  We explore the interfacial instability that results when a Newtonian fluid (a
glycerol-water mixture, inner fluid) displaces a viscoelastic fluid (a dense
cornstarch suspension, outer fluid) in a radial Hele-Shaw cell. As the ratio of
viscosities of the inner and outer fluids is increased, side branched
interfacial patterns are replaced by more stable interfaces that display
proportionate growth and finger coalescence. We correlate the average finger
spacing with the most dominant wavelength of interfacial instability, computed
using a mathematical model that accounts for viscous fingering in miscible
Hele-Shaw displacements. The model predictions on the role of viscosity ratio
on finger spacing are in close agreement with the experimental observations.
Our study lends insight into the significant contribution of the
viscoelasticity of the outer fluid on the morphology and growth of interfacial
patterns.
