Tardy Blaise L, Richardson Joseph J, Greca Luiz G, Guo Junling, Ejima Hirotaka, Rojas Orlando J
Department of Bioproducts and Biosystems, School of Chemical Engineering, Aalto University, Espoo, 02150, Finland.
Department of Materials Engineering, The University of Tokyo, Tokyo, 113-8656, Japan.
Adv Mater. 2020 Apr;32(14):e1906886. doi: 10.1002/adma.201906886. Epub 2020 Feb 16.
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 (E > 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.
粘附作用通过共价键合发生，如在反应性结构胶粘剂中，或通过非共价相互作用发生，这种相互作用在自然界中几乎无处不在。后者的一个经典例子是壁虎的脚，其分层结构增强了接触区域的摩擦力。这种结构化粘附的仿生通常通过自上而下的光刻技术来实现，该技术允许定向分离。然而，由于缺乏能够在多个长度尺度上产生强粘性、内聚性自组装的构建单元，自下而上的方法仍然难以实现。在此，引入了一个例外，即纤维素纳米晶体（CNC）的水分散体，它们在受限蒸发诱导自组装（C-EISA）过程中在固体表面之间形成超结构化的粘附层。由于与受限蒸发相关的应力，固有强度高的CNC（E>140 GPa）在多个长度尺度上排列成刚性的、向列有序的薄片。当比较平面内（≈7 MPa）和平面外（≤0.08 MPa）方向时，这种长程有序产生了显著的各向异性粘附强度。这些由自组装产生的粘附特性大大优于以前通过自上而下微加工获得的仿生粘合剂（干粘合剂，摩擦驱动），并代表了一种具有显著粘附各向异性的独特的基于流体（水性）的系统。通过使用C-EISA，新出现的特性将与胶体的性质及其分层组装紧密相关。
