Wrinkles on a textile-embedded elastomer surface with highly variable friction

A novel self-wrinkled polyurethane-acrylate wood coating with self-matting, anti-fingerprint performance and skin-tactile feeling via excimer lamp/UV curing

Wrinkled surfaces exist widely in nature and organic living world, such as plants, insects, and skin. The optical, wettability and mechanical properties of materials can be enhanced by artificially preparing regular microstructure on the surface of materials. In this study, a novel self-wrinkled polyurethane-acrylate (PUA) wood coating with self-matting, anti-fingerprint performance and skin-tactile feeling curing by excimer lamp (EX) and ultraviolet (UV) was prepared. The wrinkles were formed on the surface of PUA coating at microscopic level after excimer and UV mercury lamp irradiation. The width and height of the wrinkles on the coating surface can be controlled to adjust the coating performance by changing the curing energy. When the PUA coating samples were cured by excimer lamp and UV mercury lamp with curing energy of 25-40 mJ cm^-2 and 250-350 mJ cm^-2, the excellent coating performances were observed. The gloss value of self-wrinkled PUA coating at 20° and 60° were less than 3 GU, while at 85° was 6.5 GU, which satisfied the demanding of matting coating. Besides, the fingerprints on the coating samples could disappear in 30 s and could still have anti-fingerprint performance after 150 times of anti-fingerprint tests. Furthermore, the pencil hardness, abrasion quantity and adhesion of self-wrinkled PUA coating were 3H, 0.045 g and 0 grade respectively. Finally, the self-wrinkled PUA coating has excellent skin-tactile feeling for touching. The coating can be applied to wood substrates, and has potential application in the field of wood-based panels, furniture and leather.

Multiscale Soft Surface Instabilities for Adhesion Enhancement

Soft polymeric gels are susceptible to buckling-induced instabilities due to their great compliance to surface deformations. The instability patterns at soft interfaces have great potential in engineering functional materials with unique surface properties. In this work, we systematically investigated how swelling-induced instability patterns effectively improved the adhesive properties of soft polydimethylsiloxane (PDMS) gels. We directly imaged the formations of the surface instability features during the relaxation process of a swollen gel substrate. The features were found to greatly increase the adhesion energy of soft gels across multiple length scales, and the adhesion enhancement was associated with the variations of contact lines both inside the contact region and along the contact periphery. We expect that these studies of instability patterns due to swelling will further benefit the design of functional interfaces in various engineering applications.

Curvature-Controlled Wrinkling Surfaces for Friction

Topographical patterns endow material surfaces with unique and intriguing physical and chemical properties. Spontaneously formed wrinkling has been harnessed to generate surface topography for various functionalities. Despite promising applications in biomedical devices and robot engineering, the friction behavior of wrinkling on curved surfaces remains unclear. Herein, wrinkled surfaces are induced by sputtering metals on soft polymer microspheres. The wrinkle morphologies and length scales can be controlled precisely by tailoring the microsphere radius (substrate curvature) and film thickness. The wrinkled surfaces exhibit controlled friction property, depending on the wrinkling patterns and length scales. An increase in friction force with increasing surface roughness is generally found for dimple patterns and labyrinth patterns. The dimple patterns show the lowest friction due to strong curvature constraint. The herringbone patterns exhibit apparent friction anisotropy with respect to topographic orientation. These results will guide future design of wrinkled surfaces for friction by harnessing substrate curvature.

Photocontrollable Wrinkle Morphology Evolution on Azo-Based Multilayers for Hierarchical Surface Micropatterns Fabrication

Inspired by nature, comprehensive understanding and ingenious utilization of the self-organized wrinkling behaviors of the sandwiched multilayer bonded on substrates are important for engineering and/or functional laminated devices design. Herein, we report a facile and effective strategy to regulate the wrinkles morphology evolution and the resultant hierarchical surface micropatterns on azobenzene-based laminated multilayers by visible-light irradiation. Revealed by systematic experiments, the photocontrolled dynamic wrinkle evolutions are triggered by the reversible photoisomerization of azobenzene in the top azopolymer film and are strongly dependent on the intermediate photoinert layers (e.g., polystyrene and oxygen plasma-induced SiO _x layer) with the wrinkle-reinforcing effect or the stress relaxation acceleration effect. Interestingly, large-area well-defined hierarchical surface wrinkle patterns could be fabricated on the multilayers upon selective exposure. In the unexposed region, the wrinkles evolved into highly oriented patterns, whereas in the exposed region, they were fully erased or evolved into smaller-wavelength wrinkles. This study not only sheds light on the morphological evolution of the wrinkling laminated composites in engineering and nature but also paves a new avenue to conveniently and controllably realize the hierarchical stimulus-responsive surface patterns.

Design and fabrication of biocompatible wrinkled hydrogel films with selective antibiofouling properties

In this article, we explored the selective antibiofouling capacity acquired by functional wrinkled hydrogel films via a fine tuning of their chemical structure through the gradual insertion of hydrophobic radical groups in their network. The hydrogel consists of three main components: hydroxyethyl methacrylate (HEMA, amphiphilic monomer), trifluoroethyl methacrylate (TFMA, hydrophobic monomer), and poly(ethylene glycol) diacrylate (PEGDA, hydrophilic crosslinking agent). Interestingly, the manipulation of the chemical composition affects both, surface morphology and physicochemical characteristics of the patterns, inducing transitions between different surface microstructures, i.e. from wrinkles to creases, to folds, and to crumples. Contact angle measurements show that the insertion of TFMA produces a slight decrease in surface wettability, remaining however highly hydrophilic. By using confocal Raman spectroscopy, important information about wrinkle formation mechanism could be obtained. The procedure presented in this article involves two consecutive thermal and photopolymerization steps, generating a "pseudo" two-layer system, which contracts at different extents when is exposed to external stimuli, leading to the formation of wrinkled surfaces. Finally, bacterial and cellular adhesion/proliferation studies were carried out, evidencing that the amount of TFMA included clearly reduce the bacterial adhesion while mammalian cells are able to still proliferate.

A two-step method for fabricating large-area textile-embedded elastomers for tunable friction

Recently, shape-tunable wrinkles formed on an elastomeric sheet with a textile finely embedded in proximity to the surface have been developed for in situ control of friction depending on various situations. For their actual uses, sheets with a large area are desired. A key challenge on their fabrication is to overcome the non-uniformity of the vertical position of the textile embedded within the elastomeric sheet, which causes substantial reduction in the tunable range of friction. The defect originates from the increased difficulty, as the sheet area is scaled up, of squeezing a viscoelastic precursor liquid due to the use of a deformable elastomeric surface. Here, we report a new two-step method for a textile-embedded elastomeric sheet that avoids using the soft elastomeric surface on the squeezing process and requires post-joining to an elastomeric base sheet. The obtained sheet with a large area (180 × 180 mm), was uniform and showed a large change of friction on its strain-induced transformation between flat and wrinkled states. The relationship between the experimentally controllable parameters and the squeeze film hydrodynamics is theoretically discussed, which is generally applicable to precise embedding micro-objects at the elastomer surface.

Near-infrared light-responsive dynamic wrinkle patterns

Dynamic micro/nanopatterns provide an effective approach for on-demand tuning of surface properties to realize a smart surface. We report a simple yet versatile strategy for the fabrication of near-infrared (NIR) light-responsive dynamic wrinkles by using a carbon nanotube (CNT)-containing poly(dimethylsiloxane) (PDMS) elastomer as the substrate for the bilayer systems, with various functional polymers serving as the top stiff layers. The high photon-to-thermal energy conversion of CNT leads to the NIR-controlled thermal expansion of the elastic CNT-PDMS substrate, resulting in dynamic regulation of the applied strain (ε) of the bilayer system by the NIR on/off cycle to obtain a reversible wrinkle pattern. The switchable surface topological structures can transfer between the wrinkled state and the wrinkle-free state within tens of seconds via NIR irradiation. As a proof-of-concept application, this type of NIR-driven dynamic wrinkle pattern was used in smart displays, dynamic gratings, and light control electronics.

Switchable bumps of a bead-embedded elastomer surface with variable adhesion

An extremely simple structural design of a composite material composed of an elastomer sheet and hard beads embedded at the surface is proposed to realize a shape-tunable surface; it reversibly forms bumps/undulations in response to in-plane tensile strain applied to the surface. Tribological properties such as adhesion therefore become switchable.

A micropatterned elastomeric surface with enhanced frictional properties under wet conditions and its application

Engineered surfaces that have high friction under wet or lubricated conditions are important in many practical applications. However, it is not easy to achieve stable high friction under wet conditions because a layer of fluid prevents direct solid-solid contact. Here, we report a micropatterned elastomeric surface with superior wet friction. The surface has unique arch-shaped microstructures arrayed in a circle on the surface to provide high friction on wet or flooded surfaces. The arch-shaped micropatterned surface exhibits remarkably enhanced and stable friction under wet conditions, surpassing even the performance of the hexagonal patterns of tree frogs, owing to the large contact surface and the optimal shape of drainage channels. Robotic substrate transportation systems equipped with the micropatterned surfaces can manipulate a delicate wet substrate without any sliding in a highly stable and reproducible manner, demonstrating the superior frictional capabilities of the surface under wet conditions.

Smart wearable Kevlar-based safeguarding electronic textile with excellent sensing performance

A novel S-ST/MWCNT/Kevlar-based wearable electronic textile (WET) with enhanced safeguarding performance and force sensing ability was fabricated. Stab resistance performance tests under quasi-static and dynamic conditions show that the maximum resistance force and penetration impact energy for the WET are 18 N and 11.76 J, which represent a 90% and 50% increment with respect to the neat Kevlar, respectively. Dynamic impact resistance tests show that the WET absorbs all the impact energy. The maximum resistance force of the WET is 1052 N, which represents an improvement of about 190% with respect to neat Kevlar. With the incorporation of multi-walled carbon nanotubes (MWCNTs), the WET can achieve a stable electrical conductivity of ∼10^-2 S m^-1, and the conductivity is highly sensitive to external mechanic forces. Notably, the sensing fabric also exhibits an outstanding ability to detect and analyze external forces. In addition, it can be fixed at any position of the human body and exhibits an ideal monitoring performance. Because of its flexibility, high sensitivity to various types of deformations and excellent safeguarding performance, the WET has a strong potential for wearable monitoring devices that simultaneously provide body protection and monitor the movements of the human body under various conditions.