Binary superfluid mixtures have one interface. Three-component condensates have two — an inner and outer boundary. Rotating only the middle component drives shear at both interfaces simultaneously, but the shear and counterflow at each boundary can be tuned independently by adjusting the intercomponent interactions.
This paper exploits the two-interface geometry in a three-component Bose-Einstein condensate to map the instability landscape. In the strongly immiscible limit, Kelvin-Helmholtz instability dominates — the shear tears the interface apart. In the partially miscible regime, counter-superflow instability takes over — interpenetrating superfluid currents destabilize without requiring sharp boundaries. In between, both mechanisms coexist.
The onset condition for Kelvin-Helmholtz instability is derived from hydrodynamic pressure balance; the nonlinear evolution is tracked through Gross-Pitaevskii simulations; Bogoliubov-de Gennes analysis identifies the dominant unstable modes. The density modulations match the predicted spatial structures. The three-component system modifies the instability mechanisms relative to binary mixtures: the second interface introduces new mode couplings and parameter regimes inaccessible in two-component systems. Two interfaces isn't just twice one interface — it's a qualitatively different instability landscape.