Underwater mussel-inspired adhesive formed by simple coacervation

Mussels possess a remarkable natural ability to adhere on wet surfaces such as rocks in marine environments. This adhesion is attributed to proteins that undergo phase separation to form coacervates. These coacervates can further assemble into systems that evolve into structured fluids, where a polymer-rich viscoelastic phase becomes continuous while the solvent-rich phase remains dispersed. Here, we report a poly(acrylic acid)-based system functionalized with cationic and hydrophobic moieties that reproduces key features of this behavior. Upon pH variation, the polymer undergoes self-coacervation, leading to the formation of dense droplets. These droplets progressively sediment and coalesce, resulting in a local increase in concentration. This process is accompanied by a transition toward a continuous polymer-rich phase in which water droplets are retained, consistent with a phase inversion driven by concentration increase. Rheological measurements show that this transition is associated with a marked increase in viscoelasticity and interfacial adhesion, as quantified by tack measurements. These results highlight how the balance between electrostatic and hydrophobic interactions governs the transition from a dispersed coacervate state to a continuous viscoelastic material with adhesive properties. This bioinspired system offers a promising strategy for the development of effective underwater adhesives and opens new avenues in the design of biomimetic materials for wet-surface bonding applications.