Hi Liquid metal manoeuvred into new applications.

Hi Liquid metal manoeuvred into new applications.

A technique for controlling the surface tension of liquid metals could lead to applications in reconfigurable electronic circuits, antennas and other technologies.

Developed at North Carolina State University, the technique is said to hinge on the fact that the oxide “skin” of the metal – which can be deposited or removed – acts as a surfactant, lowering the surface tension between the metal and the surrounding fluid.

The researches used a liquid metal alloy of gallium and indium. In base, the bare alloy has a high surface tension of about 500 millinewtons (mN)/metre, which causes the metal to bead up into a spherical blob.

‘But we discovered that applying a small, positive charge – less than one volt – causes an electrochemical reaction that creates an oxide layer on the surface of the metal, dramatically lowering the surface tension from 500mN/meter to around 2mN/meter,’ said Dr Michael Dickey, an associate professor of chemical and biomolecular engineering at NC State and senior author of a paper describing the work. ‘This change allows the liquid metal to spread out like a pancake, due to gravity.’

According to NC State, the researchers also showed that the change in surface tension is reversible. If researchers change the polarity of the charge from positive to negative, the oxide is eliminated and high surface tension is restored.  The surface tension can be tuned between these two extremes by varying the voltage in small steps.

‘The resulting changes in surface tension are among the largest ever reported, which is remarkable considering it can be manipulated by less than one volt,’ Dickey said in a statement. ‘We can use this technique to control the movement of liquid metals, allowing us to change the shape of antennas and complete or break circuits. It could also be used in microfluidic channels, MEMS, or photonic and optical devices. Many materials form surface oxides, so the work could extend beyond the liquid metals studied here.’

Dickey’s lab had previously demonstrated a technique for 3D printing liquid metals, which used the oxide layer formed in air to help the liquid metal retain its shape, the opposite of what the oxide layer does to the alloy in a basic solution.

‘We think the oxide’s mechanical properties are different in a basic environment than they are in ambient air,’ said Dickey.

Researchers at NC State have developed a new method to control the interfacial energy of a liquid metal via electrochemical deposition (or removal) of an oxide layer on its surface using ~1 volt.

Hi Crumpled ‘graphene paper’ has potential to store energy.

Hi Crumpled ‘graphene paper’ has potential to store energy.

Layers of graphene can be bonded and then deformed to form a material with properties that could be useful for creating stretchable super capacitors for flexible electronic devices.

The finding is reported in the journal Scientific Reports by MIT’s Xuanhe Zhao, an assitant professor of mechanical engineering and civil and environmental engineering, and four other authors. The new, flexible superconductors should be easy and inexpensive to fabricate, the team claimed.

‘Many people are exploring graphene paper: It’s a good candidate for making supercapacitors, because of its large surface area per mass,’ Zhao said in a statement.

He said the development of flexible electronic devices, such as wearable or implantable biomedical sensors or monitoring devices, will require flexible power-storage systems.

Like batteries, supercapacitors can store electrical energy, but they primarily do so electrostatically, rather than chemically.

Now Zhao and his team have demonstrated that by crumpling a sheet of graphene paper into a mass of folds, they can make a supercapacitor that can easily be bent, folded, or stretched to as much as 800 per cent of its original size. The team has made a simple supercapacitor using this method as a proof of principle.

The material can be crumpled and flattened up to 1,000 times, the team has demonstrated, without a significant loss of performance.

‘The graphene paper is pretty robust and we can achieve very large deformations over multiple cycles,’ Zhao said. ‘

Graphene, a structure of pure carbon just one atom thick with its carbon atoms arranged in a hexagonal array, is one of the strongest materials known.

To make the crumpled graphene paper, a sheet of the material was placed in a mechanical device that first compressed it in one direction, creating a series of parallel folds or pleats, and then in the other direction, leading to a rumpled surface. When stretched, the material’s folds smooth themselves out.

Forming a capacitor requires two conductive layers – namely two sheets of crumpled graphene paper - with an insulating layer in between, which in this demonstration was made from a hydrogel material.

Like the crumpled graphene, the hydrogel is highly deformable and stretchable, so the three layers remain in contact even while being flexed and pulled.

Though this initial demonstration was specifically to make a supercapacitor, the same crumpling technique could be applied to other uses, Zhao said. For example, the crumpled graphene material might be used as one electrode in a flexible battery, or could be used to make a stretchable sensor for specific chemical or biological molecules.

The research team included Jianfeng Zang at Huazhong University of Science and Technology and Changyang Cao, Yaying Feng, and Jie Liu at Duke University. 

The work was supported by the US Office of Naval Research, the US National Science Foundation, and the National 1000 Talents Program of China.