Folding of chiral colloidal membranes*

Rod-like viruses self-assemble into one rod-length thick liquid-like monolayers in presence of depletion attraction induced by non-adsorbing polymers. The global constraint of 2-D geometry of these colloidal membranes is incompatible with local chiral interactions of the rods. Consequently, competition between these opposing tendencies leads to a variety of polymorphic transitions. We illustrate this through phenomena that cause flat colloidal membranes to transform into 3-D structures. Crystallization of colloidal membranes proceeds through the usual nucleation and growth pathway. However, the growing crystalline domains spontaneously wrinkle to satisfy local chiral interactions and on completion of the growth lead to macroscopic buckling of the membrane. In contrast, fluid colloidal membranes can be folded by simply doping them with shorter rods of same chirality. These uniformly mixed bi-disperse membranes become unstable and deform into saddle shapes. The origin of this instability lies in enhancement of Gaussian modulus of the monolayer membranes due to doping. At high doping ratio, membranes fold into higher order structures of increasing complexity. These include unduloids, tri-unduloids, unduloid-enneper surfaces as well as system spanning plumber’s nightmare-like phases. Taken together, these results show how chirality may have similar implications for conventional lipid bilayers which are rich in chiral molecules like cholesterol.