Determining the Anchoring Strength of a Capillary Using Topological Defects

We consider a smectic-A in a capillary with surface anchoring that favors parallel alignment. If the bulk phase of the smectic is the standard twist-grain-boundary phase of chiral smectics, then there will be a critical radius below which the smectic will not have any topological defects. Above this radius a single screw dislocation in the center of the capillary will be favored. Along with surface anchoring, a magnetic field will also suppress the formation of a screw dislocation. In this note, we calculate the critical field at which a defect is energetically preferred as a function of the surface anchoring strength and the capillary radius. Experiments at a few different radii could thus determine the anchoring strength.