Viscosity and positive-ion mobility near the melting transition in liquidHe4

New measurements are presented of the shear viscosity $\eta$ and the positive-ion mobility $\mu$ near the melting transition in liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. Interpreted in terms of the Stokes law for the drag on a sphere in a viscous medium, it is found, contrary to expectation, that the effective radius of the ion remains constant or decreases slightly as the melting transition is approached at constant temperature. Attempts to explain this observation have not been successful. On the other hand, an older mystery concerning the effective radius of the ion is cleared up: Ahlers and Gamota, comparing data for $\eta$ and $\mu$ at the vapor pressure curve found that the effective radius has a maximum 40 mK below the $\lambda$ transition (i.e., the lower triple point). We have observed a similar maximum on the melting curve, 40 mK below the upper triple point. It is shown that these maxima may be accounted for by an electrostrictively induced $\lambda$ transition around the ion. In this interpretation, the maximum along the melting curve serves as the first empirical evidence that the λ line extends into the region of supercooled liquid at pressures above the melting curve.