Tardy, Blaise L.; Richardson, Joseph J.; Greca, Luiz G.; Guo, Junling; Ejima, Hirotaka; Rojas, Orlando J.
Abstract: Adhesion occurs by covalent bonding, as in reactive structural adhesives, or through noncovalent interactions, which are nearly ubiquitous in nature. A classic example of the latter is gecko feet, where hierarchical features enhance friction across the contact area. Biomimicry of such structured adhesion is regularly achieved by top‐down lithography, which allows for direction‐dependent detachment. However, bottom‐up approaches remain elusive given the scarcity of building blocks that yield strong, cohesive, self‐assembly across multiple length scales. Herein, an exception is introduced, namely, aqueous dispersions of cellulose nanocrystals (CNCs) that form superstructured, adherent layers between solid surfaces upon confined evaporation‐induced self‐assembly (C‐EISA). The inherently strong CNCs (EA> 140 GPa) align into rigid, nematically ordered lamellae across multiple length scales as a result of the stresses associated with confined evaporation. This long‐range order produces remarkable anisotropic adhesive strength when comparing in‐plane (≈7 MPa) and out‐of‐plane (≤0.08 MPa) directions. These adhesive attributes, resulting from self‐assembly, substantially outperform previous biomimetic adhesives obtained by top‐down microfabrication (dry adhesives, friction driven), and represent a unique fluid (aqueous)‐based system with significant anisotropy of adhesion. By using C‐EISA, new emergent properties will be closely tied with the nature of colloids and their hierarchical assemblies.
