Electrically Induced Liquid Metal Droplet Bouncing

Abstract

Liquid metals, including eutectic gallium–indium (EGaIn), have been explored for various planar droplet operations, including droplet splitting and merging, promoting their use in emerging areas such as flexible electronics and soft robotics. However, three-dimensional (3D) droplet operations, including droplet bouncing, have mostly been limited to nonmetallic liquids or aqueous solutions. This is the first study of liquid metal droplet bouncing using continuous AC electrowetting through an analytical model, computational fluid dynamics simulation, and empirical validation to the best of our knowledge. We achieved liquid metal droplet bouncing with a height greater than 5 mm with an actuation voltage of less than 10 V and a frequency of less than 5 Hz. We compared the bouncing trajectories of the liquid metal droplet for different actuation parameters. We found that the jumping height of the droplet increases as the frequency of the applied AC voltage decreases and its amplitude increases until the onset of instability. Furthermore, we model the attenuation dynamics of consecutive bouncing cycles of the underdamped droplet bouncing system. This study embarks on controlling liquid metal droplet bouncing electrically, thereby opening a plethora of new opportunities utilizing 3D liquid metal droplet operations for numerous applications such as energy harvesting, heat transfer, and radio frequency (RF) switching.

Advances in Microscale Droplet Generation and Manipulation

“…44 It is also being used in emerging application spaces such as microfluidic acoustic metamaterial. 45 Electrowettingbased manipulation also allows operations such as droplet generation, oscillation, 46,47 merging, 48 splitting, 49 bouncing, 50 and mixing. 51 Additionally, spontaneous charging of the droplet has also been utilized for droplet manipulation applications.…”

3D actuation of foam-core liquid metal droplets

“…Although we have previously demonstrated the vertical jumping of functional magnetic LM droplets controlled using an electromagnet, 32 this approach only allows very short locomotion along the vertical direction due to the weak magnetic force and bulky experimental setups. Other groups also investigate the vertical jumping of LM droplets in an electrolyte controlled by electrochemical oxidation and reduction; 33,34 such a method only allows instant vertical movement without the ability to provide continuous and precise 3D actuation. Moreover, by employing an acid droplet and liquid metal, the spontaneous and steady rotation of droplets is achieved, 35,36 which provides a new route for the design of potential applications (e.g., micro-and nanodevices).…”

An Oscillation System Based on a Liquid Metal Droplet and Pillars under a Direct Current Electric Field

“…Over the years, many techniques have been developed to modify the charge distribution. For instance, Zavabeti et al demonstrated that pH or ionic concentration gradients across a liquid metal droplet induce both deformation and surface Marangoni flow; 20 Liu et al proposed a magnetic actuation and realized the deformation and motion of the liquid metal droplet; 21 Tang et al introduced a simple approach to rapidly vary and tune the size of liquid metal microdroplets by controlling the interfacial tension of the metal using both electrochemistry and electrocapillarity by applying an electrical potential to the liquid metal stream; 22 Idrus-Saidi et al reduced the surface tension of the droplet to almost zero using a combination of electrocapillary and vacuum filtration; 23 He et al effectively lowered the surface tension of the liquid metal droplets by applying 1.5 V voltage, causing the liquid metal to transform from a drop to a wire in NaOH solution, and the live liquid metal wire deformed under a magnetic field; 24 successfully achieved liquid metal droplet bouncing using continuous AC electrowetting; 25 and Song et al demonstrated an electrochemical approach to induce jumping behavior in liquid metal droplets. 26 The "mercury beating heart (MBH)" phenomenon, a classic deformation process of a liquid metal droplet (LMD), has been studied by many researchers since the nineteenth century.…”