We have all encountered thin elastic sheets in our day to day life and found them to be not flat. For example, plant leaves, flower petals, curtains hanging from a wall often wrinkle at their edges. Such phenomena arise because the system trades in-plane elastic energy for out of plane bending energy. Inspired by these macroscopic systems and the advent of utilizing buckling instabilities for applications, we set out to discover strategies to introduce wrinkling and buckling in microscopic sheets called colloidal membranes. Colloidal membranes are monolayers of micron long rod shaped particles within which all rods are uniformly aligned along one axis and free to diffuse within the membrane (a),(c). We showed the existence of a curvature instability that arises during the crystallization of these monolayer membranes of chiral colloidal rods. While the bulk of the membrane crystallizes, its edge remains fluid like and exhibits chiral ordering. The resulting internal stresses cause the flat membrane to buckle macroscopically and wrinkle locally (b),(d). Our work also has indirect relevance to cell membrane remodeling, which is usually driven by specialized proteins acting in conjunction with chiral building blocks of the membranes. The microscopic constituents and driving forces that assemble colloidal membranes and lipid membranes are fundamentally different. Despite these differences, both lipid membranes and colloidal membranes are governed by the same elastic energy as both systems have the same set of symmetries. Consequently, our results also point to a phase transition-driven pathway that may be relevant to membrane remodeling.
(a) Schematic of a fluid colloidal membrane. (b) Schematic of a polycrystalline colloidal membrane assembled with more than one nucleation center. Yellow rods highlight the domain walls. (c) Microscope image of a fluid colloidal membrane. Scale bar,10 μm. (d) Microscope image of solid colloidal membrane. Scale bar, 10 μm
Lachit Saikia, T. Sarkar. M. Thomas, V. A. Raghunathan, A. Sain and Prerna Sharma, Curvature Instability of Chiral Colloidal Membranes on Crystallization 8, 1160 (2017).