Fermionic and Bosonic Bubbles and Foam
  The positioning of one or more bubbles inside a many fermion or boson system, does not affect the volume, surface or curvature terms in the liquid drop expansion of the total energy. If Coulomb effects are irrelevant, the only contribution to the ground state energy of a system with one or more voids arises from quantum effects, which is similar to the Casimir effect in vacuum or in critical phenomena. We discuss the characteristics of such systems, the interplay among various effects, such as shell corrections and chaotic behavior, and briefly mention the role of the temperature and pairing. 
Aurel Bulgac, Siu A. Chin, Harald A. Forbert, Piotr Magierski, and   Yongle Yu