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Ions are used in normal liquid helium to investigate the hydrodynamics in presence of electrostriction. The Navier-Stokes equation must be modified in order to account for the spatial variations of density and viscosity around the ion because of the local pressure increase due to electrostriction. The solutions of the modified hydrodynamic equations are compared to experiments in normal liquid ⁴He. The issue matters if the liquid actually freezes around the ion. The analogies with the transport of the O₂ ^- ion in dense Neon gas are described.

At T=2.17 K and P=0, liquid helium undergoes the superfluid transition. Dose this transition belong to the same universality class of the usual liquid-vapor transition? This question has been addressed in experiments in which the ion mobility has been studied. It appears that the mobility is not singular at the transition though its slope appears to be infinite. The explanation for this result is given by solving the Navier-Stokes equations for a granular fluid composed by interwoven islands of normal fluid and superfluid. Investigations of the ion mobility at the melting transition have put into evidence the existence of an electrostriction-induced, superfluid transition in the liquid surrounding the positive ions.

This chapter presents the experimental results of the study of the mobility of ions in liquid ³He at the liquid-vapor transition. It shows how electrostriction in a region in which the compressibility is the highest affects both positive ions and negative ions, though in a quantitatively different way that depends on the different structure of the two kinds of charges. The interesting observation that the mobility minima are located on the extrapolation of the coexistence curve into the one-phase region is emphasized. The correlation of this behavior with that of O₂ ^- ions in dense argon gas is put into evidence.

Experiments on the mobility of electrons in dense helium gas elucidated how localized electron states develop when the gas density gas is increased. Up to 77 K, the density dependence of the mobility clearly shows that the formation of electron bubbles is a continuous phenomenon. Localization of electrons in bubbles also appears at high temperatures if the density is so large that the free energy of the localized state is negative enough. Percolation and hydrodynamic models have been devised to explain the continuous transition from high-mobility states to low-mobility states. It is shown that density-dependent, quantum multiple scattering effects modify the energy of the nearly free electron in a way that can be accurately described by heuristically modifying the kinetic theory prediction.