Asymmetric Surface Structure Research Articles

Insect Tarsus-Inspired Compliant Robotic Gripper With Soft Adhesive Pads for Versatile and Stable Object Grasping

Grasping multiple object types (versatile object grasping) with a single gripper is always a challenging task in robotic manipulation. Different types of grippers, including rigid and soft, have been developed to try to achieve the task. However, each gripper type is still restricted to specific object types. In nature, many insects can be observed to use only one tarsus mechanism to cope with several tasks. They have a very high grasping capability with objects and can adhere to a variety of surface types. Inspired by insect tarsus, this paper proposes a novel underactuated, single cable-driven, compliant gripper design. The structure of the gripper is based on the hornet tarsus morphology with a proportional scale. An additional pulley-like structure is introduced to increase the generated grasping torque. To maintain the ability to automatically rebound back to the original position, a torsion spring is implemented at each joint. In order to stably grasp and hold objects, soft adhesive pads with an asymmetric sawtooth-like surface structure are attached at the tarsus segments. The performance of this insect tarsus-inspired gripper with the soft pads is evaluated by grasping 35 different objects of various sizes, shapes, and weights for comparison with industrial soft and rigid grippers. The proposed gripper shows a 100% success rate in grasping all objects, while the soft and rigid gripper success rates are 81.90% and 91.43% on average, respectively. We finally demonstrate the use of our gripper installed on a robot arm for pick-and-place and pouring tasks.

Dynamics Behavior of Droplet Impact on a Controllable Curved Micropillar Array Surface Induced by a Magnetic Field.

Many fields would greatly benefit from the realization of the manipulation of droplet impact behavior by an asymmetric surface structure, such as self-cleaning, anti-icing, inkjet printing, etc. However, research on the prediction of the impact of the dynamics of small-volume droplets on the asymmetric superhydrophobic surface has been insufficient. In this study, a superhydrophobic curved micropillar array surface with controllable bending angles induced by a magnetic field was prepared. The impact and rebound behaviors of the nanoliter droplets with diameters of 100-300 μm were investigated. The experimental results showed the positive correlation between the threshold Weber number of the impact morphology transition of the droplet and the inclination angle of the micropillar. In addition, the restitution coefficient, which measures the degree of energy loss during the impact process, showed a nonmonotonic dependence on the Weber number. A critical velocity model of the impact morphology transition of the droplet on the curved micropillar array surface and a prediction model of the restitution coefficient of the droplet in different impact morphologies are suggested. Our findings will help in the design of a functional surface for manipulating the impact behavior of the droplet.

Frictional Anisotropic Locomotion and Adaptive Neural Control for a Soft Crawling Robot.

Crawling animals with bendable soft bodies use the friction anisotropy of their asymmetric body structures to traverse various substrates efficiently. Although the effect of friction anisotropy has been investigated and applied to robot locomotion, the dynamic interactions between soft body bending at different frequencies (low and high), soft asymmetric surface structures at various aspect ratios (low, medium, and high), and different substrates (rough and smooth) have not been studied comprehensively. To address this lack, we developed a simple soft robot model with a bioinspired asymmetric structure (sawtooth) facing the ground. The robot uses only a single source of pressure for its pneumatic actuation. The frequency, teeth aspect ratio, and substrate parameters and the corresponding dynamic interactions were systematically investigated and analyzed. The study findings indicate that the anterior and posterior parts of the structure deform differently during the interaction, generating different frictional forces. In addition, these parts switched their roles dynamically from push to pull and vice versa in various states, resulting in the robot's emergent locomotion. Finally, autonomous adaptive crawling behavior of the robot was demonstrated using sensor-driven neural control with a miniature laser sensor installed in the anterior part of the robot. The robot successfully adapted its actuation frequency to reduce body bending and crawl through a narrow space, such as a tunnel. The study serves as a stepping stone for developing simple soft crawling robots capable of navigating cluttered and confined spaces autonomously.

Asymmetric Colloidal Particles Fabricated by Polymerization-Induced Surface Self-Assembly Approach

In the past decades, the synthesis of asymmetric colloidal particles has been of wide concern in material science because of their anisotropic morphologies and unique properties. However, the synthesis of the asymmetric particles with controllable anisotropy still remains a big challenge because of the lack of effective methods. In this research, polymerization-induced surface self-assembly (PISSA) approach was demonstrated to be an effective method in the synthesis of colloidal particles with tunable asymmetric core–shell structures. Reversible addition–fragmentation chain transfer agents were covalently anchored onto the surfaces of silica particles and coupled onto the ends of methoxy polyethylene glycol (PEG) chains, leading to the synthesis of CTA-modified silica particles (SiO2-CTA) and PEG macro-CTA agents (PEG-CTA), respectively. PISSA process was conducted by using SiO2-CTA and PEG-CTA as co-RAFT agents in RAFT dispersion polymerization of styrene. In the RAFT polymerizations, polymer layers with PS nodules were formed on the surfaces of SiO2 particles. It was demonstrated that the formation of the nodules was related to the production of the PS homopolymer in the PISSA process. In order to control the asymmetric surface structures, RAFT dispersion polymerizations of styrene with added “free” RAFT agents were performed, and it turned out that the eccentrically positioned core–shell structures with silica cores and polymer layers were prepared. The colloidal particles experience morphology changes from surface nodules to snowman-like structures and finally to asymmetric spherical structures, depending on the monomer conversion and the feeding ratio of the co-RAFT agents. Kinetics studies were performed to investigate the mechanism of the formation of the asymmetric particles. After etching the silica cores with HF solution, asymmetric hollow capsules were fabricated.

Gradient Quasi‐Liquid Surface Enabled Self‐Propulsion of Highly Wetting Liquids

AbstractSelf‐propulsion of highly wetting liquids is important in heat exchanger, air conditioning, and refrigeration systems. However, it is challenging to achieve such a spontaneous motion as these liquids tend to wet all the surfaces due to their ultralow surface tensions. Despite that extensive asymmetric surface structures and gradient chemical coatings are developed for directional droplet transport, they will be flooded and covered by these liquids. Here, this challenge is addressed by creating a gradient quasi‐liquid surface to achieve the self‐propulsion of droplets with surface tensions down to 10.0 mN m−1. Such a surface engineered by tethering flexible polymers with gradient grafting density shows ultralow contact angle hysteresis (<1o) to highly wetting liquids. Thus, the surface can simultaneously provide sufficient driving forces through the gradient wettability and negligible retention forces through the slippery boundary lubrication for spontaneous droplet movement. Moreover, continual self‐propulsion of tiny droplets is achieved by spraying highly wetting liquids in simulated condensation conditions and demonstrates that adding temperature gradient can further accelerate the self‐propulsion. The study provides a new paradigm to promote passive removal of highly wetting droplets, leading to potential impacts in enhancing condensation heat transfer regardless of surface orientations.

Programmable and Self-Healing Light-Driven Actuators through Synergetic Use of Water-Shaping and -Welding Methods.

Shape programming is critical for the fabrication of a light-driven actuator with complex shape morphing, which demonstrates potential applications in remote-controlled light-driven soft robots. However, it remains a huge challenge to obtain light-driven actuators having advantages of complex shape morphing, self-healing function, and facile fabrication simultaneously. Here, we report a facile strategy to obtain programmable and self-healing light-driven actuators with complex shape morphing. Various initial shapes of actuators can be programmed by synergetic use of water-shaping and -welding methods, which provides unlimited opportunities for fabricating actuators with predesigned shapes and subsequently demonstrating complex shape morphing. A template transfer method is used to prepare a single-layer graphene oxide (GO) film with asymmetric surface structures, which acts as the basic actuator and has the self-healing function based on the hydrophilic property of GO. It shows bending morphing under near-infrared (NIR) light irradiation due to the photothermal effect and asymmetric morphology on the opposite surfaces. Four more types of actuators are programmed from the basic actuator through the water-shaping method, which exhibits bending, unbending, twisting, and untwisting, respectively, under NIR light illumination. In addition, an S-shape actuator and a flower-shape actuator are programmed from the basic actuators through the water-welding method. By simply turning over the S-shape actuator, it can perform a bidirectional crawling motion. Finally, two intricate bionic light-driven actuators (tendril-shape and octopus-shape) are constructed, which are unattainable from conventional fabrication methods of actuators. We believe that this study will unlock a new way to programmable, self-healing, and light-driven soft robots with tunable and complex shape morphing.

Microwave-Assisted Synthesis of PGS/PCL Nanocomposite Films: Effect of Organo-Modified Montmorillonite

Microwave-assisted synthesis of polyglycerolsebacate (PGS) - polycaprolactone (PCL) with nanoclay composite films were fabricated in this study. PGS produced via microwave irradiation technique was blended withorgano-modified montmorillonite (O-MMT) and PCL, and cast using thermally induced phase separation technique (TIPS) to produce nanocomposite films. The fabricated films underwent different qualitative characterization to identify its morphology, porosity,wettabilityandmechanical strength. FTIR results showed changes in the band peaks signifying the interaction between PCL, O-MMT and PGS. SEM images of the films displayed filamentous and non-uniform pores present on the films, which became more visible upon the addition of O-MMT and PGS.Also, the films showed improved tensile strength and elongation upon addition of PGS, but on an optimum O-MMT amount. Furthermore, the pore size of films increased upon addition of PGS, which confirmed the effect of its hydrophilic in nature. Thiswas correlated to the enhanced wettability of the films produced and the formation of asymmetric surface structure.

Liquid progressive multifocal lenses based on asymmetrical freeform surface structure using non-uniform thickness membranes

For a liquid progressive multifocal lens with an elastic membrane deformed by liquid pressure, to realize a reasonably power distribution, an asymmetric deformation characteristics of membrane surface is needed. Based on the asymmetric freeform surface structure, this paper proposed liquid progressive multifocal lenses focused by liquid with non-uniform thickness membranes and different boundary constraints. The structure and mathematical model of power distribution for the lens are introduced. The membrane deformation and the corresponding power distribution of the lenses with asymmetric freeform surface are predicted and compared under uniform pressure load and different boundary constraints. A membrane deformation testing system is constructed to analyze the power distribution of fabricated lenses through measuring the surface sag using laser displacement sensor and corresponding optical power distribution can be calculated. Experimental results show that the liquid lenses can realize as liquid progressive multifocal lenses after liquid accommodation, meanwhile, the trends of power distribution of the lenses generally agree well with simulations.

Progressive Multifocal Liquid Lenses Based on Asymmetric Freeform Surface Structure of Nonuniform Thickness Elastic Membranes with Different Constraints

For a progressive multifocal liquid lens with an elastic membrane deformed by liquid pressure, to realize a reasonably power distribution, asymmetric deformation characteristics of the membrane surface are needed. Based on the asymmetric freeform surface structure, this paper proposed progressive multifocal liquid lenses focused by liquid with nonuniform thickness membranes. The structure and mathematical model of power distribution for the lens are introduced. The membrane deformation and the corresponding power distribution of the lenses with asymmetric freeform surface are predicted and compared under uniform pressure load and different boundary conditions using the finite element method. An optical testing system is constructed to analyze the optical characteristics of the fabricated lenses through observing the focusing performance of the F target image at different regions of the lenses. Experimental results show that the liquid lenses can realize as asymmetrical progressive multifocal liquid lenses after liquid accommodation; meanwhile, the trends of power distribution of the lenses generally agree well with simulations.

Revealing How Topography of Surface Microstructures Alters Capillary Spreading

Wetting phenomena, i.e. the spreading of a liquid over a dry solid surface, are important for understanding how plants and insects imbibe water and moisture and for miniaturization in chemistry and biotechnology, among other examples. They pose fundamental challenges and possibilities, especially in dynamic situations. The surface chemistry and micro-scale roughness may determine the macroscopic spreading flow. The question here is how dynamic wetting depends on the topography of the substrate, i.e. the actual geometry of the roughness elements. To this end, we have formulated a toy model that accounts for the roughness shape, which is tested against a series of spreading experiments made on asymmetric sawtooth surface structures. The spreading speed in different directions relative to the surface pattern is found to be well described by the toy model. The toy model also shows the mechanism by which the shape of the roughness together with the line friction determines the observed slowing down of the spreading.

Dynamic Spreading and Transport of Liquid Droplets over Asymmetric Sawtooth Surface Structures

Dynamic Spreading and Transport of Liquid Droplets over Asymmetric Sawtooth Surface Structures

One Single Graphene Oxide Film for Responsive Actuation.

Graphene, because of its superior electrical/thermal conductivity, high surface area, excellent mechanical flexibility, and stability, is currently receiving significant attention and benefit to fabricate actuator devices. Here, a sole graphene oxide (GO) film responsive actuator with an integrated self-detecting sensor has been developed. The film exhibits an asymmetric surface structure on its two sides, creating a promising actuation ability triggered by multistimuli, such as moisture, thermals, and infrared light. Meanwhile, the built-in laser-writing reduced graphene oxide (rGO) sensor in the film can detect its own deformation in real time. Smart perceptual fingers in addition to rectangular-shaped and even four-legged walking robots have been developed based on the responsive GO film.

A flexible free-standing conductive polypyrrole membrane with asymmetric surface structures for biomedical application

A flexible free-standing conductive polypyrrole membrane with asymmetric surface structures for biomedical application

Janus graphene oxide nanosheets prepared via Pickering emulsion template

Janus graphene oxide (GO) nanosheets functionalized by amino-containing chemicals were prepared via Pickering emulsion template. A wax-in-water Pickering emulsion was used to mask one side of GO nanosheets in order to achieve asymmetric chemical functionalization. Janus particles were obtained by removing the oil phase. The successful reaction of epoxy groups on the surface of GO with amino-containing chemicals was confirmed by Fourier transform infrared spectroscopy (FT-IR) and thermal gravimetric analysis (TGA). The asymmetric surface structure of Janus GO nanosheets was detected by atomic force microscope (AFM) and X-ray diffraction (XRD). The efficient stabilization of an oil-in-water Pickering emulsion by Janus GO was proved. Polymer microspheres fabricated by using Janus GO as Pickering stabilizer had a more hydrophilic surface compared with those stabilized by symmetrically modified GO.

AgI Microplate Monocrystals with Polar {0001} Facets: Spontaneous Photocarrier Separation and Enhanced Photocatalytic Activity

Elucidating the facet-dependent photocatalytic activity of semiconductor photocatalysts is important in improving the overall efficiency of photocatalysis. Furthermore, combining facet control with selective deposition of oxidation and/or reduction cocatalysts on specific faces of semiconductor photocatalysts is potentially an effective strategy to synergistically optimize the functionality of photocatalysts. In the present study, high-purity wurtzite-type β-AgI platelet microcrystals with polar {0001} facets were prepared by a facile polyvinylpyrrolidone-assisted precipitation reaction. The polar-faceted AgI microplates were used as archetypes to demonstrate preferential diametric migration (i.e., effective separation) of photogenerated electrons and holes along the c axis. Such vectorial electron-hole separation stems from the asymmetric surface structures, which give rise to distinct photoexcited reaction behaviors on the ±(0001) polar facets of wurtzite-type semiconductors. Furthermore, on selective deposition of Ag and MnOx (1.5<x<2) cocatalysts on the reductive (0001) and oxidative (0001) facets, respectively, photocatalytic activity of the AgI microplates in degrading organic pollutants was dramatically enhanced thanks to the broad light-absorption range, strong dye-adsorption ability, and effective spatial separation of photocarriers.

Constructing flexible cellulose–Cu nanocomposite film through in situ coating with highly single-side conductive performance

In this work, regenerated cellulose (RC)–Cu nanocomposite films with interesting single-side conductive properties were prepared using in situ coating. The structure and morphology of the films were investigated with XRD, ATR-FTIR, XPS, TG, SEM, TEM and AFM analysis. The mechanical properties and conductive performance of the films were also examined. The copper of a cellulose–cuprammonium solution was employed as the coating source as opposed to the addition of other conductive substances. The single-side coating of Cu nanocomposite film was achieved by one-step reduction based on the asymmetric surface structure of RC film. A layer of Cu nanoparticles with the thickness of ∼1 μm was coated on the coagulant-contacting surface. The RC–Cu nanocomposite films displayed high single-sided conductivity (105.6 ± 63.9 S cm−1). It was found that the resistance could be tuned by changing the bending curvature. This study provides potential applications in areas such as electronic substrates, wearable electronics and sensors.

(Photo-)Thermally Induced Formation of Dynamic Surface Topographies in Polymer Hydrogel Networks

In this Article, we report on our approaches to creating responsive hydrogel coatings with surfaces that can be switched between a close to flat state and a state with a predesigned topographic pattern. The trigger is either temperature or, indirectly, light. The hydrogel coatings are based on the known thermal responsive poly(N-isopropylacrylamide) utilizing its solubility change at its lower critical solution temperature (LCST) at around 32 °C. Using this material in combination with controlled cross-linking, we developed three fabrication strategies. Thermally switching symmetric surface topographies were created by careful and spatial control of the cross-link density. Sensitivity to light was created by patterns of tin oxide converting absorbed light into local heat. And to broaden the application range, we introduced a method based on volumetric effects created by a corrugated substrate. The latter method allows for the formation of asymmetric or slanted surface structures.

MECHANICAL HETEROGENIZATION OF LEWIS LUNG CARCINOMA CELLS CAN IMPROVE ANTIMETASTATIC EFFECT OF DENDRITIC CELLS

The effect of mechanically heterogenized (MCH) microparticles of tumor cells (TCs) on antimetastatic action of dendritic cells (DCs) is studied in C57BL/6 mice with Lewis' carcinoma. DCs isolated from mice spleens and loaded with MCH-TCs are analyzed with flow cytometry methods. MCH-TCs are analyzed with optical and/or electron microscopy. The paper describes an original high-precision medical microvibromill with high-acceleration linear induction motor that generates magnetic levitation to produce mechanical heterogenization of TCs. MCH-TCs have a more asymmetric morphology, larger surface and higher internal structure heterogeneity, and higher concentration of free radicals with respect to conventionally treated TCs. The rate of DCs maturity, being affected by pre-incubation with MCH-TCs is found to be higher than its counterpart treated with conventional TCs. DCs loaded with MCH-TCs show a significantly higher ability to induce proliferation of allogeneic lymphocytes in mixed leukocyte reaction. The inhibition index of metastases formation increases from 42% (conventional TCs) to 66% when DCs are treated with MCH-TCs. The present results demonstrate the feasibility of increasing antimetastatic activity of DCs-based vaccines when MCH-TCs is used for their loading. Mathematical model is developed in order to simulate the processes of capture, processing and presentation of tumor antigens by DCs when using conventional TCs or MCH-TCs.

Propulsion of droplets on micro- and sub-micron ratchet surfaces in the Leidenfrost temperature regime

Spatially periodic systems with localized asymmetric surface structures (ratchets) can induce directed transport of matter (liquid/particles) in the absence of net force. Here, we show that propulsion for the directed motion of water droplets levitating on heated ratchet surfaces in the Leidenfrost (film boiling) regime is significantly enhanced as the ratchet period decreases down to micro- and sub-micrometers. At the temperature range slightly above the threshold temperature of droplet motion, sub-micron ratchets yield water droplet velocities reaching ~40 cm/s, a speed that has never been achieved with any chemical and topological gradient surfaces. This dramatic increase in the droplet velocity is attributed to an enhanced heat transfer through the local contacts between ratchet peaks and bottom of the droplet. A hydrophobic coating on the ratchet surfaces is found to further increase the droplet velocity and decrease the threshold temperature of the droplet motion. The results suggest that miniaturized ratchet surfaces can potentially be used in diverse applications requiring control over fluid transport and heat transfer such as two phase cooling systems for microprocessors and fuel injection for combustion technology and that for those applications the design of ratchet dimensions and surface chemistry are critically important.

Surface-Initiated Free Radical Polymerization at the Liquid−Liquid Interface: A One-Step Approach for the Synthesis of Amphiphilic Janus Silica Particles

Amphiphilic Janus silica particles with hydrophobic polystyrene (PS) and hydrophilic poly(sodium methacrylate) (PSMA) brushes on two hemispheres were prepared at the liquid-liquid interface by surface-initiated polymerization. After introduction of free-radical initiator modified silica particles into a mixture of styrene and water, the silica particles were adsorbed at the liquid-liquid interface. One hemisphere of a silica particle is immersed in aqueous phase, and the other one is in styrene phase. After initiation at an elevated temperature, PSMA chains grow on one hemisphere and PS chains grow on the other one. Thermogravimetric analysis and infrared spectra results confirmed the grafting of polymer brushes on the surfaces. Transmission electron microscopy was used to characterize the asymmetric surface structure and aggregation structure of the Janus particles.