Tip-splitting instabilities in the channel Saffman-Taylor flow of constantviscosity elastic fluids

Boger fluids are used to study viscous fingering growth in viscoelastic flu ids in channel Hele-Shaw flow. We have found that the viscous finger growin g in the Boger fluid is unstable to tip splitting at high velocities, in a regime where a Newtonian viscous finger is stable. No fracturelike instabil ities were observed. We show that the viscoelastic normal stress difference s arising in shear and extensional Row reach very high values at sheer and extensional rates comparable to those achieved at the tip of the finger at the onset of tip splitting, and the fluid becomes highly anisotropic. The v iscoelastic stress could affect the dynamics of the finger and induce the t ip-splitting instability.