Effects of dilute low molecular weight poly(ethylene oxide) solutions in immiscible radial viscous fingering instabilities

We experimentally study the effects of normal stress differences in the immiscible radial viscous fingering instability in a Hele-Shaw cell. Dilute low molecular weight poly(ethylene oxide) (PEO) solutions are used as the displaced fluid to focus on elastic effects without shear-thinning behavior. The molecular weight of PEO is varied to investigate the effects of normal stress differences. The experimental observations reveal that nonmonotonic and opposing effects are evident depending on the molecular weight of the PEO and the stage of the radial viscous fingering evolution. Decreases in the PEO molecular weight reduce the number of fingers and widen the finger width in the early stage. However, the increase in the PEO molecular weight promotes tip splitting and narrows finger width in the early stage but suppresses tip splitting in the intermediate stage. Weissenberg numbers are estimated at different stages of radial viscous fingering instabilities. Tip splitting occurs at the highest Weissenberg number covered in this study and suppression of tip splitting is observed at intermediate Weissenberg numbers. At low Weissenberg numbers, we observe an increased finger width and a reduced number of fingers.