Gallium sticks to new role as reversible adhesive

Some adhesives may soon have a metallic sheen and be particularly easy to unstick, following research at the Max Planck Institute for Intelligent Systems in Stuttgart, Germany, showing that the metal gallium makes an effective reversible adhesive. By inducing slight changes in temperature, researchers at the institute have found they can control whether a layer of gallium sticks or not, based on the fact that gallium transitions from a solid state to a liquid state at around 30°C.

A reversible adhesive of this kind could have applications everywhere that temporary adhesion is required, such as industrial pick-and-place processes, transfer printing, temporary wafer bonding, or for moving sensitive biological samples such as tissues and organs. Switchable adhesion could also be suitable for use on the feet of climbing robots.

As the researchers reveal in a paper in Advanced Materials, the principle behind this reversible adhesion is actually quite simple: above 30°C, gallium metal is liquid, and below 30°C it is solid. So if a drop of liquid gallium is introduced between two objects and then cooled to less than 30°C, the gallium layer solidifies and sticks the two objects together. When it is time to separate the objects, the temperature is raised to revert the gallium layer to its liquid state, allowing the objects to be pulled apart with a small amount of force.

As an adhesive, gallium works in a similar fashion to hot glue, which is widely used in DIY applications. The difference is that far less heating and cooling are required in the case of gallium; in addition, it lifts much more easily and cleanly from the surface, is highly repeatable, and is electrically conductive.

For their experiments, scientists working with Metin Sitti, director at the Max Planck Institute for Intelligent Systems, wetted the tip of a cylindrical elastomer rod with liquid gallium. They then brought the gallium droplet into contact with different materials such as glass, plastic and gold. After cooling the tip to 23°C, they found that the solidified gallium formed a strong bond between the elastomer and each of the materials.

The researchers also measured the effective binding power of gallium in both its liquid and solid states. "The behavior of these two values tells us something about the true reversibility and switchability of the adhesion process," explains Sitti. The greater the difference in the binding power between the liquid and solid state, the easier it is to reverse and switch the adhesive effect.

The team deliberately tested gallium on particularly rough and damp surfaces as well. "These are surface conditions that showed up as major weaknesses of reversible micro/nanostructured adhesives proposed recently," says Sitti. Adhesives that can bind strongly to rough or wet surfaces have always had poor reversibility, but this isn’t the case with gallium. The Stuttgart-based team have become convinced of its effectiveness in damp conditions, even testing it under water. Although its binding power and reversibility when wet are reduced compared to dry conditions, they still remain strong enough for a wide range of applications.

Sitti emphasizes that gallium's performance in damp conditions makes it ideal for biological applications. He foresees a time when gallium may be used to move individual cells, tissue samples or even organs, for example in laboratory or hospital settings.

Another possible field of application is industrial manufacturing, especially where fragile components such as ultra-thin graphene membranes or tiny electronic chips are involved. These components could be picked up by gallium-coated grippers and then set down at the precise location where they are required, such as a circuit board. In technical jargon, this kind of assembly technology is called ‘pick and place’ and is currently conducted using vacuum suction.

Sitti believes the temperature-controlled gallium adhesive has two main advantages over vacuum suction. "Wetting an object with a metallic liquid such as gallium that forms a bond when cooled slightly is a far gentler process for fragile materials than sucking them up using a vacuum," he says.

A gallium adhesive would also be more energy efficient, because once an object adheres to the gallium layer, no more energy is required to sustain the adhesive bond. Only when the adhesion needs to be reversed is the metal quickly heated to 30°C. The vacuum technique, however, requires the constant use of suction in order to maintain the adhesive effect.

To achieve rapid heating and cooling as required in their tests, the team in Stuttgart connected a Peltier element to their experimental set-up; this element either releases or absorbs heat when an electric current is applied. For practical applications in the future, however, the scientists anticipate that the adhesive bond could also be reversed remotely using infrared radiation or using electrical Joule heating through conductive wiring integrated into a surface.

Sitti sees robotics as another possible application for this adhesive. For example, climbing robots that may one day ascend wind turbines for maintenance purposes could benefit from reversible adhesives. By activating the adhesive, the robot foot would be fixed to the wall of the turbine; when taking a step, the adhesive layer between the foot and the wall would be briefly heated by means of an integrated heating element.

Another advantage of gallium as an adhesive is that it can be used for many cycles without needing to be replaced, because the liquid metal lifts completely from the substrate under proper loading and unloading conditions. No residues are left on the surface and the adhesive loses none of its own substance. "Good adhesives are generally hard to separate from the substrate," says Sitti, explaining that in gallium's case the material forms a fine oxide layer in air. This shell of gallium oxide ensures that there is no residue left behind when the adhesion is reversed.

Gallium has other advantages as well. "We can use it at different scales, from the nanometer range to microelectronics, and right up to larger applications," adds Sitti. In theory, it could even be used to lift a fully-grown person, as long as the contact surface was sufficiently large. However, it would be most cost-effective, energy efficient and practical for use with smaller objects.

Sitti and his team have already started exploring some of the potential applications of a gallium adhesive and are also working to optimize the technique. Up to now, for example, the gallium was applied to an elastomer rod around two millimeters in diameter for all the tests. "We want to test other elastomer geometries and designs with different length scales and see if we can enhance the binding strength as we do so," says Sitti. The scientists also plan to study alloys of gallium with other metals such as indium.

This story is adapted from material from Max-Planck-Gesellschaft, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.