Unduloid formation in colloidal membranes

Flat colloidal membranes composed of filamentous fd viruses were doped with shorter viruses of identical pitch and width to form metastable saddles. For sufficiently low long rod fractions in a range of osmotic pressures, these saddles coalesce into axisymmetric tubular shapes. We propose a simple steric model to explain how the introduction of short rods leads to a preferred curvature and employ an effective geometric theory to calculate these shapes, which are closely related to the unduloids studied in the theory of droplets.