Mini/Micro/Nano Scale Liquid Metal Motors

Abstract

Swimming motors navigating in complex fluidic environments have received tremendous attention over the last decade. In particular, liquid metal (LM) as a new emerging material has shown considerable potential in furthering the development of swimming motors, due to their unique features such as fluidity, softness, reconfigurability, stimuli responsiveness, and good biocompatibility. LM motors can not only achieve directional motion but also deformation due to their liquid nature, thus providing new and unique capabilities to the field of swimming motors. This review aims to provide an overview of the recent advances of LM motors and compare the difference in LM macro and micromotors from fabrication, propulsion, and application. Here, LM motors below 1 cm, named mini/micro/nano scale liquid metal motors (MLMTs) will be discussed. This work will present physicochemical characteristics of LMs and summarize the state-of-the-art progress in MLMTs. Finally, future outlooks including both opportunities and challenges of mini/micro/nano scale liquid metal motors are also provided.

Precise Regulation of Ga-Based Liquid Metal Oxidation

“…The small-sized LMPs (10 2 nm) can be produced by a more convenient and versatile ultrasonic process (Figure 4a). 1,4 In addition, different conditions will also affect the degree of oxidation under the same synthesis method. Taking ultrasound as an example, sonicating a molten Ga droplet in water will increase oxidation over time: mainly bare Ga spheres in 30 s; GaOOH crystallites partially covered within 2 min; gradually growth of GaOOH (30 min); eventually only crystals (120 min) 33 (Figure 4a).…”

“…For instance, spherical and nonspherical LMPs can be obtained by microfluidics technology (10 2 μm to 10 1 mm) supplemented with different solutions (nonoxygenated or oxygenated solution); LMPs of finer shapes can be fabricated using molding techniques (10 0 μm to 10 1 mm) with different templates, while oxide film helps to enter the pores and stabilize the shape. The small-sized LMPs (10 2 nm) can be produced by a more convenient and versatile ultrasonic process (Figure a). , In addition, different conditions will also affect the degree of oxidation under the same synthesis method. Taking ultrasound as an example, sonicating a molten Ga droplet in water will increase oxidation over time: mainly bare Ga spheres in 30 s; GaOOH crystallites partially covered within 2 min; gradually growth of GaOOH (30 min); eventually only crystals (120 min) (Figure a).…”

“…To date, various metal materials have become crucial materials for the development of human society, ranging from construction and industry manufacturing to electronics and biomedical engineering, while the revolutionary discovery of the liquid metals (LMs) with fluidity, metallicity, and peculiar interfacial characteristics will change the inherent understanding of metals and introduce a huge impact in a wide range of applications. − Compared to conventional solid metals, LMs (in molten state) consisting of a “sea” of free electron cloud coupled to positively charged metal ions, can be regarded as an amorphous solid without any crystalline. , Besides, they are also distinguished from other liquids due to characteristic metallic bonding. Up to now, Ga-based LMs as a promising class of LMs have received continuous attention in various fields, and endless research has emerged.…”

Low Melting Point Alloys Enabled Stiffness Tunable Advanced Materials

“…[ 29 ] Nowadays, LMPAs represented by gallium‐based (Ga‐based) alloys and bismuth‐based (Bi‐based) alloys are receiving increasing attention. Owing to their good biocompatibility, high boiling point, excellent thermal/electrical conductivity, reversible phase transition at around room temperature, these LMPAs have shown encouraging practical values in thermal management, [ 30 ] electronics with 3D printing, [ 31 ] biomedical application, [ 32 ] motors, [ 33 ] soft robotics, [ 34 ] catalyst, [ 35 ] and synthesis. [ 36 ] Based on current research, the discussions, presented here, mainly focus on Ga‐based and Bi‐based LMPAs.…”

Alkaline-Driven Liquid Metal Janus Micromotor with a Coating Material-Dependent Propulsion Mechanism

“…Micro/nanomotors (MNMs) are called synthetic micro-swimmers or active colloidal motors due to their controllable self-propulsion and on-demand maneuverability at micro/nanoscales. − Their on-demand maneuverability refers to the micro/nano objects with kinematic properties (including rotation, , flip, shuttle, contraction, aggregation, , and so on) under the stimulation of various external energies (light, , electricity, magnetism, heat, chemical energy, ,, etc.). People have made huge progress in MNMs’ driving power divergence and accurate maneuver manipulations, , with exciting application prospects in many fields such as drug delivery, − microsurgery, biosensing, , and pollutant degradation. ,, Especially micromotors with switchable propulsion mechanism and motion directionality have attracted attention in practical applications. , However, there are still several obstacles between their applications into biomedical field. , Gallium- and indium-based liquid metal (LM) alloys are very impressive candidates due to their good biocompatibility and low biotoxicity. ,, It offers possibility to overcome the limitations of conventional rigid metallic or inorganic material-based motors and launch tremendous opportunities for developing future-generation soft robotics. , …”