Corrosion-Resistant Functional Diamond Coatings for Reliable Interfacing of Liquid Metals with Solid Metals

Saline based microfluidic soft pressure sensor utilizing a three-dimensional focused electric field for motion and healthcare monitoring

This paper introduces the 'Spatially Focused Saline-based Pressure Sensor (SF-SaPS)', a novel soft microfluidic pressure sensor featuring a distinctive three-dimensional focusing structure. By critically reducing the cross-sectional area of the microchannel at the focused structure, the SF-SaPS achieves excellent sensitivity to pressure within the sensing region. With the spatially focused region, the SF-SaPS could detect a wide range of pressure from gentle touches to human weight, which is typically unachievable with low-conductivity sensing media such as saline, a medium inherently safe for human use. Beyond its sensitivity, the SF-SaPS exhibits sensing performance and stability comparable with conventional liquid metal-based pressure sensors. Our sensor demonstrated minimal signal drift, a rapid response time of 70 ms under cyclic loading, and 20-day long-term stability tests immersed in water. Additionally, the sensor possesses a transparency advantage unattainable by liquid metal sensors as we utilized transparent polymers and saline. A unique advantage of the SF-SaPS lies in its selective spatial and mechanical sensitivity; as the electrical resistance is highly dependent on changes in the cross-sectional area of the microchannels, the sensor has superior pressure sensitivity compared to bending and strain. Finally, various application examples highlight the SF-SaPS's advantages. By configuring the sensor in a two-axis array, the SF-SaPS facilitates pressure mapping across a plane. Additionally, it proves effective in healthcare monitoring, from radial pulse to finger movements. In conclusion, the SF-SaPS's combination of performance, stability, biocompatibility, and transparency positions this sensor as a versatile tool for applications extending beyond healthcare, as demonstrated in this study.

The Liquid Metal Age: A Transition From Hg to Ga

This review offers an illuminating journey through the historical evolution and modern-day applications of liquid metals, presenting a comprehensive view of their significance in diverse fields. Tracing the trajectory from mercury applications to contemporary innovations, the paper explores their pivotal role in industry and research. The analysis spans electrical switches, mechanical applications, electrodes, chemical synthesis, energy storage, thermal transport, electronics, and biomedicine. Each section examines the intricacies of liquid metal integration, elucidating their contributions to technological advancements and societal progress. Moreover, the review critically appraises the challenges and prospects inherent in liquid metal applications, addressing issues of recycling, corrosion management, device stability, economic feasibility, translational hurdles, and market dynamics. By delving into these complexities, the paper advances scholarly understanding and offers actionable insights for researchers, engineers, and policymakers. It aims to catalyze innovation, foster interdisciplinary collaboration, and promote liquid metal-enabled solutions for societal needs. Through its comprehensive analysis and forward-looking perspective, this review serves as a guide for navigating the landscape of liquid metal applications, bridging historical legacies with contemporary challenges, and highlighting the transformative potential of liquid metals in shaping future technologies.

Long-Term Corrosion of Eutectic Gallium, Indium, and Tin (EGaInSn) Interfacing with Diamond

Thermal transport is of grave importance in many high-value applications. Heat dissipation can be improved by utilizing liquid metals as thermal interface materials. Yet, liquid metals exhibit corrosivity towards many metals used for heat sinks, such as aluminum, and other electrical devices (i.e., copper). The compatibility of the liquid metal with the heat sink or device material as well as its long-term stability are important performance variables for thermal management systems. Herein, the compatibility of the liquid metal Galinstan, a eutectic alloy of gallium, indium, and tin, with diamond coatings and the stability of the liquid metal in this environment are scrutinized. The liquid metal did not penetrate the diamond coating nor corrode it. However, the liquid metal solidified with the progression of time, starting from the second year. After 4 years of aging, the liquid metal on all samples solidified, which cannot be explained by the dissolution of aluminum from the titanium alloy. In contrast, the solidification arose from oxidation by oxygen, followed by hydrolysis to GaOOH due to the humidity in the air. The hydrolysis led to dealloying, where In and Sn remained an alloy while Ga separated as GaOOH. This hydrolysis has implications for many devices based on gallium alloys and should be considered during the design phase of liquid metal-enabled products.

Bottom-up strategy of multi-level structured boron-doped diamond for the durable electrode in water purification

Long-term exposition of electrodes to aqueous media inevitably results in biofouling and adhesion of bacteria, reducing the electrolysis efficiency of electrodes for water treatment. To ensure technically efficient antifouling of materials for durable electrodes, hierarchical micro-/nano structured boron-doped diamond (BDD) electrodes were designed and synthesized. Multi-level structured BDD was coated on titanium mesh by a bottom-up strategy, based on a combination of self-assembly seeding and hot filament chemical vapor deposition (HFCVD) growth. The morphology of the BDD coating can be controlled by manipulating the seeding density and boron doping concentration. The designed micro/nano hierarchical structure of the BDD electrode suppressed bacterial adhesion greatly and exhibited excellent anti-biofouling efficiency with an antibacterial rate of ∼ 93 %, which entails simplified self-cleaning and durable BDD-coated electrodes. The BDD-coated electrodes were employed to electrochemically treat Escherichia coli-contaminated water, killing virtually all bacteria (≥99.9 %) in 1 min. Finally, real river water was electrochemically treated, reducing the chemical oxygen demand (COD) down to 5 mg/L in 4 h. The excellent performance shows the great potential of the structured BDD electrodes for long-term water purification.

Self-healing of fractured diamond

Materials that possess the ability to self-heal cracks at room temperature, akin to living organisms, are highly sought after. However, achieving crack self-healing in inorganic materials, particularly with covalent bonds, presents a great challenge and often necessitates high temperatures and considerable atomic diffusion. Here we conducted a quantitative evaluation of the room-temperature self-healing behaviour of a fractured nanotwinned diamond composite, revealing that the self-healing properties of the composite stem from both the formation of nanoscale diamond osteoblasts comprising sp2- and sp3-hybridized carbon atoms at the fractured surfaces, and the atomic interaction transition from repulsion to attraction when the two fractured surfaces come into close proximity. The self-healing process resulted in a remarkable recovery of approximately 34% in tensile strength for the nanotwinned diamond composite. This discovery sheds light on the self-healing capability of nanostructured diamond, offering valuable insights for future research endeavours aimed at enhancing the toughness and durability of brittle ceramic materials.

In Situ Actuators with Gallium Liquid Metal Alloys and Polypyrrole-Coated Electrodes

Gallium liquid metal alloys (GLMAs) such as Galinstan and gallium-indium eutectic (EGaIn) are interesting materials due to their high surface tensions, low viscosities, and electrical conductivities comparable to classical solid metals. They have been used for applications in microelectromechanical systems (MEMS) and, more recently, liquid metal microfluidics (LMMF) for setting up devices like actuators. However, their high tendency to alloy with the most common metals used for electrodes such as gold (Au), platinum (Pt), titanium (Ti), nickel (Ni), and tungsten-titanium (WTi) is a major problem limiting the scaleup and applicability, e.g., liquid metal actuators. Stable electrodes are key elements for many applications and thus, the lack of an electrode material compatible with GLMAs is detrimental for many potential application scenarios. In this work, we study the effect of actuating Galinstan on various solid metal electrodes and present an electrode protection methodology that, first, prevents alloying and, second, prevents electrode corrosion. We demonstrate reproducible actuation of GLMA segments in LMMF, showcasing the stability of the proposed protective coating. We investigated a range of electrode materials including Au, Pt, Ti, Ni, and WTi, all in aqueous environments, and present the resulting corrosion/alloying effects by studying the interface morphology. Our proposed protective coating is based on a simple method to electrodeposit electrically conductive polypyrrole (PPy) on the electrodes to provide a conductive alloying-barrier layer for applications involving direct contact between GLMAs and electrodes. We demonstrate the versatility of this approach by direct three-dimensional (3D) printing of a 500 μm microfluidic chip on a set of electrodes onto which PPy is electrodeposited in situ for actuation of Galinstan plugs. The developed protection protocol will provide a generic, widely applicable strategy to protect a wide range of electrodes from alloying and corrosion and thus form a key element in future applications of GLMAs.

A novel coating to avoid corrosion effect between eutectic gallium-indium alloy and heat sink metal for X-ray optics cooling

Owing to the parasitic vibration effect of the cooling medium and pipes of X-ray optics, the vibration decoupling cooling method based on eutectic gallium-indium (EGaIn) alloy has become very crucial for fourth generation synchrotron radiation advanced light sources. However, there is an issue that the corrosion of the EGaIn alloy to the heat sink metal [e.g., copper (Cu) plate] results in the solidification and the failure of eliminating the parasitic vibration effect. To deal with the problem, a novel anti-corrosion coating based on tungsten (W) is presented in this paper. It possesses better corrosion resistance performance compared with the traditional coating of nickel (Ni). The experimental investigation was carried out, in which the EGaIn alloy was exposed to several typical metal materials in conditions of various time durations and various temperatures, which were considered as controls. Furthermore, the corrosion effects are analyzed and evaluated in two aspects of micromorphology and the chemical composition by using an optical microscope and a scanning electron microscope as well as x-ray diffraction. The results show that non obvious corrosion occurred for W, 0.33 mm and 48 µm thick transition micro-area, respectively, for Cu and Ni. In addition, new substances CuGa₂ and Ni₃Ga₇ occurred, respectively, for Cu and Ni for 36 hours at 250 °C. The EGaIn alloy will freeze after corroding 18 µm substrate for Ni or 30 µm for Cu. Furthermore, the W coating that was prepared by magnetron sputtering has been implemented for feasibility validation.

Surface Tension of the Oxide Skin of Gallium-Based Liquid Metals

Gallium-based alloys have garnered considerable attention in the scientific community, particularly as they are in an atypical liquid state at and near room temperature. Though physical parameters, such as thermal conductivity, electrical conductivity, viscosity, yield stress, and surface tension, of these alloys are broadly known, the surface tension (surface free energy) of the oxide skin remains intangible due to the high yield stress of the oxide skin. In this article, we propose to employ gradually attenuated vibrations to obtain equilibrium shapes, which are analyzed along the lines of the puddle height method. The surface tension of the oxide skin was determined on quartz glass and liquid metal-phobic diamond coating to be around 350-365 mN/m, thus independent of the substrate surface or employed liquid metal (i.e., eutectic Ga-In (EGaIn) and galinstan). The similarity of the surface tension for different alloys was ascribed to the composition of the oxide skin, which predominantly comprises gallium oxides due to thermodynamic constraints. We envision that this method can also be applied to other liquid metal alloys and liquid metal marble systems facilitating modeling, simulation, and optimization processes.

Ultrathin Diamond Nanofilms-Development, Challenges, and Applications

Diamond is a highly attractive material for ample applications in material science, engineering, chemistry, and biology because of its favorable properties. The advent of conductive diamond coatings and the steady demand for miniaturization in a plethora of economic and scientific fields resulted in the impetus for interdisciplinary research to develop intricate deposition techniques for thin (≤1000 nm) and ultra-thin (≤100 nm) diamond films on non-diamond substrates. By virtue of the lowered thickness, diamond coatings feature high optical transparency in UV-IR range. Combined with their semi-conductivity and mechanical robustness, they are promising candidates for solar cells, optical devices, transparent electrodes, and photochemical applications. In this review, the difficulty of (ultra-thin) diamond film development and production, introduction of important stepping stones for thin diamond synthesis, and summarization of the main nucleation procedures for diamond film synthesis are elucidated. Thereafter, applications of thin diamond coatings are highlighted with a focus on applications relying on ultrathin diamond coatings, and the excellent properties of the diamond exploited in said applications are discussed, thus guiding the reader and enabling the reader to quickly get acquainted with the research field of ultrathin diamond coatings.

Magnetic Printing of Liquid Metal for Perceptive Soft Actuators with Embodied Intelligence

Soft actuators with perception capability are essential for robots to intelligently interact with humans and the environment. However, existing perceptive soft actuators require complex integration and coupling between the discrete functional units to achieve autonomy. Here, we report entirely soft actuators with embodied sensing, actuation, and control at the single-unit level. This is achieved by synergistically harnessing the mechanosensing and electrothermal properties of liquid metal (LM) to actuate the thermally responsive liquid crystal elastomer (LCE). We create multifunctional LM circuits on the LCE surface using a simple and facile methodology based on magnetic printing. The fluidic LM circuit can not only be utilized as a conformable resistive heater but also as a sensory skin to perceive its own deformation. Moreover, the rational design of the LM circuits makes it possible to achieve biomimetic autonomous actuation in response to mechanical stimuli such as pressure or strain. In addition, the intrinsic stretchability of LM allows us to create 3D spring-like actuators via a simple prestretch step, and complex helical motions can be obtained upon mechanical stimulation. This work provides a unique and simple design for autonomous soft robotics with embodied intelligence.