Reactive wetting enabled anchoring of non-wettable iron oxide in liquid metal for miniature soft robot.

Magnetic liquid metal (LM) soft robots attract considerable attentions because of distinctive immiscibility, deformability and maneuverability. However, conventional LM composites relying on alloying between LM and metallic magnetic powders suffer from diminished magnetism over time and potential safety risk upon leakage of metallic components. Herein, we report a strategy to composite inert and biocompatible iron oxide (Fe3O4) magnetic nanoparticles into eutectic gallium indium LM via reactive wetting mechanism. To address the intrinsic interfacial non-wettability between Fe3O4 and LM, a silver intermediate layer was introduced to fuse with indium component into AgxIny intermetallic compounds, facilitating the anchoring of Fe3O4 nanoparticles inside LM with improved magnetic stability. Subsequently, a miniature soft robot was constructed to perform various controllable deformation and locomotion behaviors under actuation of external magnetic field. Finally, practical feasibility of applying LM soft robot in an ex vivo porcine stomach was validated under in-situ monitoring by endoscope and X-ray imaging.

Ultrasonic-Enabled Nondestructive and Substrate-Independent Liquid Metal Ink Sintering.

Printing or patterning particle-based liquid metal (LM) ink is a good strategy to overcome poor wettability of LM for its circuits' preparation in flexible and printed electronics. Subsequently, a crucial step is to recover conductivity of LM circuits consisting of insulating LM micro/nano-particles. However, most widely used mechanical sintering methods based on hard contact such as pressing, may not be able to contact the LM patterns' whole surface conformally, leading to insufficient sintering in some areas. Hard contact may also break delicate shapes of the printed patterns. Hereby, an ultrasonic-assisted sintering strategy that can not only preserve original morphology of the LM circuits but also sinter circuits on various substrates of complex surface topography is proposed. The influencing factors of the ultrasonic sintering are investigated empirically and interpreted with theoretical understanding by simulation. LM circuits encapsulated inside soft elastomer are successfully sintered, proving feasibility in constructing stretchable or flexible electronics. By using water as energy transmission medium, remote sintering without any direct contact with substrate is achieved, which greatly protect LM circuits from mechanical damage. In virtue of such remote and non-contact manipulation manner, the ultrasonic sintering strategy would greatly advance the fabrication and application scenarios of LM electronics.

Erratum: A crack compensation strategy for highly stretchable conductors based on liquid metal inclusions.

[This corrects the article DOI: 10.1016/j.isci.2022.105495.].

A crack compensation strategy for highly stretchable conductors based on liquid metal inclusions.

Crack control strategies have been proven very useful for enhancing the stretchability of metal film-based stretchable conductors. However, existing strategies often suffer from the drawbacks of complicated preparation and predefined effective directions. Here, we propose a crack compensation strategy for preparing conductors featured with high stretchability by using liquid metal microparticles (LMMPs)-embedded polydimethylsiloxane (PDMS) as the substrate with a thin film of gold (Au) sputtered on the surface. LMMPs can be elongated to connect the cracked Au film upon stretching, which can form a conductive "island-tunnel" (IT) architecture to compensate for the cracks and maintain the conductivity. The high performance of the stretchable conductor is demonstrated by using it as electrodes to record surface electromyography of human brachioradialis and monitor electrocorticography signals of a rat in normal and epileptic states. The developed strategy shows the potential to provide a new perspective for the fabrication of flexible electronics.

Dependence of wetting on cavitation during the spreading of a filler droplet on the ultrasonically agitated Al substrate.

The cavitation characteristics during the spreading of a pure Sn liquid droplet subjected to ultrasonication were studied for the first time through high-speed photography to reveal the wetting mechanism. Ultrasonic vibration realized the spreading of Sn droplet on the nonwetting pure Al substrate. However, the oxide layer of the substrate at the spreading front is difficult to remove. The high-speed photography result shows that a noncavitation region consistently appears at the spreading front, because the acoustic pressure is below the cavitation threshold of 1.26 MPa. In particular, the width of the noncavitation region gradually increases as the size of the spreading area increases. Such a result accounts for the condition wherein the oxide layer at the spreading front is difficult to remove. Furthermore, the bubble density during spreading gradually decreases due to the decreased acoustic pressure of the thinned liquid. Finally, the bubble dynamics were calculated to verify the wetting mechanism.

A study on the relationship between mindfulness and work performance of web editors: Based on the chain mediating effect of workplace spirituality and digital competencies.

Introduction: Based on the job demands-resources model, this study aims to explore the relationship between mindfulness in web editors, work performance, workplace spirituality, and digital competencies. Methods: Online data from the Tencent Questionnaire Platform was used to examine the proposed research model. We distributed questionnaires to new media companies, and a total of 431 valid questionnaires were collected. Results and Discussion: The results suggested that mindfulness in web editors can improve workplace spirituality, digital competencies, and work performance. In addition, workplace spirituality was found to act as a mediator between mindfulness and work performance. And, digital competencies did not play a mediating role between mindfulness and work performance, but workplace spirituality and digital competencies played a chain mediating role between mindfulness and work performance. The study explained the internal impact mechanism of mindfulness on work performance in web editors, and proposed methods to improve mindfulness, revealing the chain mediating role of workplace spirituality and digital competencies in the impact of mindfulness on work performance, which might provide new insights into existing research. It can provide a reference for new media companies to manage the team of web editors and improve the work performance of web editors.

Extracting automata from neural networks using active learning.

Deep learning is one of the most advanced forms of machine learning. Most modern deep learning models are based on an artificial neural network, and benchmarking studies reveal that neural networks have produced results comparable to and in some cases superior to human experts. However, the generated neural networks are typically regarded as incomprehensible black-box models, which not only limits their applications, but also hinders testing and verifying. In this paper, we present an active learning framework to extract automata from neural network classifiers, which can help users to understand the classifiers. In more detail, we use Angluin's L* algorithm as a learner and the neural network under learning as an oracle, employing abstraction interpretation of the neural network for answering membership and equivalence queries. Our abstraction consists of value, symbol and word abstractions. The factors that may affect the abstraction are also discussed in the paper. We have implemented our approach in a prototype. To evaluate it, we have performed the prototype on a MNIST classifier and have identified that the abstraction with interval number 2 and block size 1 × 28 offers the best performance in terms of F1 score. We also have compared our extracted DFA against the DFAs learned via the passive learning algorithms provided in LearnLib and the experimental results show that our DFA gives a better performance on the MNIST dataset.

Bidirectional stochastic configuration network for regression problems.

To adapt to the reality of limited computing resources of various terminal devices in industrial applications, a randomized neural network called stochastic configuration network (SCN), which can conduct effective training without GPU, was proposed. SCN uses a supervisory random mechanism to assign its input weights and hidden biases, which makes it more stable than other randomized algorithms but also leads to time-consuming model training. To alleviate this problem, we propose a novel bidirectional SCN algorithm (BSCN) in this paper, which divides the way of adding hidden nodes into two modes: forward learning and backward learning. In the forward learning mode, BSCN still uses the supervisory mechanism to configure the parameters of the newly added nodes, which is the same as SCN. In the backward learning mode, BSCN calculates the parameters at one time based on the residual error feedback of the current model. The two learning modes are performed iteratively until the prediction error of the model reaches an acceptable level or the number of hidden nodes reaches its maximum value. This semi-random learning mechanism greatly speeds up the training efficiency of the BSCN model and significantly improves the quality of the hidden nodes. Extensive experiments on ten benchmark regression problems, two real-life air pollution prediction problems, and a classical image processing problem show that BSCN can achieve faster training speed, higher stability, and better generalization ability than SCN.

Ultrasonic cavitation at liquid/solid interface in a thin Ga-In liquid layer with free surface.

Cavitation in thin layer of liquid metal has potential applications in chemical reaction, soldering, extraction, and therapeutic equipment. In this work, the cavitation characteristics and acoustic pressure of a thin liquid Ga-In alloy were studied by high speed photography, numerical simulation, and bubble dynamics calculation. A self-made ultrasonic system with a TC4 sonotrode, was operated at a frequency of 20 kHz and a max output power of 1000 W during the cavitation recording experiment. The pressure field characteristic inside the thin liquid layer and its influence on the intensity, types, dimensions, and life cycles of cavitation bubbles and on the cavitation evolution process against experimental parameters were systematically studied. The results showed that acoustic pressure inside the thin liquid layer presented alternating positive and negative characteristics within 1 acoustic period (T). Cavitation bubbles nucleated and grew during the negative-pressure stage and shrank and collapsed during the positive-pressure stage. A high bubble growth speed of 16.8 m/s was obtained and evidenced by bubble dynamics calculation. The maximum absolute pressure was obtained at the bottom of the thin liquid layer and resulted in the strongest cavitation. Cavitation was divided into violent and weak stages. The violent cavitation stage lasted several hundreds of acoustic periods and had higher bubble intensity than the weak cavitation stage. Cavitation cloud preferentially appeared during the violent cavitation stage and had a life of several acoustic periods. Tiny cavitation bubbles with life cycles shorter than 1 T dominated the cavitation field. High cavitation intensities were observed at high ultrasonication power and when Q235B alloy was used because such conditions lead to high amplitudes on the substrate and further high acoustic pressure inside the liquid.

Grain refinement caused by intensified cavitation within narrow channel and its improvement to ultrasonically soldered Al joint property.

In this work, grain refinement of the aluminum soldered joint was obtained by applying cavitation within narrow channels and the possible grain refinement mechanism was proposed. Aluminum sheets with different channel widths were ultrasonically soldered by pure Sn in air. An ultrasonic system with a TC4 sonotrode, was operated at a frequency of 20 kHz and power of 1000 W during soldering. The effect of channel width on grain size, element distribution and strength of the soldered joint was studied. Results showed that the grain size decreased from 2.62 to 1.04 µm and the element ratio of Al in solder increased from 0.93 to 4.86% when the channel width decreased from 0.8 to 0.2 mm. Instant solidification of Sn grains was readily observed in the joint before cooling due to the large undercooling induced by the intensified cavitation inside the narrow channels. The random cavitation induced nucleation of Sn was believed to be mainly responsible for the grain refinement of the soldered joint. The shear strength of the joint increased from 29.5 to 48.8 MPa and the hardness increased from 16.5 to 25.2 HV due to the grain refinement of Sn and the presence of Al transferred from the substrates.

Cavitation at filler metal/substrate interface during ultrasonic-assisted soldering. Part II: Cavitation erosion effect.

Using pure Sn as filler metal, this study investigated cavitation erosion effects during ultrasonic-assisted soldering. The physical process and mechanism of cavitation erosion were revealed. Superior erosion effects were observed under long ultrasonic times, small channel widths, and high ultrasonic powers. Different vibration intensities were obtained inside filler pool. Region I, which was located far from the sonotrode, exhibited a stronger vibration intensity and better erosion effect than those of the other regions. The erosion incubation stage was shorted than 0.5 s at the channel width of 0.2 mm. Complete oxide layer removal was obtained at an ultrasonic time of 2 s under this condition. The violent cavitation stage was shorter than the erosion incubation stage, and the removal of the oxide layer mainly depended on the stable cavitation stage.

Wetting mechanism of Sn to Zr50.7Cu28Ni9Al12.3 bulk metallic glass assisted by ultrasonic treatment.

In this work, pure Sn was used to wet Zr50.7Cu28Ni9Al12.3 bulk metallic glasses (BMGs) assisted by ultrasonic treatment. Without ultrasonic treatment, pure Sn showed a non-wetting condition to BMG. Ultrasonic vibration facilitated the wetting of Sn to BMG. Prior to ultrasonication for 30 s, only physical adsorption formed at the Sn/BMG interface. Increasing ultrasonic time led to the alteration of the bond at the Sn/BMG interface from point contact to local surface contact, and to diffusion layer. Two bonding modes of order-order and order-disorder were discovered at the Sn/BMG interface. Cu content was higher than the other elements near the bonding interface. Longer diffusion distances of Sn into the BMG were obtained at high ultrasonic power, high temperature and large dip depth.

Cavitation at filler metal/substrate interface during ultrasonic-assisted soldering. Part I: Cavitation characteristics.

The cavitation characteristics at filler metal/substrate interface during ultrasonic-assisted soldering were first recorded by high-speed photography in this work. Two kinds of bubbles, steady cavitation bubbles and transient cavitation bubbles were observed. Steady cavitation bubbles did not collapse within one acoustic period and could last longer than 50 acoustic periods. Transient cavitation bubbles formed and collapsed within one acoustic period. The cavitation process was divided into two stages based on the cavitation characteristics. The first violent cavitation stage was in fact the degassing process, which lasted approximately 2700 acoustic periods and was affected by the gas content trapped inside the filler metal and the stronger vibration at the initiation stage of ultrasonic-assisted soldering. The second steady cavitation stage had obvious low bubble density and accounted for the most of the soldering process. Higher cavitation densities were observed when small channel width and large ultrasonic power were used because of larger sound pressures inside the filler metal.

Control Al/Mg intermetallic compound formation during ultrasonic-assisted soldering Mg to Al.

To prevent the formation of Al/Mg intermetallic compounds (IMCs) of Al3Mg2 and Al12Mg17, dissimilar Al/Mg were ultrasonic-assisted soldered using Sn-based filler metals. A new IMC of Mg2Sn formed in the soldered joints during this process and it was prone to crack at large thickness. The thickness of Mg2Sn was reduced to 22 µm at 285 °C when using Sn-3Cu as the filler metal. Cracks were still observed inside the blocky Mg2Sn. The thickness of Mg2Sn was significantly reduced when using Sn-9Zn as the filler metal. A 17 µm Mg2Sn layer without crack was obtained at a temperature of 200 °C, ultrasonic power of Mode I, and ultrasonic time of 2 s. The shear strengths of the joints using Sn-9Zn was much higher than those using Sn-3Cu because of the thinner Mg2Sn layer in the former joints. Sn whiskers were prevented by using Sn-9Zn. A cavitation model during ultrasonic assisted soldering was proposed.

Isolation of novel sequences targeting highly variable viral protein hemagglutinin.

Rapid evolution is a hallmark of the viral kingdom and a major concern for developing universal vaccines. The isolation of substantial numbers of viral sequence variants at highly variable viral protein domains remains a major challenge. We previously developed a combinatorial method for the isolation of novel sequences to cope with rapid viral variations at the G-H loop of Foot and Mouth Disease virus VP1 protein [1]. Here we present a modification of that method in its application in the randomization of the hemagglutinin gene from a H5N2 virus, namely: •removal of potentially stressful region which harbored a stretch of basic amino acids to increase the success rates of gene cloning, and to streamline the process of future engineering of novel viral variants.•clustered randomization in a full-length gene, as the positive rate for partial gene fragment libraries was extremely low before enrichment in the previous FMDV studies.•the use of fusion partner was avoided, which was used previously for protein expression, stabilization of clones and reduction of stresses on host cells.•the use of Poisson distribution is proposed to approximate sequencing output to achieve cost effectiveness.

Vibration characteristics of aluminum surface subjected to ultrasonic waves and their effect on wetting behavior of solder droplets.

The vibration characteristics of an aluminum surface subjected to ultrasonic waves were investigated with a combination of numerical simulation and experimental testing. The wetting behavior of solder droplets on the vibrating aluminum surface was also examined. The results show that the vibration pattern of the aluminum surface is inhomogeneous. The amplitude of the aluminum surface exceeds the excitation amplitude in some zones, while the amplitude decreases nearly to zero in other zones. The distribution of the zero-amplitude zones is much less dependent on the strength of the vibration than on the location of the vibration source. The surface of the liquid solder vibrates at an ultrasonic frequency that is higher than the vibration source, and the amplitude of the liquid solder is almost twice that of the aluminum surface. The vibration of the surface of the base metal (liquid solder) correlates with the oxide film removal effect. Significant removal of the oxide film can be achieved within 2s when the amplitude of the aluminum surface is higher than 5.4 µm or when the amplitude of the liquid solder surface is higher than 10.2 µm.

Interaction behaviors at the interface between liquid Al-Si and solid Ti-6Al-4V in ultrasonic-assisted brazing in air.

Power ultrasonic vibration (20 kHz, 6 µm) was applied to assist the interaction between a liquid Al-Si alloy and solid Ti-6Al-4V substrate in air. The interaction behaviors, including breakage of the oxide film on the Ti-6Al-4V surface, chemical dissolution of solid Ti-6Al-4V, and interfacial chemical reactions, were investigated. Experimental results showed that numerous 2-20 µm diameter-sized pits formed on the Ti-6Al-4V surface. Propagation of ultrasonic waves in the liquid Al-Si alloy resulted in ultrasonic cavitation. When this cavitation occurred at or near the liquid/solid interface, many complex effects were generated at the small zones during the bubble implosion, including micro-jets, hot spots, and acoustic streaming. The breakage behavior of oxide films on the solid Ti-6Al-4V substrate, excessive chemical dissolution of solid Ti-6Al-4V into liquid Al-Si, abnormal interfacial chemical reactions at the interface, and phase transformation between the intermetallic compounds could be wholly ascribed to these ultrasonic effects. An effective bond between Al-Si and Ti-6Al-4V can be produced by ultrasonic-assisted brazing in air.