Comments on heterogeneous nucleation in helium

We first recall some basic ideas on homogenous and heterogeneous nucleation. We then reconsider the heterogeneous nucleation of solid helium from liquid helium in the presence of walls. Recent experiments by Ruutu et al. demonstrate that the presence of walls drastically reduces the energy barrier for such "heterogeneous" nucleation events, although walls are usually more favourable to the liquid phase. In order to explain this reduction, we propose a simple model based on the unpinning of an already existing interface. In the light of such results, we then reconsider the nucleation of the B-phase of superfluid helium 3 from its A-phase. We propose a model where the B-phase nucleation is thermally activated on walls, without need of cosmic rays.