Wrinkle Structures Research Articles

Adhesive conductive hydrogels with wrinkled Janus surface and ultra-high sensitivity used as strain sensors

Conductive hydrogels have attracted enormous attention in wearable electronic devices due to considerable flexibility and similarity with human skin. Since the majority of hydrogels were fabricated by one-pot method alone and the conductivity change under strain was caused by the slight shape change and the fixed conductive path, further improvement of the sensitivity is hard. Thus, we cast the silver nanowires (AgNW) in ethanol solvent on the upper surface of polyacrylic acid/chitosan (PAA/CS) hydrogels prepared by one-pot method to obtain asymmetrical PAA/CS@AgNW hydrogels. As the construction of conductive surface, the ethanol gradually diffused into the hydrogel and caused the upper surface with ideal wrinkle structure. Thanks to the dual-effect of the design, the sensitivity was greatly enhanced with the ultra-high gauge factor (GF = 191.2, 7413, 18,720) at different pressure ranges. A series of reliable application performance (flexibility, self-healing, dry resistance and adhesion) and excellent electrical signal response (ultra-high sensitivity, identification, repetition and stability) was tested and indicated the potential applications in motion detection, gesture recognition and health monitoring.

3D printing of nanowrinkled architectures via laser direct assembly.

Structural wrinkles in nature have been widely imitated to enhance the surface functionalities of objects, especially three-dimensional (3D) architectured wrinkles, holding promise for emerging applications in mechanical, electrical, and biological processes. However, the fabrication of user-defined 3D nanowrinkled architectures is a long-pending challenge. Here, we propose a bottom-up laser direct assembly strategy to fabricate multidimensional nanowrinkled architectures in a single-material one-step process. Through the introduction of laser-induced thermal transition into a 3D nanoprinting process for leading the point-by-point nanoscale wrinkling and the self-organization of wrinkle structures, we have demonstrated the program-controlled and on-demand fabrication of multidimensional nanowrinkled structures. Moreover, the precise control of wrinkle morphology with an optimal wavelength of 40 nanometers and the regulation of the dynamic transformation of wrinkled cellular microstructures via interfacial stress mismatch engineering have been achieved. This study provides a universal protocol for constructing nearly arbitrary nanowrinkled architectures and facilitates a new paradigm in nanostructure manufacturing.

Insight into the role of functional groups on the mechanical, piezoelectric and electrical properties of an individual reduced graphene oxide wrinkle

This work aims to correlate the structural and physical properties (mechanical, piezoelectric, electrical) of individual graphene oxide (GO) wrinkle using advanced modes of atomic force microscopy (AFM). The high stiffness value of GO wrinkles in phase detection microscopy (PDM) indicates the wrinkle has a high sp2/sp3 carbon ratio and is attributed to the strain induced by the curvature of the wrinkle, which decreases the stability of functional groups. Piezo force microscopy (PFM) measurements demonstrated the strain induced and non-switchable piezoresponse on the wrinkle structure. Functional groups attached plane GO region shows switchable polarization with a piezoresponse of 6 pm/V. The ripple type wrinkles have low strain and high conductance due to their small height and two-dimensional (2D) nature. Above 5 nm height, standing collapsed and folded wrinkles transform into a three-dimensional (3D) structure that facilitates tunneling and anisotropy in electron transport. These findings show that by controlling the functional group concentration, strain, and height of an individual GO wrinkle, one can engineer the wrinkles on the GO structure to meet the needs of practical applications.

Effect of Copolymer on the Wrinkle Structure Formation and Gloss of a Phase-Separated Ternary Free-Radical/Cationic Hybrid System for the Application of Self-Matting Coatings.

Hybrid free-radical/cationic systems can generate phase-separated polymers or interpenetrating networks driven by photopolymerization. In this study, phase separation of a ternary mixture composed of a polybutadiene urethane diacrylate (PBUDA), a cycloaliphatic diepoxyde (CE), and hexanediol dimethacrylate (HDDMA) was investigated. Using systematic variations of the initial composition of the mixture, a miscibility phase diagram of the ternary mixture was established. Based on this diagram, a reactive copolymer (poly(butyl acrylate-co-glycidyl methacrylate) (PBGMA)) was introduced in a reference hybrid system to manipulate the crosslinking network, polymer morphology, and properties (e.g., roughness, gloss, strain at break, and glass transition temperature Tg). When cured as a coating, the ternary hybrid system showed a depthwise gradient of epoxy conversion, and thereby developed a mostly cured skin above a viscous sublayer of uncured monomer. This skin can develop compressive stress due to the swelling from the diffusion of unreacted monomers beneath, and if the compressive stress is significantly high, wrinkles appear on the coating’s surface. This work highlights how both skin thickness and elastic modulus impact wrinkle frequency and amplitude. It was demonstrated that these wrinkle parameters can be manipulated in the ternary system by the addition of PBGMA. We also demonstrated that by employing UV irradiation and varying the PBGMA content, it is possible to engineer coatings that range from smooth surfaces with high gloss to wrinkled topographies with a very low associated gloss.

CNTs based capacitive stretchable pressure sensor with stable performance

In recent years, with the increasing demand for artificial robots and smart devices, flexible pressure sensors have attracted much attention. In this paper, to solve the problem of insufficient stretchability and complicated microstructure fabrication process, a capacitive pressure sensor based on flexible electrodes with wrinkle structure and composite dielectric layer was designed and prepared. The as-prepared sensor exhibits a sensitivity of 2.13 kPa−1 and the response time is about 0.1 s. During the loading-unloading test, the capacitive flexible sensor showed stable sensing performance and good repeatability with minor hysteresis effect. The research is of great significance for the application of flexible sensors in fields such as soft robots, E-skin and medical monitoring in the future.

Structural Integrity Preserving and Residue-Free Transfer of Large-Area Wrinkled Graphene onto Polymeric Substrates

Wrinkled graphene offers many advantageous features resulting from modifying the structural and physical properties as well as the chemical reactivity of graphene. However, its inadequate transferability to other substrates has limited its usability. This paper reports a roll-based clean transfer approach that enables the damage-free and contamination-free transfer of large-area wrinkled graphene onto polymeric substrates without compromising the integrity of wrinkle structures. The method implements the simultaneous imidazole-assisted etching and doping of chemical vapor-deposited graphene to fabricate multilayer graphene on a thermoplastic polystyrene (PS) substrate coated with a water-soluble poly(4-styrenesulfonic acid) (PSS) sacrificial layer via a roll-based transfer process. The compliant PSS layer affords the conformal contact between the PS substrate and graphene during the wrinkle formation process, enabling the controllable fabrication of graphene wrinkle structures on a large area. The water-soluble properties of PSS simplify the typically difficult separation of wrinkled graphene from the PS substrate after its transfer onto a target substrate. This improves the transferability of wrinkled graphene, rendering the transfer process solvent-free and residue-free. This work demonstrates the feasibility of the formulated method by transferring centimeter-scale wrinkled graphene onto currently used transparent flexible substrates (i.e., polyethylene terephthalate and polydimethylsiloxane). The results indicate that the transferred wrinkled graphene possesses the desirable combination of superior stretchability, optical transmittance, sheet resistance, and electromechanical stability, rendering its suitable application to transparent flexible and stretchable electronics.

High Sensitivity and a Wide Sensing Range Flexible Strain Sensor Based on the V-Groove/Wrinkles Hierarchical Array.

Flexible strain sensors occupying a large part of human body detection and wearable electronics, which have a wide sensing range and high sensitivity, are crucial in fully monitoring human motion signals. This study proposed a strategy to construct flexible strain sensors based on the V-groove/wrinkles hierarchical array. The V-groove array was prepared on a polydimethylsiloxane (PDMS) substrate through mold transfer printing. The gold film was sputtered on the prestretching PDMS substrate, and the V-groove/wrinkles hierarchical array was formed after strain release. Compared with the sensors based on single-scale wrinkle structures and a V-groove array, the fabricated strain sensor with the hierarchical array showed high sensitivity (maximum gauge factor up to 2,557.71) and a wide sensing range (up to 45%). In addition, the dynamic characteristics of the sensor were investigated in detail, indicating that the sensor had a fast response (less than 130 ms), a low detection limit (0.1% strain), and good stability (almost no performance loss after 10,000 cycles). In practical applications, the sensor was used to detect sizable physical motion and weak physiological signals, demonstrating great potential application value in human motion detection. This study could provide new ideas for preparing high-performance flexible strain sensors.

Influence of the wrinkle surface structures on the vapor flow and keyhole stability in 20 kW high power laser welding

The stability of the extremely narrow keyhole is still the biggest challenge for the 20 kW or higher-level laser welding. The underlying causes of keyhole instability, including the wrinkle structures-induced thermal instability and vapor-induced dynamic instability, are studied in this work. A multiphase flow model integrated with a high-efficient free surface reconstruction algorithm and a high-accuracy wrinkle surface structures observation platform are built to study the morphology evolution and thermodynamic behaviors on the extremely narrow keyhole. It is found that the wrinkle structures (humps) that exist on the front keyhole wall could significantly regulate the energy distribution, and change the pressure condition in keyhole civility. The excessive energy accumulation, which is aroused by the abnormal hump distribution, is the reason for the unstable keyhole wall fluctuation. By increasing the welding speed, the intermittent-contact behavior between the laser beam and the keyhole opening is suppressed, which helps reduce the hump generation. By increasing the laser power, the drilling effect on the solid-liquid interface could be avoided, which helps to improve the downward flow efficiency of the melting layer on the front keyhole wall. An optimizing strategy based on the welding speed and laser power adjusting is offered to regulate wrinkle morphology. The experiment results show that keyhole stability is significantly improved under the optimized wrinkle surface structures.

Wrinkle-structured MXene film assists flexible pressure sensors with superhigh sensitivity and ultrawide detection range

With the rapid development of artificial intelligence and ever-increasing demand for electronic devices, constructing high-performance flexible pressure sensors is urgently needed. In this study, a novel flexible pressure sensor with superhigh sensitivity and ultrawide detection range is tactfully designed and prepared based on wrinkle-structured MXene film. Due to the unique wrinkle structure, the prepared pressure sensor exhibits outstanding performance including high sensitivity of nearly 860 kPa−1, wide detection range (0.5 Pa ‒ 30 kPa), short response/recover time (40/53 ms) and high stability (11600 cycles). A multi-scale working mechanism is proposed to explain the sensing process. Interestingly, the prepared sensor can easily detect the smallest movement of a millet (2.4 mg) in real time. Based on these advantages, the prepared pressure sensor is expected to show significant potentials in future physiological detection and precision instrument monitoring systems.

Preparation of Nano-silica with Radial Wrinkle Structures for Self-cleaning and Superhydrophobic Coatings

Preparation of Nano-silica with Radial Wrinkle Structures for Self-cleaning and Superhydrophobic Coatings

Study of turbulent flame characteristics of water vapor diluted hydrogen-air micro-mixing combustion

Hydrogen has been regarded as an important alternative fuel, especially in gas turbine applications. However, the premixed combustion of pure hydrogen has problems such as easily flashback and high NOx emissions. A more reasonable method of combustion is to use micro-mixing technology combined with water vapor dilution. This work experimentally investigated the influences of dilution rate (D) and equivalence ratio (φ) on the flame characteristics of hydrogen-air micro-mixing combustion diluted with water vapor. Results show that increasing D reduces the flame area by reducing the downstream OH radical signal, which is contrary to the effect of CO2 dilution. The flame local curvature radius shows that the micro-mixing combustion is a high turbulence intensity combustion, and the convex structure is more frequent. Moreover, increasing φ or decreasing D leads to a decrease in the instantaneous average curvature radius (Rave), indicating more small-scale wrinkle structures are generated. D and φ have a similar influence on the fluctuations of Rave and flame area in the time domain, while the fluctuation of the flame area is more evident near the lean-burn limit. When D = 25%, the frequency characteristics of the flame structure and chemical reactions are synchronized.

Surface mechanical behavior of water-spread poly(styrene)–poly(ethylene glycol) (PS–PEG) micelles at the air–water interface: Effect of micelle size and polymer end/linking group chemistry

Hypothesis:The surface mechanical properties of poly(styrene)-poly(ethylene glycol) (PS–PEG) micelles are influenced by the PEG corona structure. Changes in micelle aggregation number as well as changes in the PEG end group and linking group chemistry of the PS–PEG block copolymer are expected to alter PEG corona characteristics and therefore affect surface mechanical properties of the resulting micelle film. Experiments:Different sized micelles comprised of PS–PEG block copolymer chains were formulated by equilibrating micelles in different ratios of acetone/water mixtures and subsequently removing acetone using dialysis. Additionally, micelles of a similar size and PS–PEG molecular weight but slightly different chemistry were formulated. The micelles were characterized using dynamic light scattering (DLS), transmission electron microscopy (TEM), 1H NMR, surface pressure–area isotherms and Brewster angle microscopy (BAM). Findings:The reduction in micelle aggregation number results in the subsequent monolayer having higher compressibility moduli and bending stiffnesses and collapsing at lower surface pressures. Micelle hydrophobicity was shown to improve readsorption of micelles to interface after collapse. Analysis of Brewster angle microscopy images of out-of-plane wrinkle structures which formed upon monolayer collapse indicates the presence of continuous 1 nm thick PEG layer which allows micelle monolayers to bend under high compression.

Superior Performance in Liquid Crystal Alignment of Polystyrene-Block-Poly(ethylene-ran-butylene)-Block-Polystyrene-Graft-Maleic Anhydride Film Irradiated with Ion Beam

We studied the change of the surface and liquid crystal (LC) alignment characteristics of Polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene-graft-maleic anhydride (SEBS-g-MA) film according to ion beam (IB) irradiation. Atomic force microscopy (AFM) analysis revealed that IB irradiation reduces peaks and creates a wrinkled structure. X-ray photoelectron spectroscopy (XPS) analysis revealed that IB irradiation disintegrates the C–C bonds and makes C–O bonds, creating a dipole-dipole interaction, which improves the LC alignment characteristics. Through polarized optical microscopy (POM) analysis, it was found that the best LC alignment characteristics can be obtained after 120 s of IB irradiation. It was found that the anisotropy caused by the ion beam contributed more to the liquid crystal alignment in homogeneous direction than the one caused by the wrinkle structure.

Fatigue-free Cu films on wrinkled soft substrates

The fatigue behavior of nanocrystalline Cu films on polydimethylsiloxane (PDMS) substrates was studied. It is revealed that the Cu films on wrinkled PDMS substrates exhibit ultrahigh fatigue resistance without electrical failure. The electrical resistance either remains constant during cyclic loading or slightly increases and rapidly reaches saturation, which is attributed to the buffering effect of wrinkle structures on crack initiation and the suppression effect of surface undulations on crack propagation. The film thickness-dependence of fatigue resistance is sensitive to the amount of the strain amplitude. The fatigue resistance increases with decreasing film thickness at small strain amplitudes because of higher strength and smaller local strain concentration, while the fatigue resistance first increases and then decreases as the film thickness increases at large strain amplitudes, which is related to the extrinsic deformation capability of Cu films.

Antifatigue Hydration-Induced Polysaccharide Hydrogel Actuators Inspired by Crab Joint Wrinkles.

Joint wrinkles in animals facilitate frequent bending and contribute to the duration of the joint. Inspired by the morphology and function of joint wrinkles, we developed a bionic hydration-induced polymeric actuator with constructed wrinkles at the selected area. Specifically, we adopt electrical writing to create defined single and double cross-linking regions on chitosan (CS) hydrogel. The covalent cross-linking network was constructed by electrical writing-induced covalent cross-linking between CS chains and epichlorohydrin. Subsequent treatment of sodium dodecyl sulfate allows electrostatic cross-linking at the unwritten area with the simultaneous formation of surface wrinkles. The resulting single and double cross-linking hydrogel demonstrates spontaneous deformation behaviors by the influx and efflux of H2O to the electrostatic cross-linking domain under different ion concentrations. Importantly, the wrinkle structure endows the hydrogel with extraordinary antifatigue bending performance. By regulating the surface morphology and spatial cross-linking, we can design novel biomimetic polysaccharide hydrogel actuators with fascinating functions.

Ultra-Sensitive, Self-Healable, and Robust Silicone Elastomer-Based Flexible Sensor with Wrinkle and Crack Structures for Wearable Electronics

Ultra-Sensitive, Self-Healable, and Robust Silicone Elastomer-Based Flexible Sensor with Wrinkle and Crack Structures for Wearable Electronics

Shape instabilities of islands in smectic films under lateral compression.

Smectic liquid crystals are fluids, and in most rheological situations they behave as such. Nevertheless, when thin freely floating films of smectic A or smectic C materials are compressed quickly in-plane, they resist such stress by buckling similar to solid membranes under lateral stress. We report experimental observations of wrinkling and bulging of finite domains within the films, so-called islands, and give a qualitative explanation of different observed patterns. Depending on the external stress and their dimensions, the islands can expel a specifically shaped bulge in their center, form radial wrinkles or develop target-like wrinkle structures. When the external stress is relaxed, these patterns disappear reversibly.

Strain‐Insensitive Self‐Powered Tactile Sensor Arrays Based on Intrinsically Stretchable and Patternable Ultrathin Conformal Wrinkled Graphene‐Elastomer Composite

AbstractTissue‐like intrinsically stretchable electronics have attracted ever‐increasing study attention in recent years as they can form intimate interfaces with skin, endowing devices monitoring tactile and physiological signals with the negligibly constraining movement of the human body. However, harsh mechanical deformation inevitably leads to degradation of or even destroys the electronic properties of the devices. Strain‐insensitive self‐powered triboelectric tactile sensor arrays based on wafer‐scale patterned and intrinsically stretchable nanoscale thin conformal wrinkled graphene‐elastomer composite material are demonstrated here. By regulating the wrinkle structure of the composite, the stretchability performance of the material can be optimized. The fabrication process of the composite can be readily incorporated into photolithography and shadow mask techniques without high temperature, annealing, etching, or organic solvents operating. An intrinsically stretchable semitransparent pressure sensor array is created, which can be stretched to 100% strain without visible signals output degradation. The theoretical modeling points out that the unique conformal wrinkle structure is the key element that attributes to the strain‐insensitive property of the device. This work offers an alternative approach for the design of novel graphene‐based strain‐insensitive stretchable soft electronic devices.

2D/3D ZIF‐9/Mo15S19 S‐Scheme Heterojunction for Productive Photocatalytic Hydrogen Evolution

Flower‐like Mo15S19 nanoparticles are successfully attached on the surface of the lamellar cobalt‐based zeolite imidazole framework (ZIF‐9) according to the physical mixing process herein described. The existence of lamellar ZIF‐9 not only avoids the stacking of spherical flower‐like Mo15S19 nanoparticles but also enhances the adsorbability of the ZIF‐9/Mo15S19 system for Eosin‐Y (EY) molecules, which injects more vitality into the hydrogen production reaction. In addition, the lamellar ZIF‐9 can be freely stacked and form a regular rich wrinkle structure. Benefitting from this special structure, Mo15S19 nanoparticles with spherical flower‐like shape can be adsorbed more firmly between the layers of ZIF‐9. This 2D/3D interlaced structure makes the system more stable, which also can improve the adsorption capacity of EY molecules. In addition, the construction of an S‐scheme heterojunction between ZIF‐9 and Mo15S19 is another nonnegligible driving force for the enhanced photocatalytic performance in this system. The rational establishment of the S‐scheme heterojunction provides a more convenient transmission channel for the photogenerated charge. This work uses ZIF‐9 as the carrier of Mo15S19 and promotes the catalytic performance of the composite ZIF‐9/Mo15S19 by adjusting the evacuation of the spherical Mo15S19 nanoparticles and constructing a heterojunction that can promote the flow ability of charges.

A highly elastic conductive film prepared by bidirectional AS-LBL method

Development of elastic conductive film is critical to prepare flexible electronic devices. Herein, bidirectional elastic conductive films based on polyurethane (PU) and different dimensional fillers were prepared by using an novel alternating stretching layer-by-layer self-assembly (AS-LBL) technology. It is shown that wrinkle structures were constructed in the conductive films, which allows them to cope with the deformation under bidirectional stretching to a certain extent, while maintaining stable conductivity. Basically, the one-dimensional MWCNTs exhibit the better construction speed of the conductive path than zero-dimensional Ag NPs. In addition, under the synergy of Ag NPs and MWCNTs, the conductivity of PU/Ag-MWCNTs films reaches the best. After adsorbing 6 layers of Ag-MWCNTs, the conductivity of the film increased to 0.24 S cm−1. More importantly, during the stretching process, the PU/MWCNTs and PU/Ag-MWCNTs films show a more stable conductivity than PU/Ag. As the strain increases from 0 to 500%, the conductivity of PU/Ag-MWCNTs films decreases by only 9.6%. However, during the stretch-release cycle, the zero-dimensional small-sized Ag NPs display a more superior ability of conductive path reconstruction than MWCNTs. When the elongation is 100%, after 100 stretching cycles, the conductivity of PU/Ag film is only reduced by 8%.