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.

Abstract:The excess thermal noise generated in polymer solutions through narrow capillaries is studied in detail for aqueous solutions of poly(ethylene oxide), (m w ~ 4" 106), of varying concentration. With increasing flow rate, the excess noise level increases, the noise spectrum assuming a 1/f~-form with e ~ 1.5. Within a critical flow range, distinct peaks appear in the spectrum, their frequencies being multiples of a fundamental frequency. The latter frequency ()Co) is found to increase with the flow rate; this variation, as well as that brought about by varying concentration and capillary dimensions, can be accommodated in a single curve correlating f0 with the shear rate at the capillary wall. No such correlation was found for the total noise level. The value off0 appeared to be determined by transversal oscillations of the liquid stream entering the capillary. Addition of small amounts of silica pamcies (Aerosil) led to the disappearance of the peaks in the spectrum.

No 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 ...

No abstract