Evidence for Condensation ofHe3Atoms on the Surface of Bubbles in LiquidHe4

A phenomenological model is presented to explain the reduction in the critical velocity for vortex-ring creation by negative ions in liquid helium when small concentrations of ${\mathrm{He}}^3$ impurities are added. The model is based on the assumption that the surface state for a ${\mathrm{He}}^3$ atom has a lower energy than the bulk state. This results in an enhanced concentration of ${\mathrm{He}}^3$ atoms on the surface of the negative ions. This change in the surface affects the flow of liquid helium in the boundary layer around the ion. The effects of the difference in the surface of the positive and negative ions on the critical velocity is discussed in both the continuous and instantaneous vortex-ring creation models.