Charged complexes at the surface of liquid helium
  Charged clusters in liquid helium in an external electric field form a two-dimensional system below the helium surface. This 2D system undergoes a phase transition from a liquid to a Wigner crystal at rather high temperatures. Contrary to the electron Wigner crystal, the Wigner lattice of charged clusters can be detected directly. 
A.M. Dyugaev, P.D. Grigoriev, P. Wyder