Coalescence of crystalline drops

We present the first experimental analysis of drop coalescence in a case where the dynamics is not governed by viscous dissipation in the bulk nor by the inertia of the fluid flow, only by the geometry and mobility of surfaces. We found such a situation in the physics of 3He crystals near 0.32 K where the latent heat of crystallization vanishes. Two crystalline drops of 3He coalesce if their crystalline orientations are identical: a neck forms after the contact at time t=0, and the shape evolves towards that of one convex crystal by local growth and melting in a fraction of a second. We have found that the neck radius initially increases as t(1/3), as predicted by Maris. This behavior is also expected for superfluid drops. It is clearly distinguished from the logarithmic behavior and from the t(1/2) power law which have been predicted by Eggers et al. in more usual situations.