Liquid metal flows smoothly at room temperatures-领域趋势-仿生智能材料研究组  
Liquid metal flows smoothly at room temperatures 文献论文 柔性智能
lchaoxu 2020-8-28 1833

A photo of a droplet of liquid metal and a second image of the same metal flowing in a smooth stream after a voltage is applied


Researchers in the US have eliminated the instabilities in liquid metal streams for the first time, creating steady, controllable cylindrical “wires” that flow at room temperatures. Minyung Song and colleagues at North Carolina State University achieved the result by applying a small voltage to the metal, which allowed them to drastically lower its surface tension.

When fluids are pumped out of a nozzle, they typically minimize their surface energy by breaking up into droplets. This behaviour is driven by the Rayleigh-Plateau instability, and it occurs particularly rapidly for liquid metals, which have surface tensions that are significantly higher (and viscosities that are far lower) than room-temperature liquids like water. The effects of the instability can be suppressed by firing liquids from nozzles at high velocities, but for metals, such streams are short-lived, and travel only slightly further before breaking into droplets.

In their study, Song and members of a research team led by physicist Karen Daniels and chemist Michael Dickey overcame the issue by injecting a liquid stream of eutectic gallium indium (EGaIn) into a water-based electrolyte bath. At the same time, they applied a small voltage across the liquids. This applied potential triggered an oxidation reaction at the interface between the liquids, producing a thin, flowing oxide skin on the metal surface.

Once formed, this skin reduced the metal’s surface tension from over 500 mN/m to less than 0.1 mN/m – significantly lower than that of water. Subsequently, the EGaIn stream was able to retain its initial cylindrical shape for far longer, and over greater distances, before breaking up into droplets. The researchers compare this effect to adding soap molecules to water – although in this case, the metal’s original surface tension could be restored simply by removing the applied voltage.

Hair-like filaments

The researchers used this technique to produce flowing cylindrical “wires” of EGaIn on demand and in real time. These streams had numerous advantageous properties, including long lifetimes and velocities of around 1cm/s – far slower than the nozzle speeds required in previous techniques. In addition, the wires could be thinned down to diameters of just 0.1mm, forming hair-like filaments that flowed and bent over long distances.

Furthermore, the team found that the stream’s surface tension (and consequently its shape) could be finely tuned by varying the applied voltage. Beyond producing cylindrical wires, they could also generate diverse morphologies including balloons, clustered blobs, and tree-like fractals. Such a high degree of control over these shapes could give researchers powerful new tools for studying and manipulating the behaviours of fluids.

As well as having implications for fundamental research, Song and colleagues believe that their technique, which they describe in PNAS, could also make it possible to construct thin, stretchable conductive wires by coating electrolytes containing liquid metal streams with elastic sheaths. They now hope to explore the numerous potential applications for this technology through their future research.

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