Visualizing, manipulating, and imprinting $\pi$-wall defects in self-assembled colloidal membranes

Geometric frustration and the resulting topological defects play an important role in determining the structural, mechanical and optical properties of materials. Here we describe the behavior of a new type of defect, called a $\pi$-wall, in a model system of colloidal membranes composed of chiral rod-like \textit{fd} viruses. We use complimentary optical microscopy techniques to study the structure and energetics of $\pi$-walls, and develop a model based on the analogy between liquid-crystals and superconductors to determine the structure and energetics of $\pi$-walls. We then focus on $\pi$-wall formation, showing that $\pi$-walls naturally assemble through a unique coalescence process in which chiral frustration plays an essential role. $\pi$-walls can also be artificially created and engineered using externally applied optical forces.