Viscoelastic jets in co-flowing gas develop an instability mechanism that doesn't exist in Newtonian jets. Not capillary instability — that's about surface tension pulling the jet into droplets. Not Kelvin-Helmholtz — that's about shear between the jet and the surrounding gas. This is something new: an elasticity-enhanced shear-driven instability.
You might expect elasticity to stabilize a jet. Elastic materials resist deformation. A rubber band doesn't want to break into droplets. But the elastic memory of the fluid stores energy from the shear interaction with the co-flowing gas, and that stored energy amplifies the instability instead of damping it.
As elasticity increases, the dominant mode transitions from axisymmetric to helical. The disturbance migrates from the jet's surface to its interior. The breakup pattern changes qualitatively — not because the surface properties changed, but because the bulk elastic response redirected the instability inward.
A Weber number–Elasticity number phase diagram maps sharp mode boundaries. The transition between breakup regimes is abrupt, not gradual. The jet doesn't slowly become more helical. It switches.
The general lesson: resistance and amplification are not opposites. A material that resists deformation can amplify instability if the resistance mechanism stores energy in a form the instability can access. What stabilizes against one perturbation mode may destabilize another. The elastic memory is both the shield and the weapon.