Unified theory of chiral smectic A monolayers and $\pi$-wall defects

Monodisperse suspensions of the rodlike chiral \textit{fd} viruses are condensed into one rod length thick colloidal monolayers of aligned rods by depletion forces. Twist deformations of the molecules are expelled to the monolayer edge as in a chiral smectic A (Sm-A*) liquid crystal, and a cholesteric (Ch) region forms at the edge. Coalescence of two such isolated monolayers results in a cholesteric wall sandwiched between two regions of aligned \textit{fd} viruses, dubbed $\pi$-wall defects. Based on the analogy of Sm-A* with superconductors, we develop a unified theory of the $\pi$-wall defects and the monolayer edge structure. Our model yields the molecular tilt profiles, the local thickness change, and the crossover from Sm-A*-to-Ch behavior across the monolayer and the $\pi$-wall. These allow us to determine the line tension as a function of the depletant polymer concentration and the chirality of the viruses, in agreement with experiment.