Programmable liquid materials

Liquid metals such as eutectic gallium indium alloy (EGaIn), due to their unique attributes such as voltage controlled surface tension, high liquid-state conductivity and liquid-solid phase transition at room temperature, open new avenues towards the development of programmable liquid materials.

Researchers at the University of Sussex and Swansea University, UK have exploited the self-locomotion, self-rotation, voltage controlled surface tension, high liquid-state conductivity and deformation characteristics of EGaIn and modulated its stiffness and density so that it would be possible to program the liquid metal in to a desired shape.

A liquid metal blob (certain quantity) in an electrolyte solution is highly conductive. In the absence of contact with any of the electrodes, external force and an applied voltage, the electrolyte induces a uniform charge distribution on the blob’s surface (Fig. 1(a)). However, when the blob is in contact with the anode (Fig. 1(b)) and a suitable voltage is applied to one or more of the other electrodes, the difference in the conductivity between the electrolyte and the liquid metal alters the charge distribution on the blob’ surface. The formation of an electric double-layer (EDL) at the blob’s interface enables deformation of liquid metal in the direction of the electric field (Fig. 1(b)). Hence, it would be possible to deform the liquid metal from anode (high voltage electrode) to cathode (low voltage electrode) to any desired shape (Fig. 1(c)).

Fig. 1 Influence of electric field on the deformation of liquid metal: (a) A liquid metal blob far from the electrodes and in absence of field is subject to no force; (b) Deformation of the blob upon contact with anode and an electric voltage is applied across it; and (c) Deformation of liquid metal to a desired shape.

The basic electrode array control algorithm to deform liquid metal in to a desired shape is shown in Fig. 2. In this system arrangement, the cathode attracts the liquid metal and the anode keeps the liquid metal in a wet and flat state (having the lowest surface tension). Hence, by switching only one selected electrode as cathode (low voltage) and setting all other electrodes as anodes (high voltage), the movement of liquid metal can be controlled. The relative voltage difference decides the speed of liquid metal deformation.

Fig. 2 Basic electrode array control algorithm to make alphabet letter “S”.

Three main problems that are inherent to liquid metal deformation still remains to be solved:

• H₂ evolution at the cathode causes the liquid metal to branch out as multiple trees
• Higher surface tension of liquid metals at smaller size leads to splitting
• Liquid metal body interference stops its movement towards the cathode

The programmable liquid materials will find applications in soft robotics and shape changing, reconfigurable electronic circuits and display domains.

T.S.N. Sankara Narayanan

For a more detailed information, the reader may kindly refer: Yutaka Tokuda et al., Programmable Liquid Matter: 2D Shape Deformation of Highly Conductive Liquid Metals in a Dynamic Electric Field, Proceedings of the Interactive Surfaces and Spaces on ZZZ -ISS ’17 (2017). DOI: 10.1145/3132272.3134132  

Synthesis of atomically thin metal oxides at room temperature using liquid metals – A novel approach to expand the realm of 2D materials

Metals when exposed to air under ambient conditions leads to the formation of self-limiting atomically thin oxide layer at the metal-air interface, which is considered to be a naturally occurring two-dimensional (2D) material. However, isolation of 2D metal oxides from the metal surface poses considerable challenges.

Researchers at RMIT University Australia, Queensland University of Technology,  Australia and California NanoSystems Institute, University of California, USA have shown that it would be possible to synthesis atomically thin metal oxides (2D metal oxide) at room-temperature using liquid metals as reaction environment (Reference: Ali Zavabeti et al., A liquid metal reaction environment for the room-temperature synthesis of atomically thin metal oxides. Science, 2017; 358 (6361): 332 DOI: 10.1126/science.aao4249)

In this study galinstan (liquid metal alloy containing gallium, indium and tin) was used as a reaction environment. Galinstan alloyed ~1 wt % of elemental hafnium, aluminum, or gadolinium served as the precursors for the formation of their respective oxides (HfO₂, Al₂O₃ and Gd₂O₃). The choice of these alloying elements were made on the basis of thermodynamic considerations (Gibbs free energy (ΔG_f) value).

Two different methods were proposed for isolating the surface oxides; (i) van der Waals (vdW) exfoliation technique; and (ii) gas injection method.

The van der Waals (vdW) exfoliation technique is quite similar to the method for obtaining monolayer of graphene which involves touching the liquid metal droplet with a solid substrate. The liquid nature of the parent metal allows a clean delamination of the oxide layer (Fig. 1). This technique is suitable for the production of high-quality thin oxide sheets on substrates.

The second technique relies on the injection of pressurized air into the liquid metal, in which the metal oxide forms rapidly on the inside of air bubbles and rose through the liquid metal. When the released air bubbles pass through deionized water placed above the liquid metal, allows dispersion of the oxide sheets in the aqueous suspension. Subsequently, the suspension can be subjected to drop casting to prepare 2D metal oxide films on suitable substrates (Fig. 2). This technique is highly scalable and hence suitable for the synthesis of the target oxide nanosheets with high yield.

Fig. 1 Schematic representation of the van der Waals exfoliation technique. The pristine liquid metal droplet is first exposed to an oxygen-containing environment. Touching the liquid metal with a suitable substrate allows transfer of the interfacial oxide layer.

Fig. 2 Schematic representation of the gas injection method (left), photographs of the bubble bursting through the liquid metal (center), and an optical image of the resulting sheets drop-cast onto a SiO₂/Si wafer (right)

The findings of the study indicate that oxide layers formed on liquid metals can be manipulated by an appropriate choice of alloying elements based on Gibbs free energy. The two method proposed to isolate the 2D nanosheets require simple experimental set-up and allows either a direct deposition on solid surfaces or formation of an aqueous suspension that can be drop cast over a variety of substrates. The methodology outlined in this study provides a novel pathway for the synthesis and easy isolation of 2D materials that was previously inaccessible.

The 2D materials, viz., HfO₂, Al₂O₃ and Gd₂O₃, synthesized in this study hold promise for applications in energy storage, such as supercapacitors and batteries. HfO₂ can be used as an ultrathin insulator dielectric material for the fabrication of field-effect transistors.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer Ali Zavabeti et al., Science, 2017; 358 (6361): 332 DOI: 10.1126/science.aao4249