Zoom on a rolled-up vortex during the nonlinear stage of the Kelvin-Helmholtz instability for an initially sheared plasma with a density jump. The colours indicate the passive tracer of the two plasmas (blue and yellow), initially on both sides of the velocity shear. Small-scale vortices at x ~ 60, y ~ 40 (expressed in ion inertial length units) are a signature of a secondary Kelvin-Helmholtz instability that develops in the large-scale vortex. The simulation was performed with the Two-fluid code.