Anomalous ion radius near the λ transition

Measurements have been made of the shear viscosity eta and the positive ion mobility mu along the melting curve in liquid ⁴He, passing through the lambda triple point. The effective radius of the ion, defined by Stokes' law, shows a maximum 40 mK below the triple point. This and similar behaviour observed previously by Ahlers and Gamota at saturated vapour pressure are explained by the presence of normal fluid in the immediate vicinity of the ion due to the electrostrictive increase in pressure. The maximum observed at the melting curve is evidence that the lambda line continues into the region of supercooled liquid.