Viscous fingering in polymer solutions

Characteristics of viscous fingering patterns in polymer solutions were investigated by radially pushing air in a radial Hele‐Shaw cell containing hydroxypropyl methyl cellulose (HPMC) solutions. Low and high molecular weight HPMC samples were used. An increase in molecular weight easily yielded chain entanglements, which led to a strong shear thinning and an increase in elasticity. For the low molecular weight HPMC solutions, only the dense‐branching patterns were observed over the entire injection pressure range. When isopropyl alcohol was added into the HPMC solutions, leading to less elasticity, the pattern was tip‐splitting one, which is observed for Newtonian fluids. On the other hand, for the high molecular weight HPMC solutions, the resulting patterns showed a systematic change from dense‐branching to skewering patterns through tip‐splitting patterns as the injection pressure increased. The measured tip velocity was compared with the modified Darcy's law, but the coincidence is poor. The velocity corrected by the displaced area ratio provided a good linear relation.