We present results of an experimental study of Saffman-Taylor fingers in dilute solutions of polystyrene in dioctyl phthalate (DOP). While fingers in pure DOP obey classical scaling laws, we observe transitions to narrow fingers in polysterene solutions of concentrations as low as 1 part per million. The onset of narrowing occurs at a shear rate which roughly equals the inverse of the polymer Zimm relaxation time and depends only weakly on concentration. The onset of instability occurs earlier than in Newtonian fluids and is often initiated by small-amplitude oscillations.

• Received 29 October 1991

DOI:https://doi.org/10.1103/PhysRevA.45.R2165