Optical Observations of Disorder in Solid Helium 4

It has been proposed that the supersolidity of helium 4 is associated with disorder in the solid samples. We report optical observations of this disorder in samples grown by different methods. We show that, when grown by the blocked capillary method as in most experiments showing anomalous phenomena, solid helium 4 can be polycrystalline with grain sizes down to the micrometer range, much smaller than the 0.1 mm found in some other experiments. We analyze the properties of grain boundaries, and in particular the existence of liquid channels at the contact of grain boundaries with walls. Our analysis includes measurements of the contact angle of the liquid-solid interface with these walls, which exhibits a large hysteresis. Furthermore, we predict that similar channels should exist at the junction between grain boundaries.