Zhendong Dai

A highly porous nafion membrane templated from polyoxometalates-based supramolecule composite for ion-exchange polymer-metal composite actuator

This paper reports a new technique for fabricating a highly porous hybrid SiO2/Nafion membrane, from which a high electromechanical performance ion-exchange polymer-metal composite (IPMC) actuator is achieved. This technique uses polyoxometalate (POM)-based supramolecule composite carried by amorphous SiO2 particles to form a hybrid membrane with the polar Nafion ionomer. The resultant composite (POM-Nafion) is then incubated and degraded in a base solution, forming a porous membrane. Attenuated total reflectance fourier transform infrared spectroscopy (ATR-FTIR) and scanning electron microscopy (SEM) were used to monitor the degradation process. ATR-FTIR results demonstrated that the POM-based composite was removed while SiO2 remained in the porous membrane after degradation. SEM images showed that there were a great quantity of channels with sizes of 0.4–1.4 μm and pores with sizes of 300–700 nm in the newly-formed porous Si-Nafion membrane. With the same Nafion content, the porous Si-Nafion membrane and self-casting Nafion membrane were both made into IPMC actuators. A laser displacement sensor, a force sensor and a high speed camera were assembled into a measurement system to evaluate the performance of the IPMC actuator. Results showed that the maximum blocking forces from the Si-Nafion membrane actuator were 3.78 g for 2.5 V driving voltage and 2.39 g for 1.5 V, which increase by 2.97 and 4.19 times, respectively, when compared to the self-casting Nafion membrane actuator. The maximum displacement outputs were 7.2 mm for 2.5 V driving voltage and 5.9 mm for 1.5 V. Compared to the self-casting Nafion membrane actuator, these outputs increased by 4.8 and 9.7 times, respectively. When actuated in the air, the Si-Nafion membrane actuator exhibited stable working time of 480 s and 710 s for the voltages of 2.5 V and 1.5 V, respectively. Compared to the self-casting Nafion membrane actuator, these data increased by 4.36 and 2.22 times, respectively.

The Tribological Behaviors of Self-Lubricating Composites as Filler in Copper Foam

A copper foam–based (CFB) self-lubricating composite was developed as a friction pair material. The composite consists of copper foam with three-dimensionally interpenetrated pores and solid lubricant fillers (polytetrafluoroethylene [PTFE] and MoS2). The thermal and electrical conductivity, bending strength, and tribological properties of the new composite were investigated. The friction and wear properties were investigated on a M-2000 model ring-on-block test rig. An electric field was imposed between the sample and ring to monitor the tribochemical reaction and formation of transfer film by means of contact resistance. Measurement of friction temperature was carried out by means of three thermocouples embedded in the material. The friction coefficients of the CFB composite decrease slightly, and the wear rate substantially decreased overall compared with that of the homologous polymers. The optical and scanning electron micrographs (SEM) of the frictional surfaces show the worn surface of the CFB composite. The main wear mechanism was three-body abrasion, caused and promoted by plastic deformation, abrasive wear, and fatigue spalling. X-ray photoelectron spectroscopy (XPS) analysis showed that the reactions of MoS2 with Cu accompanied by their respective oxidation reactions are involved in the identifiable copper area.

Centrifugation-Assisted Fog-Collecting Abilities of Metal-Foam Structures with Different Surface Wettabilities

The collection of water from fog is a simple and sustainable means of obtaining freshwater for human and animal consumption. Herein, we address the use of metal foam in fog collection and present a novel fog-collecting device fabricated from copper foam. This device, which can also be used in other liquid-gas separation applications, is a 3D extension of biologically inspired 1D and 2D materials. The network structure of the 3D material effectively increased the contact area and interaction time of the skeleton structure and fog compared to those of traditional 2D fog-collecting materials. The main aspects investigated in this study were the influences of the inertial centrifugal force generated by rotating the metal-foam samples and the use of samples with different surface wettabilities on the fog-collecting performance. Superhydrophilic and superhydrophobic samples were found to have higher collection efficiencies at low and high rotational speeds, respectively, and a maximum efficiency of 86% was achieved for superhydrophobic copper foam (20 pores per inch) rotated at 1500 rpm.

An ionic electro-active actuator made with graphene film electrode, chitosan and ionic liquid

A newly developed ionic electro-active actuator composed of an ionic electrolyte layer sandwiched between two graphene film layers was investigated. Scanning electronic microscopy observation and x-ray diffraction analysis showed that the graphene sheets in the film stacked in a nearly face-to-face fashion but did not restack back to graphite, and the resulting graphene film with low sheet resistance (10 Ω sq−1) adheres well to the electrolyte membrane. Contact angle measurement showed the surface energy (37.98 mJ m−2) of the ionic electrolyte polymer is 2.67 times higher than that (14.2 mJ m−2) of the Nafion membrane, contributing to the good adhesion between the graphene film electrode and the electrolyte membrane. An electric double-layer is formed at the interface between the graphene film electrode and the ionic electrolyte membrane under the input potential, resulting in a higher capacitance of 27.6 mF cm−2. We report that this ionic actuator exhibits adequate bending strain, ranging from 0.032 to 0.1% (305 to 945 μm) as functions of voltage.

The role of fore‐ and hindlimbs during jumping in the Dybowski's frog (Rana dybowskii)

Anurans are well known for their jumping abilities, making use of their strong hindlimbs. In contrast, the function of the forelimbs during take-off has rarely been studied. We measured the ground reaction forces exerted by forelimbs and hindlimbs during short jumps in the Dybowskis frog Rana dybowskii. Take-off occurred in two phases. Phase one (from the initial time until the forelimbs took off), which lasts a relatively long time (63.2 ± 4.1% of the total take-off phase, N = 20), provides sufficient time for the forelimbs to elevate the body to a suitable posture to deliver the best take-off angle. Phase two (from the forelimbs lift-off until hindlimbs lift-off) was dominated by the hindlimbs which provided a constant and fast elevation. The force angle (angle of the resultant vector from fore-aft and normal force components towards the plane of the substrate) of the hindlimbs and body trajectory was variable before the forelimbs lifted off of the substrate and then primarily followed the direction of the line from the foot-substrate point to the center of mass (COM). The preparation angle adopted when the forelimbs lifted off of the substrate was a good predictor of the take-off angle. The total normal force oscillated around body weight (BW) before the forelimb normal force peaked. The BW shifted from the hindlimbs to the forelimbs during the initial phase of take-off. A simple lever model suggests that the forelimbs are responsible for raising the COM, thus influencing the take-off angle in short jumps.

Electromechanical performance of an ionic polymer–metal composite actuator with hierarchical surface texture

Two stainless steel templates were fabricated using electric-spark machining, and a hierarchical surface texture of ionic polymer was produced using both polishing and replication methods, which produced microscale and nanoscale groove-shaped microstructures at the surface of the polymer. The surface morphology of the Nafion membrane and metal electrode were observed using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). SEM and EDS line-scan analysis indicated that the interfacial surface area was considerably increased and an excellent metal electrode was obtained with the production of a hierarchical surface texture. The displacement, blocking force, and electric current were measured using home-built apparatus. The results revealed that the combined polishing and replication method significantly improved the electromechanical performance of the ionic polymer–metal composite (IPMC). Compared with sandblasted Nafion-based IPMC, the blocking force, displacement, and electric current of the replicated Nafion-based IPMC were 4.39, 2.35, and 1.87 times higher, respectively. The IPMC fabricated in this work exhibited a competitive blocking force compared with recently reported actuators.

The Mechanics and Trajectory Control in Locust Jumping

Locusts (Locusta migratoria manilensis) are characterised by their flying ability and abiding jump ability. Research on the jumping mechanics and behavior of locusts plays an important role in elucidating the mechanism of hexapod locomotion. The jump gestures of locusts were observed using high-speed video camera at 250 fps. The reaction forces of the hindlegs were measured using two three-dimensional sensors, in case the two hindlegs attached on separated sensor plates. The jump gestures and reaction forces were used to illustrate the locust jumping mechanism. Results show that the trajectory control is achieved by rapid rolling and yawing movements of the locust body, caused by the forelegs, midlegs and hindlegs in different jumping phases. The final jump trajectory was not determined until hind tarsi left platform. The horizontal co-impulse between two hindlegs might play a key role in jump stability and accuracy. Besides, the angle between two hindlegs affects the control of jump trajectory but has a little effect on the elevation angle of a jump, which is controlled mechanically by the initial position of the hindlegs. This research lays the groundwork for the probable design and development of biomimetic robotics.

Fabrication and adhesion of a bio-inspired microarray: capillarity-induced casting using porous silicon mold

Inspired by the setal microstructure found on the geckos toe-pads, a highly dense array of high-aspect-ratio (HAR) artificial setae has been developed with a novel mold-casting technique using a porous silicon (PSi) template. To overcome the high fluid resistance in the HAR capillary pores, the PSi template surface is modified with a monolayer coating of dimethylsilane. The coating exhibits similar chemical composition and surface energy to the precursor of the poly(dimethylsiloxane) (PDMS) replica. The compatibility between the template and the replica addresses the major challenge of molding HAR microstructures, resulting in high-resolution replicas of artificial PDMS microsetae with complicated geometry resembling a real geckos setae. The artificial setae are characterized by a mean radius of 1.3 μm, an aspect ratio of 35.1, and a density of ~4.7 × 105 per mm2. Results from adhesion characterizations reveal that with increasing preload, the shear adhesion of micro-setae continually increases while the normal adhesion decreases. The unique adhesion performance is caused by both van der Waals forces and the elastic resistance of PDMS setae. With further structural optimizations and the addition of an actuation mechanism, artificial setal arrays might eventually demonstrate the fascinating adhesion performances of the gecko for mimetic devices such as wall-climbing devices.

Mechanical and Frictional Properties of the Elytra of Five Species of Beetles

The mechanical and frictional properties of different parts of the elytra of five species of beetle were measured using a nano-indenter and a micro-tribometer. The surface microstructures of the elytra were observed by optical microscopy and scanning white light interferometry. The surface microstructures of the elytra of all five species are characterized as non-smooth concavo-convex although specific morphological differences demonstrate the diversity of beetle elytra. Young’s modulus and the hardness of the elytral materials vary with the species of beetle and the sampling locations, ranging from 1.80 GPa to 12.44 GPa, and from 0.24 GPa to 0.75 GPa, respectively. In general, both the Young’s modulus and the hardness are lower in samples taken from the center of the elytra than those taken from other regions, which reflects the functional heterogeneity of biological material in the process of biological evolution. The elytra have very low friction coefficient, ranging from 0.037 to 0.079, which is related to their composition and morphology. Our measurements indicate that the surface texture and its mi-crostructural size of beetle elytra contribute to anti-friction effects.

Transfer of vertically aligned carbon nanotube arrays onto flexible substrates for gecko-inspired dry adhesive application

The geckos extraordinary climbing ability has inspired scientists to develop synthetic dry adhesives that mimic the structure and function of gecko feet. The vertically aligned carbon nanotube (VACNT) array has been considered a potential candidate for developing gecko-inspired dry adhesive materials due to its outstanding structural and mechanical properties. However, the limited choices of growth substrates and poor interfacial bonding between VACNTs and growth substrates have restricted their application as gecko-inspired dry adhesive materials. Here, we report a versatile transfer method for transferring VACNT arrays onto flexible polymer substrates using a thermal oxidation process. This transfer method mainly focused on using the VACNT array as a gecko-inspired dry adhesive. Using the transfer method developed in our study, VACNT array-based gecko-inspired dry adhesive materials with improved adhesion property, structural stability, and self-cleaning ability can be developed. A thermal oxidation process was used to obtain free-standing VACNT arrays, resulting in the production of top-transferred and bottom-transferred structural VACNT array-based dry adhesive materials. The shear adhesive strength of the transferred VACNT array was enhanced using this method. The interfacial bonding strength of the transferred VACNT array increased nearly 15 times according to nanoscratch tests. The flexible structure of the transferred VACNT array exhibited better antifouling properties by mimicking digital hyperextension (DH) motion, which is a natural peeling motion of the gecko foot with a unique dynamic self-cleaning mechanism. These findings show that our method is an efficient method for transferring VACNT arrays and an important process for fabricating gecko-inspired VACNT array-based dry adhesive materials with high structural stability, self-cleaning ability, and high adhesive strength.