Formation of Positively Charged Liquid Helium Clusters in Supercritical Helium and their Solidification upon Compression.
Positively charged ions were produced in supercritical helium at temperatures from 6 to 10 K and up to 2 MPa using a corona discharge. Their mobility was measured via current-voltage curves, and the hydrodynamic radius was derived using Stokes law. An initial increase and subsequent decrease of hydrodynamic radius was observed and interpreted in terms of growth, compression and solidification of ion clusters. The mobility was modeled using a van der Waals-type thermodynamic state equation for the ion-in-helium mixed system and a temperature-dependent Millikan-Cunningham factor, describing experimental data both in the Knudsen and the Stokes flow region. Regions of maximum hydrodynamic radius and large compressibility were interpreted as boiling points. These points were modeled over a large range of pressures and found to match the Frenkel line of pure helium up to 0.7 MPa, reflecting similarity of density fluctuations in pure supercritical helium and gas-liquid phase transitions of ionic helium clusters.
†G2ELab-GreEn-ER, Equipe MDE, 21 avenue des Martyrs, CS 90624, 38031 Grenoble Cedex 1, France.|§Laboratory for Materials, Organization and Properties (LabMOP), Campus Universitaire - El Manar, 2092 Tunis, Tunisia.
Hejer Gharbi
HG
Tarchouna
†G2ELab-GreEn-ER, Equipe MDE, 21 avenue des Martyrs, CS 90624, 38031 Grenoble Cedex 1, France.
Nelly
N
Bonifaci
†G2ELab-GreEn-ER, Equipe MDE, 21 avenue des Martyrs, CS 90624, 38031 Grenoble Cedex 1, France.
Frédéric
F
Aitken
‡Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom.
Luis Guillermo
LG
Mendoza Luna
‡Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom.
Klaus
K
von Haeften
cluster size; helium cluster ions; liquid−solid phase transitions; mobility; supercritical helium