Reversible Anisotropic Wet Adhesion Enabled by Rigid Amphiphiles

Abstract Wet adhesives such as glues are extensively used in everyday life and various engineering processes to integrate two different objects together. Usually a firm adhesion is desired in all applications, but the strong adhesion often brings up difficulties in revising an undesired adhesion or reusing the adhered objects. To solve this dilemma, we report an anisotropic wet adhesive that combine the advantages of strong adhesion and easy deadhesion together by incorporating a rigid amphiphile glycyrrhizic acid (GA) and a flexible polyelectrolyte poly (diallyldimethylammonium) chloride (PDDA) into the glue. The rigidity of the GA molecules allows formation of oriented hydrophobic GA bilayers on smooth surfaces under the mild pressure required for adhesion. The van der Waals forces between the hydrophobic chains are much weaker than hydrogen bonds, coordination, and electrostatic forces occurred between the glue and the adhered substrate. As a result, the shear force along the hydrophobic chain, which is vertical to the surface, is 6 ~ 13 times smaller than that along the surface. This immediately generates an anisotropic adhesion, which makes it possible to revise or detach an undesired adhesion by applying a gentle pulling force vertical to the surface. Reversible adhesion is possible by pressing back the detached part, and the undesired adhesives can be facilely cleaned with water. As the GA molecule is replaced by other rigid ones, similar anisotropic adhesion is observed. We envision the rigid molecules facilitated anisotropic wet glue would open a new paradigm in developing smart wet adhesives for building sustainable societies.