Superhydrophobic surfaces from hierarchically structured wrinkled polymers

Simple Method for Creating Hydrophobic Ultraflexible Photovoltaics

Optoelectronic devices, such as photodetectors and photovoltaics, are susceptible to surface contamination or water damage that can lead to reductions in performance or stability. Applying superhydrophobic coatings to these devices can introduce self-cleaning behavior and water resistance to extend their lifetime and improve their efficiency. However, existing methods for inducing superhydrophobicity have not been compatible with ultraflexible devices because of their thickness and complexity requirements. In this work, we introduce a procedure for inducing superhydrophobic and self-cleaning behavior on ultraflexible components using a combination of shrinkage-induced wrinkles and a low-surface-energy coating. We apply these techniques to an ultraflexible organic photovoltaics and demonstrate excellent hydrophobicity and self-cleaning behavior.

Smart Polymer Surfaces with Complex Wrinkled Patterns: Reversible, Non-Planar, Gradient, and Hierarchical Structures

This review summarizes the relevant developments in preparing wrinkled structures with variable characteristics. These include the formation of smart interfaces with reversible wrinkle formation, the construction of wrinkles in non-planar supports, or, more interestingly, the development of complex hierarchically structured wrinkled patterns. Smart wrinkled surfaces obtained using light-responsive, pH-responsive, temperature-responsive, and electromagnetic-responsive polymers are thoroughly described. These systems control the formation of wrinkles in particular surface positions and the reversible construction of planar-wrinkled surfaces. This know-how of non-planar substrates has been recently extended to other structures, thus forming wrinkled patterns on solid, hollow spheres, cylinders, and cylindrical tubes. Finally, this bibliographic analysis also presents some illustrative examples of the potential of wrinkle formation to create more complex patterns, including gradient structures and hierarchically multiscale-ordered wrinkles. The orientation and the wrinkle characteristics (amplitude and period) can also be modulated according to the requested application.

Supramolecular Engineering of Crystalline Fullerene Micro-/Nano-Architectures

Fullerenes are a molecular form of carbon allotrope and bear certain solubility, which allow the supramolecular assembly of fullerene molecules-also together with other complementary compound classes-via solution-based wet processes. By well-programmed organizing these building blocks and precisely modulating over the assembly process, supramolecularly assembled fullerene micro-/nano-architectures (FMNAs) are obtained. These FMNAs exhibit remarkably enhanced functions as well as tunable morphologies and dimensions at different size scales, leading to their applications in diverse fields. In this review, both traditional and newly developed assembly strategies are reviewed, with an emphasis on the morphological evolution mechanism of FMNAs. The discussion is then focused on how to precisely regulate the dimensions and morphologies to generate functional FMNAs through solvent engineering, co-crystallization, surfactant incorporation, or post-fabrication treatment. In addition to C₆₀ -based FMNAs, this review particularly focuses on recently fabricated FMNAs comprising higher fullerenes (e.g., C₇₀ ) and metallofullerenes. Meanwhile, an overview of the property modulation is presented and multidisciplinary applications of FMNAs in various fields are summarized, including sensors, optoelectronics, biomedicines, and energy. At the end, the prospects for future research, application opportunities, and challenges associated with FMNAs are proposed.

Growth-Induced Wrinkles and Dotlike Patterns of a Swollen Fluoroalkylated Thin Film by the Reaction of Surface-Attached Polymethylhydrosiloxane

The design of hydrophobic surfaces requires a material which has a low solid surface tension and a simple fabrication process for anchoring and controlling the surface morphology. A generic method for the spontaneous formation of robust instability patterns is proposed through the hydrosilylation of a fluoroalkene bearing dangling chains, R_f = C₆F₁₃(CH₂)₃-, with a soft polymethylhydrosiloxane (PMHS) spin-coated gel polymer (0.8 μm thick) using Karstedt catalyst. These patterns were easily formed by an irreversible swelling reaction due to the attachment of a layer to various substrates. The buckling instability was created by two different approaches for a gel layer bound to a rigid silicon wafer substrate (A) and to a soft nonswelling silicone elastomer foundation (B). The observations of grafted R_f-PMHS films in the swollen state by microscopy revealed two distinct permanent patterns on various substrates: dotlike of wavelength λ = 0.4-0.7 μm (A) or wrinkle of wavelength λ = 4-7 μm (B). The elastic moduli ratios of film/substrate were determined using PeakForce quantitative nanomechanical mapping. The characteristic wavelengths (λ) of the patterns for systems A and B were quantitatively estimated in relation to the thickness of the top layer. A diversity of wrinkle morphologies can be achieved by grafting different side chains on pristine PMHS films. The water contact angle (WCA) hysteresis of fluorinated chain (R_f) was enhanced upon roughening the surfaces, giving highly hydrophobic surface properties for water with static/hysteresis WCAs of 136°/74° in the resulting wrinkle (B) and 119°/41° in the dotlike of lower roughness (A). The hydrophobic properties of grafted films on A with various mixtures of hexyl/fluoroalkyl chains were characterized by static CA: WCA 104-119°, ethylene glycol CA 80-96°, and n-hexadecane CA 17-61°. A very low surface energy of 15 mN/m for R_f-PMHS was found on the smoother dotlike pattern.

Simple Microcontact Printing Technique to Obtain Cell Patterns by Lithography Using Grayscale, Photopolymer Flexographic Mold, and PDMS

Microcontact printing using PDMS embossing tools and its variations have aroused the interest of a wide spectrum of research fields, hence the feasibility of defining micro and nanoscale patterns. In this work, we have proposed and demonstrated a novel lithography method based on grayscale patterns printed in a flexographic photopolymer mold and transferred to epoxy resin and a single PDMS stamp to obtain different microprint pattern structures. The geometry of the patterns can be modified by adjusting the layout and grayscale of the stamp patterns. The functionality of this contact printing methodology was validated by generating human induced pluripotent stem cells (hiPSC) patterns. These specific micropatterns can be very useful for achieving complex differentiation in cell lines such as hiPSC. Microfabrication through the new technique provides a promising alternative to conventional lithography for constructing complex aligned surfaces; these structures could be used as components of biological patterns or microfluidic devices.

Surface Wrinkling for Flexible and Stretchable Sensors

Recent advances in nanolithography, miniaturization, and material science, along with developments in wearable electronics, are pushing the frontiers of sensor technology into the large-scale fabrication of highly sensitive, flexible, stretchable, and multimodal detection systems. Various strategies, including surface engineering, have been developed to control the electrical and mechanical characteristics of sensors. In particular, surface wrinkling provides an effective alternative for improving both the sensing performance and mechanical deformability of flexible and stretchable sensors by releasing interfacial stress, preventing electrical failure, and enlarging surface areas. In this study, recent developments in the fabrication strategies of wrinkling structures for sensor applications are discussed. The fundamental mechanics, geometry control strategies, and various fabricating methods for wrinkling patterns are summarized. Furthermore, the current state of wrinkling approaches and their impacts on the development of various types of sensors, including strain, pressure, temperature, chemical, photodetectors, and multimodal sensors, are reviewed. Finally, existing wrinkling approaches, designs, and sensing strategies are extrapolated into future applications.

Laser-Induced Fast Assembly of Wettability-Finely-Tunable Superhydrophobic Surfaces for Lossless Droplet Transfer

Rose-petal-like superhydrophobic surfaces with strong water adhesion are promising for microdroplet manipulation and lossless droplet transfer. Assembly of self-grown micropillars on shape-memory polymer sheets with their surface adhesion finely tunable was enabled using a picosecond laser microprocessing system in a simple, fast, and large-scale manner. The processing speed of the wettability-finely-tunable superhydrophobic surfaces is up to 0.5 cm²/min, around 50-100 times faster than the conventional lithography methods. By adjusting the micropillar height, diameter, and bending angle, as well as superhydrophobic chemical treatment, the contact angle and adhesive force of water droplets on the micropillar-textured surfaces can be tuned from 117.1° up to 165° and 15.4 up to 200.6 μN, respectively. Theoretical analysis suggests a well-defined wetting-state transition with respect to the micropillar size and provides a clear guideline for microstructure design for achieving a stabilized superhydrophobic region. Droplet handling devices, including liquid handling tweezers and gloves, were fabricated from the micropillar-textured surfaces, and lossless liquid transfer of various liquids among various surfaces was demonstrated using these devices. The superhydrophobic surfaces serve as a microreactor platform to perform and reveal the chemical reaction process under a space-constrained condition. The superhydrophobic surfaces with self-assembled micropillars promise great potential in the fields of lossless droplet transfer, biomedical detection, chemical engineering, and microfluidics.

Advances in the development of superhydrophobic and icephobic surfaces

Superhydrophobicity and icephobicity are governed by surface chemistry and surface structure. These two features signify a potential advance in surface engineering and have recently garnered significant attention from the research community. This review aims to simulate further research in the development of superhydrophobic and icephobic surfaces in order to achieve their wide-spread adoption in practical applications. The review begins by establishing the fundamentals of the wetting phenomenon and wettability parameters. This is followed by the recent advances in modeling and simulations of the response of superhydrophobic surfaces to static and dynamic droplets contact and impingement, respectively. In view of their versatility and multifunctionality, a special attention is given to the development of these surfaces using nanocomposites. Furthermore, the review considers advances in icephobicity, its comprehensive characterization and its relation to superhydrophobicity. The review also includes the importance of the use of superhydrophobic surface to combat viral and bacterial contamination that exist in fomites.

Altering the Surface Properties of Metal Alloys Utilizing Facile and Ecological Methods

The development of a superhydrophobic and, even, water-repellent metal alloy surface is reported utilizing a simple, fast, and economical way that requires minimum demands on the necessary equipment and/or methods used. The procedure involves an initial irradiation of the metallic specimen using a femtosecond laser, which results in a randomly roughened surface, that is subsequently followed by placing the item in an environment under moderate vacuum (pressure 10^-2 mbar) and/or under low-temperature heating (at temperatures below 120 °C). The effects of both temperature and low pressure on the surface properties (water contact angle and contact angle hysteresis) are investigated and surfaces with similar superhydrophobicity are obtained in both cases; however, a significant difference concerning their water-repellent ability is obtained. The surfaces that remained under vacuum were water-repellent, exhibiting very high values of contact angle with a very low contact angle hysteresis, whereas the surfaces, which underwent thermal processing, exhibited superhydrophobicity with high water adhesion, where water droplets did not roll off even after a significant inclination of the surface. The kinetics of the development of superhydrophobic behavior was investigated as well. The findings were understood when the surface roughness characteristics were considered together with the chemical composition of the surface.

Template-assisted self-assembly of achiral plasmonic nanoparticles into chiral structures

The acquisition of strong chiroptical activity has revolutionized the field of plasmonics, granting access to novel light-matter interactions and revitalizing research on both the synthesis and application of nanostructures. Among the different mechanisms for the origin of chiroptical properties in colloidal plasmonic systems, the self-assembly of achiral nanoparticles into optically active materials offers a versatile route to control the structure-optical activity relationships of nanostructures, while simplifying the engineering of their chiral geometries. Such unconventional materials include helical structures with a precisely defined morphology, as well as large scale, deformable substrates that can leverage the potential of periodic patterns. Some promising templates with helical structural motifs like liquid crystal phases or confined block co-polymers still need efficient strategies to direct preferential handedness, whereas other templates such as silica nanohelices can be grown in an enantiomeric form. Both types of chiral structures are reviewed herein as platforms for chiral sensing: patterned substrates can readily incorporate analytes, while helical assemblies can form around structures of interest, like amyloid protein aggregates. Looking ahead, current knowledge and precedents point toward the incorporation of semiconductor emitters into plasmonic systems with chiral effects, which can lead to plasmonic-excitonic effects and the generation of circularly polarized photoluminescence.

Recent Progress of Bioinspired Scalephobic Surfaces with Specific Barrier Layers

Bioinspired superwettable surfaces have been widely harnessed in diverse applications such as self-cleaning, oil/water separation, and liquid transport. So far, only a little work is focused on scalephobic capability of those superwettable surfaces. However, the troublesome scale deposition will inevitably be observed in our daily production and life, greatly reducing heat transfer efficiency and inhibiting the liquid transport. To address this annoying problem, as the emerging strategy, specific barrier layers are introduced onto superwettable surfaces to reduce or even avoid the direct contact between scale and the surfaces. In this feature article, we first provide the basic concept of bioinspired scalephobic surfaces with specific barrier layers. Then, we briefly introduce the typical fabrication methods of scalephobic surfaces. Later, we summarize recent progress of bioinspired scalephobic surfaces with specific barrier layers. Furthermore, we point out the guiding theory and criteria for the stability of barrier layers. Finally, we put forward the forecast on the existing problems and future direction in bioinspired scalephobic surfaces.

Laser Direct Writing via Two-Photon Polymerization of 3D Hierarchical Structures with Cells-Antiadhesive Properties

We report the design and fabrication by laser direct writing via two photons polymerization of innovative hierarchical structures with cell-repellency capability. The structures were designed in the shape of “mushrooms”, consisting of an underside (mushroom’s leg) acting as a support structure and a top side (mushroom’s hat) decorated with micro- and nanostructures. A ripple-like pattern was created on top of the mushrooms, over length scales ranging from several µm (microstructured mushroom-like pillars, MMP) to tens of nm (nanostructured mushroom-like pillars, NMP). The MMP and NMP structures were hydrophobic, with contact angles of (127 ± 2)° and (128 ± 4)°, respectively, whereas flat polymer surfaces were hydrophilic, with a contact angle of (43 ± 1)°. The cell attachment on NMP structures was reduced by 55% as compared to the controls, whereas for the MMP, a reduction of only 21% was observed. Moreover, the MMP structures preserved the native spindle-like with phyllopodia cellular shape, whereas the cells from NMP structures showed a round shape and absence of phyllopodia. Overall, the NMP structures were more effective in impeding the cellular attachment and affected the cell shape to a greater extent than the MMP structures. The influence of the wettability on cell adhesion and shape was less important, the cellular behavior being mainly governed by structures’ topography.

The effects of bio-inspired micro/nano scale structures on anti-icing properties

Ice formation and accumulation have detrimental effects on commercial surfaces and people's lives. The ice adhesion strength decreases with increasing surface hydrophobicity, and the superhydrophobicity of a surface can be constructed by a combination of low surface free energy and high surface roughness. Conversely, the characteristics of biological surfaces have aroused wide attention as a result of the superhydrophobicity of plants and animals, deriving from the synergistic effects of chemical compositions and multi-scale hierarchical structures. Therefore, inspired by bio-mimetic studies on biological surfaces, a lot of artificial bio-inspired superhydrophobic surfaces have been broadly designed and constructed. Herein, we aim to summarize the fundamental theories of surface wettability and recent progress in the fabrication of bio-inspired surfaces. The bio-inspired surfaces prepared by different facile methods not only have superhydrophobicity, but also have anti-icing/icephobic properties. In the end, some challenges and problems in the future study and advancement of bio-inspired superhydrophobic surfaces are proposed.

Bio-Inspired Design of Bi/Tridirectionally Anisotropic Sliding Superhydrophobic Titanium Alloy Surfaces

Many biological surfaces with the multi-scale microstructure show obvious anisotropic wetting characteristics, which have many potential applications in microfluidic systems, biomedicine, and biological excitation systems. However, it is still a challenge to accurately prepare a metal microstructured surface with multidirectional anisotropy using a simple but effective method. In this paper, inspired by the microstructures of rice leaves and butterfly wings, wire electrical discharge machining was used to build dual-level (submillimeter/micrometer) periodic groove structures on the surface of titanium alloy, and then a nanometer structure was obtained after alkali-hydrothermal reaction, forming a three-level (submillimeter/micrometer/nanometer) structure. The surface shows the obvious difference of bidirectional superhydrophobic and tridirectional anisotropic sliding after modification, and the special wettability is easily adjusted by changing the spacing and angle of the inclined groove. In addition, the results indicate that the ability of water droplets to spread along parallel and perpendicular directions on the submillimeter groove structure and the different resistances generated by the inclined groove surface are the main reasons for the multi-anisotropic wettability. The research gives insights into the potential applications of metal materials with multidirectional anisotropic wetting properties.

Preparation and Characterization of Tris(trimethylsiloxy)silyl Modified Polyurethane Acrylates and Their Application in Textile Treatment

Three series of silicone modified polyurethane acrylate (SPUA) prepolymers were prepared from dicyclohexylmethane-4, 4'-diisocyanate (HMDI), PPG1000, triethylene glycol (TEG), 2-hydroxyethyl acrylate (HEA), and multi-hydroxyalkyl silicone (MI-III) with tris(trimethylsiloxy)silyl propyl side groups. Their structures were confirmed by 1H NMR, 13C NMR, and Fourier transformed infrared (FTIR) analysis, and SPUA films were obtained by UV curing. The properties of films were investigated by attenuated total reflection (ATR)-FTIR, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), water contact angle (WCA), thermogravimetric analysis (TGA), differential scanning calorimeter (DSC), water and hexane resistance, and tensile testing. The results showed that the structures and dosages of MI-III could influence the polymerization properties, surface properties, water and n-hexane resistance, and thermal and tensile properties of SPUA. For instance, the surface aggregation of tris(trimethylsiloxy)silyl propyl groups (even ~2.5 wt%) could endow SPUA films with less microphase separation, good hydrophobicity, lipophilicity, thermal stability, and mechanical properties. Interestingly, obvious regular winkles appeared on the surfaces of SPUAIII films, which are characterized by relatively high WCA values. However, relatively smooth were observed on the surfaces of SPUAIII films, which also exhibit lower water absorption ratio values. Furthermore, the ordinary cotton textiles would be transformed into hydrophobic and oleophilic textiles after treating with SPUA simply, and they were used in the oil/water separation study. Among them, consistent with water and hexane resistance analysis of SPUA films, SPUAII treated cotton textiles are characterized by relatively small liquid absorption capacity (LAC) values. Thus, phenyl groups and side-chain tris(trimethylsiloxy)silyl propyl groups are helpful to improve the hydrophobicity and lipophilicity of SPUA films. SPUAII-5 (even with 5 wt% MII) treated cotton textiles could efficiently separate the oil/water mixture, such as n-hexane, cyclohexane, or methylbenzene with water. Thus, this material has great potential in the application of hydrophobic treatment, oil/water separation, and industrial sewage emissions, among others.

Surface Chemical Functionalization of Wrinkled Thiol-ene Elastomers for Promoting Cellular Alignment

Wrinkled polymer surfaces find broad applicability; however, the polymer substrates are often limited to poly(dimethylsiloxane) (PDMS), which limits spatial control over wrinkle features and surface chemistry. An approach to surface functionalization of wrinkled elastomer substrates is demonstrated through versatile, multistep thiol-ene click chemistry. The elastomer is formed using a thiol-Michael reaction of tetrathiol with excess diacrylates while wrinkle formation is induced through a second free radical UV polymerization of the acrylates on the surface of the elastomer. Due to oxygen inhibition of the free radical polymerization, pendant acrylates at the surface remain unreacted and are subsequently functionalized with a multi-functional thiol, which can be further reacted through a number of thiol-X 'click' reactions. As a demonstration, these thiol surfaces are further modified to either promote cell adhesion of human mesenchymal stem cells (hMSCs) through coupling with RGDS-containing peptides or surface passivation through reaction with hydrophilic hydroxyl ethyl acrylate moieties. Through engineering a combination of surface chemistry and surface topography, hMSCs exhibited increased spreading and cell density on RGDS-functionalized surfaces and a two-fold increase in cell alignment when cultured on wrinkled substrates. Gradient functionalized surfaces created by tuning the wrinkle wavelength with UV irradiation enabled rapid screening of the effect of topography on the hMSCs. Further, this novel application of click chemistry enables simultaneous tuning of wrinkle topology and surface chemistry towards targeted material applications.

Anti-Reflective Coating Materials: A Holistic Review from PV Perspective

The solar photovoltaic (PV) cell is a prominent energy harvesting device that reduces the strain in the conventional energy generation approach and endorses the prospectiveness of renewable energy. Thus, the exploration in this ever-green field is worth the effort. From the power conversion efficiency standpoint of view, PVs are consistently improving, and when analyzing the potential areas that can be advanced, more and more exciting challenges are encountered. One such crucial challenge is to increase the photon availability for PV conversion. This challenge is solved using two ways. First, by suppressing the reflection at the interface of the solar cell, and the other way is to enhance the optical pathlength inside the cell for adequate absorption of the photons. Our review addresses this challenge by emphasizing the various strategies that aid in trapping the light in the solar cells. These strategies include the usage of antireflection coatings (ARCs) and light-trapping structures. The primary focus of this study is to review the ARCs from a PV application perspective based on various materials, and it highlights the development of ARCs from more than the past three decades covering the structure, fabrication techniques, optical performance, features, and research potential of ARCs reported. More importantly, various ARCs researched with different classes of PV cells, and their impact on its efficiency is given a special attention. To enhance the optical pathlength, and thus the absorption in solar PV devices, an insight about the advanced light-trapping techniques that deals with the concept of plasmonics, spectral modification, and other prevailing innovative light-trapping structures approaching the Yablonovitch limit is discussed. An extensive collection of information is presented as tables under each core review section. Further, we take a step forward to brief the effects of ageing on ARCs and their influence on the device performance. Finally, we summarize the review of ARCs on the basis of structures, materials, optical performance, multifunctionality, stability, and cost-effectiveness along with a master table comparing the selected high-performance ARCs with perfect AR coatings. Also, from the discussed significant challenges faced by ARCs and future outlook; this work directs the researchers to identify the area of expertise where further research analysis is needed in near future.

Bioinspired Multiscale Wrinkling Patterns on Curved Substrates: An Overview

The surface wrinkling of biological tissues is ubiquitous in nature. Accumulating evidence suggests that the mechanical force plays a significant role in shaping the biological morphologies. Controlled wrinkling has been demonstrated to be able to spontaneously form rich multiscale patterns, on either planar or curved surfaces. The surface wrinkling on planar substrates has been investigated thoroughly during the past decades. However, most wrinkling morphologies in nature are based on the curved biological surfaces and the research of controllable patterning on curved substrates still remains weak. The study of wrinkling on curved substrates is critical for understanding the biological growth, developing three-dimensional (3D) or four-dimensional (4D) fabrication techniques, and creating novel topographic patterns. In this review, fundamental wrinkling mechanics and recent advances in both fabrications and applications of the wrinkling patterns on curved substrates are summarized. The mechanics behind the wrinkles is compared between the planar and the curved cases. Beyond the film thickness, modulus ratio, and mismatch strain, the substrate curvature is one more significant parameter controlling the surface wrinkling. Curved substrates can be both solid and hollow with various 3D geometries across multiple length scales. Up to date, the wrinkling morphologies on solid/hollow core-shell spheres and cylinders have been simulated and selectively produced. Emerging applications of the curved topographic patterns have been found in smart wetting surfaces, cell culture interfaces, healthcare materials, and actuators, which may accelerate the development of artificial organs, stimuli-responsive devices, and micro/nano fabrications with higher dimensions.

Drawing High-Definition and Reversible Hydrogel Paintings with Grayscale Exposure

High-definition and arbitrary grayscale hydrogel paintings that can appear reversibly with hydration/dehydration are realized through spatially grayscale exposure. Spatio-temporally grayscale images are used to guide the exposure of a hydrogel processor to dictate the gradient cross-linking density spatially, which thereafter results in the heterogeneity of hydrogels in swelling ratio, mechanical properties, and especially visible light transmittance, leading to swelling-induced patterns by gradient and local visible light scattering difference based on tunable mesh size and microphase separation. The resultant grayscale hydrogel patterns, with visible light transmittance adjustable, are reversible during hydration (in 1-2 s) and dehydration and possess the feature size of 70 μm and more pattern information compared with previous hydrogel patterning. Uniquely, the patterns can be realized not only on the outmost surface of hydrogels as usual but inside. Combining the unique grayscale exposure with three-dimensional printing technology, arbitrary hydrogel patterns that have plenty of details and even vary at different layers are fabricated readily, indicating its broad potential in smart anti-counterfeiting of security field, mechanics, engineering, and many others.

A Highly Sensitive and Stretchable Yarn Strain Sensor for Human Motion Tracking Utilizing a Wrinkle-Assisted Crack Structure

With the booming development of flexible electronics, the need for a multifunctional and high-performance strain sensor has become increasingly important. Although significant progress has been made in designing new microstructures with sensing capabilities, the tradeoff between sensitivity and workable strain range has prevented the development of a strain sensor that is both highly sensitive and also stretchable. Here, a wrinkle-assisted crack microstructure is designed and fabricated via prestretching the multiwalled carbon nanotubes ink (CNTs ink)/polyurethane yarn (PU yarn). This designed structure originates from the mismatch in Young's modulus and elasticity between the CNTs ink and PU yarn during the stretching process. The structure endows the sensor with combined characteristics of a high sensitivity toward stretching strain (gauge factor of 1344.1 at 200% strain), an ultralow limit of detection (<0.1% strain), excellent durability (>10 000 cycles), a wide workable strain range (0-200%), and outstanding response and stability toward bending deformation. This high-performance strain sensor will see widespread improved performance across applications such as intelligent fabrics, electrical skins, and fatigue detection for full-range human motion monitoring.

Designing Hierarchical Nanostructures from Conformable and Deformable Thin Materials

This Perspective focuses on the design of hierarchical structures in deformable thin materials by patterning mechanical instabilities. Fabrication of three-dimensional (3D) structures with multiple length scales-starting at the nanoscale-can result in on-demand surface functionalities from the modification of the mechanical, chemical, and optical properties of materials. Conventional top-down lithography, however, cannot achieve 3D patterns over large areas (>cm²). In contrast, a bottom-up approach based on controlling strain in layered nanomaterials conformally coated on polymeric substrates can produce multiscale structures in parallel. In-plane and out-of-plane structural hierarchies formed by conformal buckling show unique structure-function relationships. Programmable hierarchical surfaces offer prospects to tune global- and local-level characteristics of nanomaterials that will positively impact applications in nanomechanics, nanoelectronics, and nanophotonics.

Influence of surface topography attributes on settlement and adhesion of natural and synthetic species

Surface topographies of various sizes, shapes, and spatial organization abound in nature. They endow properties such as super-hydrophobicity, reversible adhesion, anti-fouling, self-cleaning, anti-glare, and anti-bacterial, just to mention a few. Researchers have long attempted to replicate these structures to create artificial surfaces with the functionalities found in nature. In this review, we decompose the attributes of surface topographies into their constituents, namely feature dimensions, geometry, and stiffness, and examine how they contribute (individually or collectively) to settlement and adhesion of natural organisms and synthetic particles on the surface. The size of features that comprise the topography affects the contact area between the particle and surface as well as its adhesion and contributes to the observed adsorptive properties of the surface. The geometry of surface perturbations can also affect the contact area and gives rise to anisotropic particle settlement. Surface topography also affects the local stiffness of the surface and governs the adhesion strength on the surface. Overall, systematically studying attributes of surface topography and elucidating how each of them affects adhesion and settlement of particles will facilitate the design of topographically-corrugated surfaces with desired adsorption characteristics.

Simple and Affordable Way To Achieve Polymeric Superhydrophobic Surfaces with Biomimetic Hierarchical Roughness

A water contact angle greater than 150° together with a sliding angle less than 10° is a special surface phenomenon that appears on superhydrophobic surfaces. In this paper, a brief introduction of the development history and present research on superhydrophobic surfaces was given. Polymeric superhydrophobic surfaces with biomimetic hierarchical roughness were fabricated by a simple method of hot embossing without any chemical treatments. Stainless steel meshes with different mesh numbers were used as template. Moreover, the influences of processing parameters, including mesh number, mold temperature, and pressure, were deeply investigated. Hierarchical microplatforms, microfibers, and oriented arrayed nanowrinkles structure on them, which were resembled with the nanowrinkles structure and hierarchical roughness on a red rose petal, were observed by a scanning electron microscope. A water contact angle of 154° can be achieved after parameter optimization. The method proposed in this study offered a fine and affordable choice for the fabrication of polymeric superhydrophobic surfaces. With the rapid development of functional applications in micro- and nanodevices, this method will show greater superiority in large-area and large-scale production due to its advantages of low cost, high efficiency, and high reliability.

Fabrication of sub-20 nm patterns using dopamine chemistry in self-aligned double patterning

A self-aligned double patterning approach using a dopamine chemistry-inspired coating technique has been developed for the fabrication of sub-20 nm patterns. Poly(methyl methacrylate) (PMMA) films were patterned by nanoimprint lithography to form relief features. A thin layer of polydopamine (PDA) was conformally deposited on the surface of the PMMA pattern sidewalls to form a spacer layer. After etching the surface of the PDA layer from the horizontal surfaces and subsequently removing the PMMA template, free-standing PDA sidewall patterns remained that essentially doubled the original PMMA pattern density with decreased feature dimensions as compared to the initial PMMA template structures. The critical dimension of the PDA patterns can be tuned to ∼20 nm by controlling the PDA deposition conditions and further reduced to ∼13 nm by thermal carbonization of the PDA. Both simple lines and more complex rhombic ring features were fabricated by this technique to demonstrate its capacity for replicating arbitrary patterns. This work represents a simple and scalable strategy for preparing well-defined nanostructures with feature sizes usually only accessible via complex leading edge lithographic methods.

Contactless Control of Local Surface Buckling in Photoaligned Gold/Liquid Crystal Polymer Bilayers

Wrinkling is a powerful technique for the preparation of surface structures over large areas, but it is difficult to simultaneously control the direction, period, and amplitude of the wrinkles without resorting to complicated procedures. In this work, we demonstrate a wrinkling system consisting of a liquid crystal polymer network and a thin layer of gold, in which the direction of the wrinkles is controlled by the alignment of the liquid crystal molecules and the average amplitude and period are controlled by a high-intensity UV irradiation. The UV exposure represses the amplitude and period dictated by the total exposure. Using photoalignment and photomasks, we demonstrate an unprecedented control over the wrinkling parameters and were able to generate some striking optical patterns. The mechanism of the wrinkle suppression was investigated and appears to involve localized photodegradation at the polymer-gold interface, possibly due to the formation of mechanoradicals.

Reversible Surface Patterning by Dynamic Crosslink Gradients: Controlling Buckling in 2D

Harnessing the self-organization of soft materials to make complex, well-ordered surface patterns in a noninvasive manner is challenging. The wrinkling of thin films provides a compelling strategy to achieve this. Despite much attention, however, a simple, single-step, reversible method that gives rise to controlled, two-dimensional (2D) ordered, continuous, and discontinuous patterns has proven to be elusive. Here a novel, robust method is described to achieve this using an ultraviolet-light-sensitive anthracene-containing polymer thin film. The origin of the patterns is the local buckling of the thin film, where the control over the topology is given by laterally patterning out-of-plane gradients in the crosslink density of the film. The underlying buckling mechanics and formation of the surface features are well-described by finite element analysis. By illuminating the film with a photomask, local and long-range patterns that can be both continuous and discontinuous are able to be written. Furthermore, the patterning is fully reversible over multiple cycles. The results demonstrate a simple strategy for erasable storage of information in a surface topography that has applications in memory, anticounterfeiting, and plasmonics.

Small degree of anisotropic wetting on self-similar hierarchical wrinkled surfaces

We studied the wetting behavior of multiscale self-similar hierarchical wrinkled surfaces. The hierarchical surface was fabricated on poly(dimethylsiloxane) (PDMS) substrates by manipulating the sequential strain release and combined plasma/ultraviolet ozone (UVO) treatment. The generated structured surface shows an independently controlled dual-scale roughness with level-1 small-wavelength wrinkles (wavelength of 700-1500 nm and amplitude of 50-500 nm) resting on level-2 large-wavelength wrinkles (wavelength of 15-35 μm and amplitude of 3.5-5 μm), as well as accompanying orthogonal cracks. By tuning the aspect ratio of hierarchical wrinkles, the degree of wetting anisotropy in hierarchical wrinkled surfaces, defined as the contact angle difference between the parallel and perpendicular directions to the wrinkle grooves, is found to change between 3° and 9°. Through both experimental characterization (confocal fluorescence imaging) and theoretical analyses, we showed that the wetting state in the hierarchical wrinkled surface is in the Wenzel wetting state. We found that the measured apparent contact angle is larger than the theoretically predicted Wenzel contact angle, which is found to be attributed to the three-phase contact line pinning effect of both wrinkles and cracks that generates energetic barriers during the contact line motion. This is evidenced by the observed sudden drop of over 20° in the static contact angles along both perpendicular and parallel directions after slight vibration perturbation. Finally, we concluded that the observed small degree of wetting anisotropy in the hierarchical wrinkled surfaces mainly arises from the competition between orthogonal wrinkles and cracks in the contact line pinning.

Buckling Instabilities in Polymer Brush Surfaces via Postpolymerization Modification

We report a simple route to engineer ultrathin polymer brush surfaces with wrinkled morphologies using post-polymerization modification (PPM), where the length scale of the buckled features can be tuned from hundreds of nanometers to one micrometer using PPM reaction time. We show that partial crosslinking of the outer layer of the polymer brush under poor solvent conditions is critical to obtain wrinkled morphologies upon swelling. Characterization of the PPM kinetics and swelling behavior via ellipsometry and the through-thickness composition profile via time-of-flight secondary ion mass spectroscopy (ToF-SIMS) provided keys insight into parameters influencing the buckling behavior.

Wood-mimetic skins prepared using horseradish peroxidase catalysis to induce surface wrinkling of chitosan film upon drying

We previously developed bio-based wrinkled surfaces induced by wood-mimetic skins upon drying in which microscopic wrinkles were fabricated on a chitosan (CS) film by immersing it in a phenolic acid solution, followed by horseradish peroxidase (HRP)-catalyzed surface reaction and drying. However, the detailed structure of the resulting wood-mimetic skins, including crosslinking mode and thickness, has not been clarified due to the difficulty of the analysis. Here, we prepare wrinkled films using ferulic acid (FE), vanillic acid (VA), and homovanillic acid (HO) and characterize their structures to clarify the unknown characteristics of wood-mimetic skin. Chemical and structural analyses of wood-mimetic skins prepared using VA and HO indicate that the crosslinking structure in the skin is composed of ionic bonds between CS and an oligophenolic residue generated by the HRP-catalyzed reaction on the CS surface. Moreover, the quantity of these ionic bonds is related to the skin hardness and wrinkle size. Finally, SEM and TOF-SIMS analyses indicate that the skin thickness is on the submicron order (<200nm).

Multidirectional Wrinkle Patterns Programmed by Sequential Uniaxial Strain with Conformal yet Nontraceable Masks

Surface wrinkling is a promising route to control the mechanical, electrical, and optical properties of materials in a wide range of applications. However, previous artificial wrinkles are restricted to single or random orientation and lacks selectivity. To address this challenge, this study presents multidirectional wrinkle patterns with high selectivity and orientation through sequential uniaxial strain with conformal polymeric shadow masks. The conformal but nontraceable polymeric stencil with microapertures are adhered to a flat substrate prior to oxidation, which forms discrete and parallel wrinkles in confined domains without any contamination. By fully investigating the process, this study displays compound topography of wrinkles consisting of wrinkle islands and surrounding secondary wrinkles on the same surface. With this topography, various diffusion properties are presented: from semi-transparent yet diffusive films to multidirectional diffusive films, which will be available for new types of optical diffuser applications.

Self-similar Hierarchical Wrinkles as a Potential Multifunctional Smart Window with Simultaneously Tunable Transparency, Structural Color, and Droplet Transport

Smart window has immense potential for energy savings in architectural and vehicular applications, while most studies focus on the tunability of a single property of optical transmittance. Here we explore harnessing dynamically tunable hierarchical wrinkles for design of a potential multifunctional smart window with combined structural color and water droplet transport control. The self-similar hierarchical wrinkles with both nanoscale and microscale features are generated on a prestrained poly(dimethylsiloxane) elastomer through sequential strain release and multistep oxygen plasma treatment. We show that the hierarchically wrinkled elastomer displays both opaqueness and iridescent structural color. We find that restretching/releasing the elastomer leads to the reversible and repeatable switch from opaqueness to transparency, arising from the flattening of large wrinkles (micrometer scale), while a nonvanishing structural color occurs due to the nondisappearing small wrinkles (nanoscale). The unique features of combined reversible large wrinkles and irreversible small wrinkles during hierarchical wrinkling are well reproduced by corresponding finite element simulation. The criteria for generating self-similar hierarchical wrinkles is revealed through a simplified theoretical model and validated by experiments. In addition to its tunable optical property, we further show its ability in control of water droplet transport on demand through mechanical stretching and release. We find that an initially pinned water droplet on the tilted hierarchically wrinkled surface starts to slide when the surface is stretched, and becomes pinned again upon strain release. Such a process is reversible and repeatable. The hierarchically wrinkled surface could find broad potential applications not only in multifunctional smart windows with additional features of aesthetics and water collection, but in microfluidics, design of slippery surfaces, and directional water transportation.

Programming Feature Size in the Thermal Wrinkling of Metal Polymer Bilayer by Modulating Substrate Viscoelasticity

We report a novel strategy for creating stress-induced self-organized wrinkles in a metal polymer bilayer with programmable periodicity (λ_S) varying over a wide range, from ∼20 μm down to ∼800 nm by modulating the viscoelasticity of the bottom polymer layer. Substrates with different viscoelasticity are prepared by precuring thin films of a thermo-curable poly dimethylsiloxane (PDMS) elastomer (Sylgard 184) for different durations (t_P) prior to deposition of the top aluminum layer by thermal evaporation. Precuring of the Sylgard 184 film for different durations leads to films with different degrees of viscoelasticity due to variation in the extent of cross-linking of the polymer matrix. The λ_S as well as the amplitude (a_S) of the wrinkles progressively decrease with an increase in the extent of elasticity of the film, manifested as an increase in the storage modulus (G'). Based on the variation in the rate of decay of λ_S with G', we identify three clearly distinguishable regimes over predominantly viscous, viscoelastic, and elastic bottom layers. While λ_S and a_S drop with an increase in G' for both the first and the third regimes, it remains nearly independent of G' for the intermediate regime. This is attributed to the difference in the mechanisms of wrinkle formation in the different regimes. We finally show that simultaneous modulation of λ_S and a_S can be used to engineer surfaces with different wettability as well as anti-reflection properties.

Stretching-induced wrinkling in plastic-rubber composites

We examine the mechanics of three-layer composite films composed of an elastomeric layer sandwiched between two thin surface layers of plastic. Upon stretching and releasing such composite films, they develop a highly wrinkled surface texture. The mechanism for this texturing is that during stretching, the plastic layers yield and stretch irreversibly whereas the elastomer stretches reversibly. Thus upon releasing, the plastic layers buckle due to compressive stress imposed by the elastomer. Experiments are conducted using SEPS elastomer and 50 micron thick LLDPE plastic films. Stretching and releasing the composites to 2-5 times their original length induces buckles with wavelength on the order of 200 microns, and the wavelength decreases as the stretching increases. FEM simulations reveal that plastic deformation is involved at all stages during this process: (1) during stretching, the plastic layer yields in tension; (2) during recovery, the plastic layer first yields in-plane in compression and then buckles; (3) post-buckling, plastic hinges are formed at high-curvature regions. Homogeneous wrinkles are predicted only within a finite window of material properties: if the yield stress is too low, the plastic layers yield in-plane, without wrinkling, whereas if the yield stress is too high, non-homogeneous wrinkles are predicted. This approach to realizing highly wrinkled textures offers several advantages, most importantly the fact that high aspect ratio wrinkles (amplitude to wavelength ratios exceeding 0.4) can be realized.

Hydrogels with Lotus Leaf Topography: Investigating Surface Properties and Cell Adhesion

The interactions of cells with the surface of materials is known to be influenced by a range of factors that include chemistry and roughness; however, it is often difficult to probe these factors individually without also changing the others. Here we investigate the role of roughness on cell adhesion while maintaining the same underlying chemistry. This was achieved by using a polymerization in mold technique to prepare poly(hydroxymethyl methacrylate) hydrogels with either a flat topography or a topography that replicated the microscale features of lotus leaves. These materials were then assessed for cell adhesion, and atomic force microscopy and contact angle analysis were then used to probe the physical reasons for the differing behavior in relation to cell adhesion.

Facile Selective and Diverse Fabrication of Superhydrophobic, Superoleophobic-Superhydrophilic and Superamphiphobic Materials from Kaolin

As the starting material, kaolin is selectively and diversely fabricated to the superhydrophobic, superoleophobic-superhydrophilic, and superamphiphobic materials, respectively. The wettability of the kaolin surface can be selectively controlled and regulated to different superwetting states by choosing the corresponding modification reagent. The procedure is facile to operate, and no special technique or equipment is required. In addition, the procedure is cost-effective and time-saving and the obtained super-repellent properties are very stable. The X-ray photoelectron spectroscopy analysis demonstrates different changes of kaolin particles surfaces which are responsible for the different super-repellency. The scanning electron microscopy displays geometric micro- and nanometer structures of the obtained three kinds of super-repellent materials. The results show that kaolin has good applications in many kinds of superwetting materials. The method demonstrated in this paper provides a new strategy for regulating and controlling the wettability of solid surfaces selectively, diversely, and comprehensively.

Controlling self-patterning of acrylate films by photopolymerization

Acrylate formulations can spontaneously generate surface patterns by UV-curing in open-air.

Bio-inspired hierarchically structured polymer fibers for anisotropic non-wetting surfaces

A rice leaf-like hierarchically textured polymer fiber arrays for anisotropic non-wetting surfaces.

Biobased Wrinkled Surfaces Induced by Wood Mimetic Skins upon Drying: Effect of Mechanical Properties on Wrinkle Morphology

We previously developed biobased wrinkled surfaces based on wood mimetic skins in which microscopic wrinkles were fabricated on a chitosan film by immersion in a phenolic acid solution, horseradish peroxidase-catalyzed surface reaction, and drying. Here, we prepared a diverse range of wrinkled films by immersion treatment at 30, 40, 50, and 60 °C in p-coumaric acid and then investigated the correlation between wrinkle morphology and mechanical properties. Wrinkle wavelengths gradually decreased as the immersion temperature increased as well as the previous report. In order to clarify the mechanisms responsible for the different wrinkle morphologies, the films were subjected to elastic moduli measurement and GPC analysis after immersion treatment. These experiments provided evidence that the chitosan around the film surface decomposed along with the immersion process. The decomposition was accelerated by higher immersion temperature, suggesting that higher temperatures led to the formation of softer skins, inducing smaller wrinkles. In fact, wrinkle morphologies with this system were predominately determined by the hardness of the wood mimetic skins. This phenomenon is consistent with the fundamentals of surface wrinkling in nature. This study is the first to demonstrate that artificial wrinkling triggered by water evaporation can be controlled by precise control of the surface hardness of soft material.