Couette flow of dilute solutions of macromolecules: embryo cells and overstability

Abstract

The outer cylinder was kept stationary whilst the inner rotated. Weak circulatory motion (‘embryo cells’) was observed in all the fluids investigated at Taylor numbers Ta less than the critical Taylor number Tac. These cells grew with increasing Ta, then burst into vigorous Taylor vortices at Ta = Tac. In solutions of rigid elongated molecules having concentrations above a minimum concentration, an axial oscillation (overstability?) of the embryo cells was observed. The oscillation did not occur in solutions of flexible folded molecules nor in the solvent. The oscillation was observed over a range Ta0 ≤ Ta ≤ Tac. For concentrations above the minimum Ta0/Tac decreased with increasing concentration.Other phenomena of the flow were also studied. It was found for solvent and solutions that the torque on the outer cylinder increased relatively less rapidly with the speed of rotation of the inner cylinder when \[ Ta/Ta_c > 1.08 \] than it did when \[ 1 < Ta/Ta_c\leqslant 1.08. \] In this latter range there was a torque reduction in the flow of the solutions relative to that in the flow of solvent. At sufficiently high speeds of rotation circumferential waves occurred in the fully developed Taylor cells (at Ta = Taw) and it was found that when R1/R2 = 0·95 then Taw/Tac = 1·5 for the solvent, whereas for the solutions Taw/Tac ≥ 1·5. When R1/R2 = 0·90 then Taw/Tac = 1·25, and for the solutions Taw/Tac > 1·25. No experiment was done at Ta > Taw. The normal-stress coefficients of the solution have been evaluated from the results.