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.