  The injection of a fluid into another fluid causes a spatiotemporal pattern
along the injection front. Viscous fingering is a well-known example when the
replaced material is a viscous fluid. Notably, most fluids are, in reality,
viscoelastic, i.e., they behave as an elastic solid over short timescales. For
this reason, it is important to study the situation when the replaced fluid is
viscoelastic. In this study, we observe extensive tip-splitting in the
fingering pattern when an incompressible organic liquid was injected into an
oleophilic Hele--Shaw cell filled with an aqueous viscoelastic fluid made of a
wormlike micellar solution. The tip-splitting led to thin fingers with a
characteristic size comparable to four times the cell thickness. We examined
the material properties and suggest that the thin fingering pattern observed in
our current system is due to the delamination of viscoelastic fluid from the
bottom substrate surface. Our result shows that the effect of interfacial
energy in the existing solid layer should be considered in the injection
process.
