Anisotropic surface effects on the formation of chiral morphologies of nanomaterials

Nanomaterials with chiral morphologies hold promise for a wide diversity of technologically important applications in such fields as micro/nano-electromechanical systems and medical engineering. Understanding the mechanisms underlying the formation of chiral morphologies of natural and synthesized materials remains an issue of crucial significance. In this study, a refined Kirchhoff rod model taking into account anisotropic surface effects is employed to describe quasi-one-dimensional nanomaterials with complicated spatial morphologies. It is shown that anisotropic surface stresses can induce the formation of rich morphologies of nanomaterials. A general shape equation of nanowires is derived by the variational method of energy. Thereby, the effects of anisotropic surface properties, bulk elastic properties and cross-sectional sizes on the chiral morphologies of nanomaterials are quantitatively investigated, and the conditions for the formation of binormal nanohelices are given. The physical mechanism addressed in this study is verified by our recent experiments on tuning the twisting chirality of polymer lamellae via surface treatments. Our analysis suggests that one can design and adjust the morphology of synthesized nanohelices by tailoring or functionalizing their surfaces during fabrication. This study is also helpful in interpreting the formation of such artificial and biological chiral materials as the flagella of bacterial and self-assembled helical ribbons.