  The molecules in lyotropic membranes are typically aligned with the surface
normal. When these molecules are chiral, there is a tendency for the molecular
direction to twist. These competing effects can reach a compromise by producing
helicoidal defects in the membranes. Unlike thermotropic smectics, the centers
of these defects are hollow and thus their energy cost comes from the line
energy of an exposed lamellar surface. We describe both the
twist-grain-boundary phase of chiral lamellar phases as well as the isolated
helicoidal defects.
