AB interface in superfluid 3He and Casimir effect
  The friction force on the moving interface between two different vacuum states of superfluid 3He is considered at low temperature. Since the dominating mechanism of the friction is the Andreev reflection of the massless "relativistic" fermions, which live on the A-phase side of the interface, the results are similar to that for the perfectly reflecting mirror moving in the quantum vacuum. 
G.E. Volovik