New underwater adhesive gets stronger in water

Although many everyday objects are held together by adhesives, their importance often goes unnoticed until a shoe sole comes loose or a piece of furniture breaks. For engineers, however, adhesives present a major challenge: conventional products generally perform poorly underwater, complicating efforts to maintain ocean infrastructure such as pipelines.

The problem arises because water molecules coat surfaces and create a repulsive barrier that prevents adhesives from forming strong bonds. Water can also erode and wash away adhesive materials over time.

In a new study, researchers have developed a specially engineered underwater adhesive that is unusually strong and partially reusable. The material works through a mechanism based on a supramolecular ionic liquid and solvent-exchange-mediated self-assembly.

The term "supramolecular chemistry" refers to the study of complex structures formed through interactions among multiple molecules rather than through conventional molecular bonds. In this adhesive, exposure to water triggers the molecular components to reorganize and form strong bonds between submerged surfaces.

The researchers combined flexible and rigid molecular components in a supramolecular ionic liquid known as BP16TPB. They then mixed it with the powerful solvent dimethyl sulfoxide (DMSO), which efficiently dissolves BP16TPB and breaks some of its molecules into mobile charged particles.

When exposed to water, these particles reorganize and become locked into a new, stable structure. The resulting network is strengthened by the reformation of hydrogen bonds as well as another type of molecular interaction known as π–π stacking.

The adhesive also exploits the Marangoni effect, which describes how liquids respond to changes in surface tension. This phenomenon can produce the so-called "tears of wine" effect and plays an important role in surface coating, floating actuators and the movement of liquids through channels narrower than a human hair.

Crucially, the new adhesive has strong hydrophobic properties that repel the layer of water normally formed on submerged surfaces and often acting as a barrier to adhesion.

The combination of hydrophobicity and non-covalent interactions within the supramolecular structure drives the transformation of a flowable precursor into a dense, water-resistant network, converting the initially fluid material into a strong adhesive.

The researchers tested the adhesive on a wide range of water-submerged materials, including ceramics, epoxy and plastics.

The material demonstrated exceptionally strong adhesion, reaching a bonding strength of 1.1 megapascals after just 10 seconds underwater—substantially outperforming conventional underwater adhesives.

It also showed remarkable durability. During more than three years of continuous underwater immersion, the adhesive was able to support a 2-kilogram load.

The researchers said the long-term static-load test provides strong proof of concept, demonstrating the material's resistance to water-induced surface degradation and structural creep over a period of several years.

The adhesive was also partially reusable. Tests involving detaching and reattaching the material underwater showed that it could maintain reliable adhesion over eight cycles.

The adhesive remained functional in a range of electrolyte solutions, including acidic, alkaline and saline environments. However, its performance was sensitive to temperatures above 70°C.

The researchers said the study not only introduces a high-performance underwater adhesive but also offers a strategy for developing environmentally adaptable smart materials with potentially broad applications.