Liquid Metal Gallium Micromachines Speed Up in Confining Channels

Abstract

One of the great challenges in the field of tiny machines is the capability of engineering liquid metal micromachines to autonomously move within confining channels to perform various complex tasks. Herein, liquid metal gallium micromachines that significantly increase their velocity in confining channels with adaptive deformation under exposure to an electric field are presented. The liquid metal gallium micromachines move toward the negative electrode under the propulsion of hydrogen bubbles, which is obviously different from the previous report that liquid metal gallium alloy (i.e., Galinstan) micromachines move to the positive electrode, owing to the surface tension gradient. More importantly, the liquid metal gallium micromachine can adaptively deform and accelerate in confined channels. The velocity of liquid metal gallium micromachines increases with the decrease in the width of channels. It is found that the speed-up motion of liquid metal gallium micromachines is caused by the enhanced electro-osmosis effect in confining channels. The findings not only help understand the role of the confinement effect on the motion of liquid metal micromachines but also provide a novel strategy to manipulate the motion velocity of liquid metal micromachines for specific applications. Micromachines capable of converting chemical energy and other forms of energies to mechanical movement have received much attention over the past decades. [1-9] These micromachines hold promise in the fields of microfabrication, [10-12] material assembly, [13,14] detection, [15,16] sensing, [17,18] detoxification, [19] and targeted drug delivery. [20-22] Most of the current microma-chines are composed of rigid materials, due to the limitation of deformation; however, it remains a challenge for the specific applications of the reported micromachines. As is well known, the liquid metal gallium and gallium-based alloys display fluidic and metallic properties and have deformability, a low melting point, and a supercooling effect at room temperature. [23] With the release of the Hollywood film "Terminator 2," more and more studies have focused on how to construct a liquid metal micromachine with a T-1000-like motion and deformation capability. Recently, several liquid metal-based micromachines were reported, which can autonomously move in fluids under the propulsion of bubble, [24,25] pressure, [26] ionic gradient, [27] ultrasound field, [28] electrical field, [29-31] and magnetic field. [32,33] To achieve the applications in complex environments, liquid metal micro-machines are often required to move in confining channels. However, to our best knowledge, the influence of the confining environment on the motion of the liquid metal-based micromachine has not been reported yet, which is important for the precise control of their movement behavior and future applications. Herein, we report that the liquid metal gallium droplet micromachines propelled by the generated hydrogen bubbles move toward the negative electrode in sodium hydroxide (NaOH) solution under exposure to an electric field, but they speed up with the adaptive deformation in the confining channels. The movement velocities of the gallium droplet micromachine increase with the width of the microfluidic channels. We demonstrate that this speed-up effect in the confining channels is ascribed to the increasing electro-osmotic effect. Our work provides a new proof for the underlying mechanism of electrically driven liquid metal micromachines and also their motion manipulation. Figure 1A schematically illustrates the autonomous movement of the liquid metal Galinstan and gallium micromachines in the NaOH solution under a direct current (DC) electric field at 35 C. Liquid metal Galinstan and gallium micromachines with a mass of 50 mg were submerged, respectively, in a polydimethylsiloxane (PDMS) channel with dimensions of 12 cm long  10 mm wide  10 mm high, containing 1 M NaOH solution. After applying a DC electric field with a voltage of 30 V, the liquid metal Galinstan micromachine moved toward the positive electrode at a speed of 117.2 mm s À1 (Figure 1B). The corresponding top and side images in Figure 1C demonstrate that the shape of the liquid metal Galinstan micromachine deformed during the movement. Interestingly, under the same conditions, the liquid metal gallium micromachine moved toward the negative electrode with a velocity of 2.8 mm s À1 (Figure 1D). We observed that the liquid metal gallium micromachine not