Excess thermal noise and low-frequency oscillations during capillary flow of aqueous solutions of poly(ethylene oxide)

Abstract

The excess thermal noise generated in polymer solutions through narrow capillaries is studied in detail for aqueous solutions of poly(ethylene oxide),\((\bar M_w \approx 4 \cdot 10^6 )\), of varying concentration. With increasing flow rate, the excess noise level increases, the noise spectrum assuming a 1/fα-form with α ≈ 1.5. Within a critical flow range, distinct peaks appear in the spectrum, their frequencies being multiples of a fundamental frequency. The latter frequency (f0) 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 correlatingf0 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 particles (Aerosil) led to the disappearance of the peaks in the spectrum.