  The Rayleigh-Plateau instability (RPI) is a classical hydrodynamics
phenomenon that prevents a jet of liquid to flow indefinitely within air or
another liquid. Here, we show how adding a magnetic force field makes possible
its suppression. Enclosing the jet in a ferrofluid held by magnetic forces
allows flow focusing without sheath flow, which completely avoids dripping
failure at small flow rates and provides conditional stability for a continuous
fluid jet. Highly deformable liquid interfaces withstanding spatial and time
varying flow conditions within a large parameter space can be realized.
