be published show that these oscillations are associated with a periodic movement of the flowing solutions at the capillary entrance. The present method may thus be considered as a new way to study flow instabilities and related phenomena. ExperimentalThe experimental set-up has been described in detail earlier [1]. It consisted of two glass reservoirs (volume ca. 100 ml) with the glass capillary placed between them (180 ° entry, length 2.4 or 5mm, diameter 0.2mm). The flow through the capillary was produced by applying vacuum to one of the reservoirs. The electrodes (platinum, 20 x 20 x l mm) were placed 30mm from the capillary ends; their position in the reservoir did not influence the results [1 ].The noise was measured with a preamplifier and an FFTfrequëncy analyser (Princeton Applied Research ...

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.…”