Defect Topology in Colloidal Smectics

Abstract

Colloidal smectics -- layered structures formed in dense suspensions of rod-like particles -- often exhibit grain boundaries, across which the layer orientation changes by $90^{\circ}$. Motivated by this feature, we develop a layer-based topological framework that treats orthogonal grain boundaries as constituents of the ground state rather than as exceptional defect structures. Extending the layer-based approach for ordinary smectics, we reduce the smectic structure to layers, half-layers, and domain walls. We classify the topology of defects and their combination rules based on this structure. In two dimensions, point defects are described by semi-directed cycle graphs. Although the disclination charge remains a valid topological invariant, it does not uniquely classify defects, as distinct graphs may share the same charge. In three dimensions, line defects are classified by their transverse graph structure, while point defects exhibit qualitatively different behavior. In particular, we show that the hedgehog disclination charge is not a topological invariant, but instead varies continuously under smooth deformations of the layer structure.