Transverse instability of bubbles in viscoelastic channel flows

The trajectory of a stream of bubbles is measured in a microfluidic pressure-driven flow. For Newtonian fluids with or without surfactants, or viscoelastic fluids without surfactants, the bubbles maintain stable positions equally spaced along the channel center line. However, for viscoelastic fluids with surfactant, a transverse instability is observed, which has the characteristics of a linear instability, whose growth rate increases with the speed of the flow. The results are shown to be consistent with the action of shear-induced normal stresses in the fluid, where the magnitude increases with shear rate and so is augmented by surfactants that make the interface rigid.