PHY. Rev. Let.: magic liquid metal snowflake

Finger like branching patterns can be produced by a variety of diffusion mechanisms: viscous diffusion, directional solidification, zero surface tension fluid, bacterial colony growth, dielectric breakdown and so on Liquid metal has many potential applications because of its special properties in the fields of heat, optics, electricity and so on However, it is known that the surface tension of any room temperature metal fluid is very large, which makes it difficult to flow, so it is unlikely to form finger branch pattern A team from North Carolina State University found that liquid metal droplets can grow into snowflakes When they tried to use a drop of liquid gallium indium alloy as an electrode in an electrochemical cell, they found some strange phenomena: within seconds, the metal began to diffuse into a snowflake shape This is very unusual behavior for liquids with high surface tension, because they are difficult to diffuse, let alone form such patterns Device diagram and process diagram of liquid metal snowflake (source: PHY Rev Let.) however, applying medium voltage will cause the reaction between gallium indium alloy and surrounding electrolyte (aqueous solution of sodium hydroxide) The metal surface will be oxidized one by one, which will reduce its surface tension Similarly, sodium hydroxide will also reduce the surface tension of water Because of gravity, the liquid metal diffuses into a disk, and the interference on its surface continues to break its edge into individual "petals" As they grow, petals split into more branches, forming a complex pattern of fingerlike branches Dynamic analysis of the formation of liquid metal Snowflakes (source: PHY Rev Let.) the largest liquid metal snowflakes are about 20 times the size of the original droplets, and are rapidly formed in less than half a minute Interestingly, the researchers found that this effect can only occur in a narrow voltage range of about 2V If the voltage is too low than 2V, the surface oxidation effect of the metal droplet is too weak to disturb its shape If the voltage is too high, the oxide layer on the surface of the liquid metal will become too thick without the phenomenon of metal droplet diffusion and fracture The relationship between potential and volume (source: PHY Rev Let.) shows that the surface chemical oxidation process effectively reduces the interfacial tension, and drives the instability of liquid alloy gallium indium until the oxide becomes too thick and hinders further oxidation This behavior of liquid metal is of great interest for a variety of reasons: it shows that the instability of liquid metal can be produced by adjusting the oxide layer, and the instability of metal can be finally stopped; its surface can be used to locate and control the pressure stress at the liquid interface with appropriate voltage; it has the potential to be applied to equipment requiring reconfigurable shape conductor Moreover, the first mock exam shows a new self similar dynamic model The key to the analysis is to determine the correlation between local effects and non local effects The ability to change the shape of a metal in a simple, low-power and scalable way can help create new electronic devices Liquid metals have been used in small vehicles, such as wheels Future work will focus on the quantitative control of compressive stress, including the development of time dynamics and application potential If we can control the shape of liquid metal accurately, it can help us design new soft electronic products Paper link: https://journals.aps.org/prl/abstract/10.1103/physrevlett.119.174502