The Richtmyer–Meshkov instability (RMI) occurs when two fluids of different density are impulsively accelerated.  Normally this is by the passage of a shock wave.  The development of the instability begins with small amplitude perturbations which initially grow linearly with time.  This is followed by a nonlinear regime with bubbles appearing in the case of a light fluid penetrating a heavy fluid, and with spikes appearing in the case of a heavy fluid penetrating a light fluid.  A chaotic regime eventually is reached and the two fluids mix.  This instability can be considered the impulsive-acceleration limit of the Rayleigh–Taylor instability.