Tunable open-channel microfluidics on soft poly(dimethylsiloxane) (PDMS) substrates with sinusoidal grooves

Network of Inorganic Nanocrystals Can Swell: Study of Swelling-Induced Surface Instability

A unique organic-inorganic hybrid network composed of inorganic nanocores (ranging from semiconductors to metallic ones) interconnected through organic molecules can be produced by crosslinking the organic ligands of colloidal inorganic nanocrystals in assemblies. This work reports that this network, which is conventionally considered an inorganic film, can swell when exposed to a solvent because of the interaction between the solvent and the organic linkage within the network. Intriguingly, this work discovers that drying the solvent of the swollen organic-inorganic hybrid network can significantly affect the morphology owing to the swelling-induced compress stress, which is widely observed in various organic network systems. This work studies the surface instability of crosslinked organic-inorganic hybrid networks swollen by various organic solvents, which led to buckling delamination. Specifically, this work investigates the effects of the i) solvent-network interaction, ii) crosslinking density of the network, and iii) thickness of the film on the delamination behavior of the crosslinked network.

Light-Induced Surface Wrinkling on Azo-Based Composite Films

Here we report a simple micro/nano patterning strategy based on light-induced surface wrinkling. Namely, we fabricated a film/substrate system composed of polydimethylsiloxane (PDMS) as a soft substrate and non-photosensitive polymer polystyrene (PS) mixed with azo-polymer (polydisperse orange 3, PDO3) as a stiff film. Taking advantage of the photo-thermal effect and photo-softening effect of PDO3, we fabricated various microstructured wrinkling morphologies by a simple light illumination. We investigated the influence of two exposure modes (i.e., static selective exposure and dynamic moving exposure), the illumination conditions, the composition of the blended film, and the film thickness on the resulting wrinkling patterns. It is highly expected that this azo-based photosensitive wrinkling system will be extended to functional systems for the realization of light-induced surface micro/nanopatterning.

Tailoring Ordered Wrinkle Arrays for Tunable Surface Performances by Template-Modulated Gradient Films

Complex wrinkled microstructures are ubiquitous in natural systems and living bodies. Although homogeneous wrinkles in film-substrate bilayers have been extensively investigated in the past 2 decades, tailoring heterogeneous wrinkles by a facile method is still a challenge. Here, we report on the controllable heterogeneous wrinkles in template-modulated thickness-gradient metal films sputter-deposited on polydimethylsiloxane substrates. It is found that the stress of the gradient film is strongly position-dependent and the wrinkles are always restricted in thinner film regions. The morphological characteristic and formation mechanism of the heterogeneous wrinkles are analyzed and discussed in detail based on the stress theory. Ordered wrinkle arrays are achieved by adjusting the deposition time, copper grid period, template shape, and lifting height. The surface performances (e.g., the friction property) are well controlled by the wrinkle arrays. This work could promote better understanding of the spontaneously heterogeneous wrinkles in template-modulated gradient films and controllable fabrication of various wrinkle arrays by independently tuning film deposition conditions and template parameters.

Anisotropic Wettability on One-Dimensional Nanopatterned Surfaces: The Effects of Intrinsic Surface Wettability and Morphology

Large-scale molecular dynamic simulations were conducted to study anisotropic wettability on one-dimensional (1D) nanopatterned surfaces. Hexadecane (C₁₆H₃₄) and decane (C₁₀H₂₂) nanodroplets were used as wetting liquids. Initially, surfaces with various intrinsic wettability (oleophobic and oleophilic) were produced using surface lattice size as a control parameter. These surfaces were subsequently patterned with 1D grooves of different sizes, and their anisotropic wettability was examined. The results show that anisotropic wettability strongly depends on intrinsic surface wettability and surface morphology. The results also demonstrate that the anisotropy in the contact angle is negligible for oleophobic surfaces. However, the anisotropy becomes more evident for oleophilic surfaces and increases with the degree of oleophilicity. Results suggest that anisotropy also depends on the surface morphology, including the patterns' width and height. Monitoring the droplet shape showed that more significant droplet distortion was associated with higher anisotropy. A clear association was lacking between the roughness ratio, r, and the degree of anisotropy. The observed average contact angle for 1D patterned oleophilic surfaces disagreed with the predicted values from the Wenzel theory. However, the theory could correctly predict the state of the droplet being Cassie-Baxter or Wenzel.

Tunable Optical Diffusers Based on the UV/Ozone-Assisted Self-Wrinkling of Thermal-Cured Polymer Films

Tunable optical diffusers have attracted attention because of the rapid development of next generation stretchable optoelectronics and optomechanics applications. Flexible wrinkle structures have the potential to change the light path and tune the diffusion capability, which is beneficial to fabricate optical diffusers. The generation of wrinkles usually depends on an external stimulus, thus resulting in complicated fabricating equipment and processes. In this study, a facile and low-cost method is proposed to fabricate wrinkle structures by the self-wrinkling of thermal-cured polymer for tunable optical diffusers. The uncured polydimethylsiloxane (PDMS) precursors were exposed to UV/ozone to obtain hard silica layers and then crosslinked via heating to induce the wrinkle patterns. The wrinkle structures were demonstrated as strain-dependent tunable optical diffusers and the optical diffusion of transmitted light via the deformable wrinkle structures was studied and adjusted. The incident light isotropically diffused through the sample at the initial state. When the wrinkle structures deformed, it showed a more pronounced isotropic optical diffusion with uniaxial tensile strain. The optical diffusion is anisotropical with a further increase in uniaxial tensile strain. The proposed method of fabricating wrinkles by UV/ozone-assisted self-wrinkling of thermal-cured polymer films is simple and cost-effective, and the obtained structures have potential applications in tunable optical diffusers.

Soft skin layers for reconfigurable and programmable nanowrinkles

Wrinkling skin layers on pre-strained polymer sheets has drawn significant interest as a method to create reconfigurable surface patterns. Compared to widely studied metal or silica films, softer polymer skins are more tolerant to crack formation when the surface topography is tuned under applied strain. This Mini-review discusses recent progress in mechano-responsive wrinkles based on polymer skin materials. Control over the skin thickness with nanometer accuracy allows for tuning of the wrinkle wavelength and orientation over length scales from nanometer to micrometer regimes. Furthermore, soft skin layers enable texturing of two-dimensional electronic materials with programmable feature sizes and structural hierarchy because of the conformal adhesion to the substrates. Soft skin systems open prospects to tailor a range of surface properties via external stimuli important for applications such as smart windows, microfluidics, and nanoelectronics.

Evaluation of surface layer stability of surface-modified polyester biomaterials

Surface modification of biomaterials is a strategy used to improve cellular and in vivo outcomes. However, most studies do not evaluate the lifetime of the introduced surface layer, which is an important aspect affecting how a biomaterial will interact with a cellular environment both in the short and in the long term. This study evaluated the surface layer stability in vitro in buffer solution of materials produced from poly(lactic-co-glycolic acid) (50:50) and polycaprolactone modified by hydrolysis and/or grafting of hydrophilic polymers using grafting from approaches. The data presented in this study highlight the shortcomings of using model substrates (e.g., spun-coated films) rather than disks, particles, and scaffolds. It also illustrates how similar surface modification strategies in some cases result in very different lifetimes of the surface layer, thus emphasizing the need for these studies as analogies cannot always be drawn.

Viscoelastic and Poroelastic Relaxations of Soft Solid Surfaces

Understanding surface mechanics of soft solids, such as soft polymeric gels, is crucial in many engineering processes, such as dynamic wetting and adhesive failure. In these situations, a combination of capillary and elastic forces drives the motion, which is balanced by dissipative mechanisms to determine the rate. While shear rheology (i.e., viscoelasticity) has long been assumed to dominate the dissipation, recent works have suggested that compressibility effects (i.e., poroelasticity) could play roles in swollen networks. We use fast interferometric imaging to quantify the relaxation of surface deformations due to a displaced contact line. By systematically measuring the profiles at different time and length scales, we experimentally observe a crossover from viscoelastic to poroelastic surface relaxations.

Mechanics of reversible wrinkling in a soft dielectric elastomer

A parallel array of wrinkles can be generated in a simple parallel plate capacitor arrangement with a soft dielectric elastomer plate constrained all around its periphery and coated with flexible electrodes. The direction of the wrinkles is predetermined and the phenomenon is reversible in a range of applied potentials. A model of the wrinkled plate as a prestretched neo-Hookean membrane with superposed small out-of-plane bending displacements yields good estimates of the potential range for wrinkling as well as the number of parallel wrinkles. The mechanics is controlled by the thickness and aspect ratios, prestretch, and a nondimensional electric potential.

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.

Ultrafast Self-Propelled Directional Liquid Transport on the Pyramid-Structured Fibers with Concave Curved Surfaces

Self-propelled directional liquid transport (SDLT) has been observed on many natural substrates, serving as an efficient strategy to utilize surrounding liquids for a better habitat to the local environment. Drawing inspiration, various artificial materials capable of SDLT have been developed. However, the liquid transport velocity is normally very low (ca. 3-30 μm/s), which limits its practical applications. Herein, we developed novel pyramid-structured fibers with concave curved surfaces (P-concave curved-fiber, PCCF), which enable the ultrafast SDLT. Specifically, the liquid transport velocity can be up to ∼28.79 mm/s on a dry tri-PCCF, over 50 times faster than that on the surface of Sarracenia trichome (∼520 μm/s). The velocity is even faster on a wet fiber by two times (∼47.34 mm/s). Here, the Laplace pressure difference (F_L) induced by the tapered structure determines the liquid transport direction. It is proposed that both the capillary rises imparted by the concave curved surfaces and the oriented microridges/valleys and the enhanced F_L aroused by the reduced cross-sectional area accelerate the SDLT on surfaces of the PCCFs. Consequently, the PCCF takes a different liquid transport strategy with a convex-shaped advancing meniscus, differing from that on traditional conical fibers. Moreover, the as-developed PCCF is also applicable for underwater ultrafast SDLT of oil. We envision that the result will open a new perspective for fabricating a fibrous system for microfluidic and liquid manipulation.

Soft Skin Layers Enable Area-Specific, Multiscale Graphene Wrinkles with Switchable Orientations

This paper reports a method to realize crack-free graphene wrinkles with variable spatial wavelengths and switchable orientations. Graphene supported on a thin fluoropolymer and prestrained elastomer substrate can exhibit conformal wrinkling after strain relief. The wrinkle orientation could be switched beyond the intrinsic fracture limit of graphene for hundreds of cycles of stretching and releasing without forming cracks. Mechanical modeling revealed that the fluoropolymer layer mediated the structural evolution of the graphene wrinkles without crack formation or delamination. Patterned fluoropolymer layers with different thicknesses produced wrinkles with controlled wavelengths and orientations while maintaining the mechanical integrity of graphene under high tensile strain.

Tunable adhesion and slip on a bio-mimetic sticky soft surface

Simultaneous tuning of wettability and adhesion of a surface requires intricate procedures for altering the interfacial structures. Here, we present a simple method for preparing a stable slippery surface, with an intrinsic capability of varying its adhesion characteristics. Cross-linked PDMS, an inherent hydrophobic material commonly used for microfluidic applications, is used to replicate the structures on the surface of a rose petal which acts as a high adhesion solid base and is subsequently oleoplaned with silicone oil. Our results demonstrate that the complex hierarchical rose petal structures can arrest dewetting of the silicone oil on the cross linked PDMS base by anchoring the oil film strongly even under flow. Further, by tuning the extent of submergence of the rose petal structures with silicone oil, we could alter the adhesion characteristics of the surface on demand, while retaining its slippery characteristics for a wide range of the pertinent parameters. We have also demonstrated the possible fabrication of gradient adhesion surfaces. This, in turn, may find a wide variety of applications in water harvesting, droplet maneuverability and no-loss transportation in resource-limited settings.

Bioinspired multistructured paper microfluidics for POCT

The rapid development of and the large market for medical diagnostics necessitate point-of-care testing (POCT) with superior sensitivity, miniaturization, multiple functionalities and high integration. Thus, flexible substrates with complex structures that provide multiple functions are in demand. Herein, we present multistructured pseudo-papers (MSPs) as a platform for building flexible microfluidics. Flexible and freestanding MSPs are generated by the self-assembly of colloidal silica crystals or core-shell copolymer elastic colloidal crystals on microcavity PDMS molds to form photonic crystals (PCs). Nitrocellulose (NC) multistructured pseudo-papers (NC MSPs) were obtained by etching SiO₂ PCs after NC precursor infiltration, while elastic copolymer (EC) multistructured pseudo-papers (EC MSPs) were directly peeled off the mold; both types of freestanding MSPs have ordered micropillars and nanocrystal structures and presented unique properties such as pumpless liquid transport and fluorescence and chemiluminescence (CL) enhancement. MSPs with designed patterns were fabricated by patterned PDMS molds, and complicated microfluidic chips were used to generate MSPs by utilizing these patterns as liquid channels. The MSPs were used for fabricating microfluidic sensors for human cardiac marker and cancer marker sensing; the features of these bioinspired MSPs indicate their potential for sensitive sensing, which will enable them to find broader applications in many fields.

Microstructured hybrid scaffolds for aligning neonatal rat ventricular myocytes

In cardiac tissue engineering (TE), in vitro models are essential for the study of healthy and pathological heart tissues in order to understand the underpinning mechanisms. In this scenario, scaffolds are platforms that can realistically mimic the natural architecture of the heart, and they add biorealism to in vitro models. This paper reports a novel and robust technique to fabricate cardiovascular-mimetic scaffolds based on Parylene C and Polydimethylsiloxane (PDMS). Parylene C is employed as a mask material for inducing hybrid and non-hybrid micropatterns to the PDMS layer. Hybrid architectures present striped hydrophobic/hydrophilic surfaces, whereas non-hybrid scaffolds only corrugated topographies. Herein, we demonstrate that wavy features on PDMS can be obtained at the micro- and nanoscale and that PDMS can be integrated into the microfabrication process without changing its intrinsic physical properties. A study of the effects of these scaffolds on the growth of Neonatal Rat Ventricular Myocytes (NRVMs) cultures reveals that cell alignment occurs only for the case of hybrid architectures made of hydrophilic PDMS and hydrophobic Parylene C.

Pattern selection when a layer buckles on a soft substrate

If a neo-Hookean elastic layer adhered to a neo-Hookean substrate grows equibiaxially, it will buckle into a topographic pattern. Here, we combine higher order perturbation theory and finite element numerics to predict the pattern formed just beyond the buckling threshold. More precisely, we construct a series of solutions corresponding to hexagonal, square and stripe patterns, and expand the elastic energy for each pattern as a Landau-like energy series in the topography amplitude. We see that, for square and stripe patterns, the elastic energy is invariant under topography inversion, making the instabilities supercritical. However, since patterns of hexagonal dents are physically different to patterns of hexagonal bumps, the hexagonal energy lacks this invariance. This lack introduces a cubic term which causes hexagonal patterns to be formed subcritically and are hence energetically favoured. Our analytic calculation of the cubic term allows us to determine that dents are favoured in incompressible systems, but bumps are favored in sufficiently compressible systems. Finally, we consider a stiff layer sandwiched between an identical substrate and superstrate pair. This system has topography inversion symmetry, so hexagons form supercritically, and square patterns are favoured. We use finite element calculations to verify our theoretical predictions for each pattern, and confirm which pattern is selected. Previous work has used a simplified elastic model (a plate & a linear elastic substrate) that possesses invariance under topography inversion, and hence incorrectly predicted square patterns. Our work demonstrates that large strain geometry is sufficient to break this symmetry and explain the hexagonal dent patterns observed in buckling experiments.

Bioinspired Flexible Volatile Organic Compounds Sensor Based on Dynamic Surface Wrinkling with Dual-Signal Response

Living systems can respond to external stimuli by dynamic interface changes. Moreover, natural wrinkle structures allow the surface to switch dynamically and reversibly from flat to rough in response to specific stimuli. Artificial wrinkle structures have been developed for applications such as optical devices, mechanical sensors, and microfluidic devices. However, chemical molecule-triggered flexible sensors based on dynamic surface wrinkling have not been demonstrated. Inspired by human skin wrinkling, herein, a volatile organic compound (VOC)-responsive flexible sensor with a switchable dual-signal response (transparency and resistance) is achieved based on a multilayered Ag nanowire (AgNW)/SiO_x /polydimethylsiloxane (PDMS) film. Wrinkle structures can form dynamically in response to VOC vapors (such as ethanol, toluene, acetone, formaldehyde, and methanol) due to the instability of the multilayer induced by their different swelling capabilities. By controlling the modulus of PDMS and the thickness of the SiO_x layer, tunable sensitivities in resistance and transparency of the device are achieved. Additionally, the proximity mechanism of the solubility parameter is proposed, which explains the high selectivity of the device toward ethanol vapor compared with that of other VOCs well. This stimuli-responsive sensor exhibits the dynamic visual feedback and the quantitative electrical signal, which provide a novel approach for developing smart flexible electronics.

Microfluidics Assisted Fabrication of Three-Tier Hierarchical Microparticles for Constructing Bioinspired Surfaces

Construction of textured bioinspired surfaces with refined structures that exhibit superior wetting properties is of great importance for many applications ranging from self-cleaning, antibiofouling, anti-icing, oil/water separation, smart membrane, and microfluidic devices. Previously, the preparation of artificial surfaces generally relies on the combination of different approaches together, which is a lack of flexibility to control over the individual architecture unit, the surface topology, as well as the complex procedure needed. In this work, we report a method for rapid fabrication of three-tier hierarchical microunits (structures consisting of multiple levels) using a facile droplet microfluidics approach. These units include the first-tier microspheres consisting of the second-tier close-packed polystyrene (PS) nanoparticles decorated with the third-tier elegant polymer nanowrinkles. These nanowrinkles on the PS nanoparticles are formed according to the interfacial instability induced by gradient photopolymerization of N-isopropylacrylamide (NIPAM) monomers. The formation process and topologies of nanowrinkles can be regulated by the photopolymerization process and the fraction of carboxylic groups on the PS nanoparticle surface. Such a hierarchical microsphere mimics individual units of bioinspired surfaces. Therefore, the surfaces from self-assembly of these fabricated two-tier and three-tier hierarchical microunits collectively exhibit "gecko" and "rose petal" wetting states, with the micro- and nanoscale structures amplifying the initial hydrophobicity but still being highly adhesive to water. This approach offers promising advantages of high-yield fabrication, precise control over the size and component of the microspheres, and integration of microfluidic droplet generation, colloidal nanoparticle self-assembly, and interfacial polymerization-induced nanowrinkles in a straightforward manner.

Polymer spreading on substrates with nanoscale grooves using molecular dynamics

Understanding how liquid polymer interacts with and spreads on surfaces with nanoscale texture features is crucial for designing complex nanoscale systems. We use molecular dynamics to simulate different types of polymer as they spread on substrates with a single nanoscale groove. We study how groove design affects the potential energy of a substrate and how this governs polymer spreading and orientation. Based on our simulations, we show that groove shape, polymer chemistry, and polymer molecule entanglement are the three parameters that determine polymer spreading on a nanoscale groove. We provide a molecular-level explanation of the underlying physical mechanisms, and we illustrate this fundamental understanding by designing a network of grooves to engineer user-specified polymer spreading and coverage. This work has implications for nanoscale systems and devices that involve the design of complex groove networks with an ultrathin polymer coating, including micro and nanoelectromechanical devices, nanoimprint lithography, flexible electronics, antibiofouling coatings, and hard disk drives.

Shape Evolution of Droplets Growing on Linear Microgrooves

Anisotropic spreading of liquids and elongated droplet shapes are often encountered on surfaces decorated with a periodic micropattern of linear surface topographies. Numerical calculations and wetting experiments show that the shape evolution of droplets that are slowly growing on a surface with parallel grooves can be grouped into two distinct morphological regimes. In the first regime, the liquid of the growing droplet spreads only into the direction parallel to the grooves. In the second regime, the three-phase contact line advances also perpendicular to the grooves, whereas the growing droplets approach a scale-invariant shape. Here, we demonstrate that shapes of droplets in contact with a large number of linear grooves are identical to the shapes of droplets confined to a plane chemical stripe, where this mapping of shapes is solely based on the knowledge of the cross section of the linear grooves and the material contact angle. The spectrum of interfacial shapes on the chemical stripe can be exploited to predict the particular growth mode and the asymptotic value of the base eccentricity in the limit of droplets covering a large number of grooves. The proposed model shows an excellent agreement with experimentally observed base eccentricities for droplets on grooves of various cross sections. The universality of the model is underlined by the accurate match with available literature data for droplet eccentricities on parallel chemical stripes.

Ring Wrinkle Patterns with Continuously Changing Wavelength Produced Using a Controlled-Gradient Light Field

We report a facile method to prepare gradient wrinkles using a controlled-gradient light field. Because of the gradient distance between the ultraviolet (UV) lamp and polydimethylsiloxane (PDMS) substrate during UV/ozone treatment, the irradiance reaching the substrate continuously changed, which was transferred into the resulting SiOx film with a varying thickness. Therefore, wrinkles with continuously changing wavelength were fabricated using this approach. It was found that the wrinkle wavelength decreased as the distance increased. We fabricated 1-D wrinkle patterns and ring wrinkles with a gradient wavelength. The ring wrinkles were prepared using radial stresses, which were achieved by pulling the center of a freely hanging PDMS film. The resulting wrinkles with changing wavelength can be used in fluid handling systems, biological templates, and optical devices.

Tailored topography: a novel fabrication technique using an elasticity gradient

A facile methodology to create a wrinkled surface with a tailored topography is presented herein. The dependency of the elasticity of poly(dimethyl)siloxane (PDMS) on the curing temperature has been exploited to obtain a substrate with an elasticity gradient. The temperature gradient across the length of PDMS is created by a novel set-up consisting of a metal and insulator connected to a heater and the highest usable (no degradation of PDMS) temperature gradient is used. The time-dependent temperature distributions along the substrate are measured and the underlying physics of the dependence of the PDMS elasticity on the curing temperature is addressed. The PDMS substrate with the elasticity gradient is first stretched and subsequently oxidized by oxygen plasma. Upon relaxation, an ordered wrinkled surface with continuously varying wavelength and amplitude along the length of PDMS is obtained. The extent of hydrophobicity recovery of this plasma oxidized PDMS with varying elasticity has been studied. The change in the wavelength and amplitude of the regular patterns on the substrate can be controlled by varying operational parameters like applied pre-strain, plasma power and the heater temperature. It has been found that the spatial distributions of the topography and the hydrophobicity collectively decide the resultant wettability of the substrate. Such surfaces with gradients in the substructure dimensions demonstrate different wetting characteristics that may lead to a wide gamut of applications including droplet movement, cell adhesion and proliferation, diffraction grating etc.

Arcuate wrinkling on stiff film/compliant substrate induced by thrust force with a controllable micro-probe

Wrinkling patterns are widely observed in nature and can be used in many high-tech applications such as microfluidic channel, self-assembly ordered microstructures and improved adhesives. In order to use the wrinkling patterns for these applications, it is necessary to precisely control the formation and geometry of the wrinkles. In this paper, we investigate the localized wrinkling of a stiff film/compliant substrate system subjected to a thrust force with a controllable micro-probe. A thin Au film is deposited onto a thick PDMS substrate attached to a glass to form the stiff film/compliant substrate system. And a micro-probe is controlled by a piezoelectric microrobotic system to exert a point force onto the stiff film/compliant substrate to demonstrate the evolution of the localized wrinkles. The experiments show that the film will wrinkle into orthoradial morphology spontaneously when it is deformed in the vertical direction, and then it will wrinkle into arcuate morphology with deformation in the horizontal direction. Since the compressive stress and tensile stress of the film are generated simultaneously, the evolution of the arcuate wrinkles is always accompanied by some radial cracks. The morphological characteristic, formation mechanism and dynamic evolution of the arcuate wrinkles are demonstrated and discussed in detail.

Effects of Stiff Film Pattern Geometry on Surface Buckling Instabilities of Elastic Bilayers

Buckling instabilities-such as wrinkling and creasing-of micropatterned elastic surfaces play important roles in applications, including flexible electronics and microfluidics. In many cases, the spatial dimensions associated with the imposed pattern can compete with the natural length scale of the surface instabilities (e.g., the wrinkle wavelength), leading to a rich array of surface buckling behaviors. In this paper, we consider elastic bilayers consisting of a spatially patterned stiff film supported on a continuous and planar soft substrate. Through a combination of experimental and computational analyses, we find that three surface instability modes-wrinkling, Euler buckling, and rigid rotation-are observed for the stiff material patterns, depending on the in-plane dimensions of the film compared to the natural wrinkle wavelength, while the intervening soft regions undergo a creasing instability. The interplay between these instabilities leads to a variety of surface structures as a function of the pattern geometry and applied compressive strain, in many cases yielding contact between neighboring stiff material elements because of the formation of creases in the gaps between them.

Capillary Coatings: Flow and Drying Dynamics in Open Microchannels

Capillary flow and drying of polymer solutions in open microchannels are explored over time scales spanning seven orders of magnitude: from capillary filling (10^-3-10 s) to the formation of a dry thin film (a "capillary coating"; 10²-10³ s). During capillary filling, drying-induced changes (increased solids content and viscosity) generate microscale pinning events that impede contact line motion. Three unique types of pinning are identified and characterized, each defined by the specific location(s) along the contact line at which pinning is induced. Drying is shown to ultimately pin the contact line permanently, and the associated total flow distances and times are revealed to be strong functions of channel width and drying rate. In general, lower drying rates coupled with intermediate channel widths are found to be most conducive to longer flow distances and times. After the advancing contact line permanently pins, internal flows driven by uneven evaporation rates continue to drive polymer to the contact line. This phenomenon promotes a local accumulation of solids and persists until all motion is arrested by drying. The effects of channel width and drying rate are investigated at each stage of this capillary coating process. These results are then applied to case studies of two functional inks commonly used in printed electronics fabrication: a PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)) ink and a graphene ink. Although drying is shown to permanently arrest flow in both inks, both systems exhibit an increased resistance to pinning unexplained by mechanisms identified in aqueous polymer systems. Instead, arguments based on chemistry, particle size, and rheology are used to explain their novel behavior. These case studies provide insight into how functional inks can be better designed to optimize flow distances and maximize overall dry film uniformity in capillary coatings.

Surface Instability of Bilayer Hydrogel Subjected to Both Compression and Solvent Absorption

The bilayered structure of hard thin film on soft substrate can lose stability and form specific patterns, such as wrinkles or creases, on the surface, induced by external stimuli. For bilayer hydrogels, the surface morphology caused by the instability is usually controlled by the solvent-induced swelling/shrinking and mechanical force. Here, two important issues on the instability of bilayer hydrogels, which were not considered in the previous studies, are focused on in this study. First, the upper layer of a hydrogel is not necessarily too thin. Thus we investigated how the thickness of the upper layer can affect the surface morphology of bilayer hydrogels under compression through both finite element (FE) simulation and theoretical analysis. Second, a hydrogel can absorb water molecules before the mechanical compression. The effect of the pre-absorption of water before the mechanical compression was studied through FE simulations and theoretical analysis. Our results show that when the thickness of the upper layer is very large, surface wrinkles can exist without transforming into period doublings. The pre-absorption of the water can result in folds or unexpected hierarchical wrinkles, which can be realized in experiments through further efforts.

Patterning of Wrinkled Polymer Surfaces by Single-Step Electron Irradiation

A novel yet simple approach to fabricate and pattern wrinkled surfaces on polymers is presented. Only by irradiating an electron beam onto a polymer, wrinkles are created on the polymer surface. Electron irradiation produces a bilayer polymeric structure comprising a degrading upper layer and a pristine bottom layer. Electron irradiation also increases the polymer surface temperature to a point much higher than the glass-transition temperature of the upper layer, leading to drastic thermal expansion of the upper layer. As a result, significant compressive force is applied to form surface wrinkles. The mechanism behind the wrinkle formation and the effects of electron irradiation parameters on the wrinkle characteristics are discussed. In addition, by this electron irradiation approach, a patterned wrinkle structure is uniquely prepared.

Evolution of local wrinkles near defects on stiff film/compliant substrate

Disordered wrinkles are widely observed in stiff film deposited onto a thermally expanded polymer when compressive stress exceeds the critical wrinkling stress of the film. Highly ordered wrinkles can be fabricated by introducing regularly arranged patterns on the polymer before deposition. However, the study on the morphological evolution of localized wrinkling patterns near defects on the stiff film/compliant substrate is neglected. In this paper, we show two morphological transitions of the local wrinkles induced by defects on an Au film/PDMS substrate. The observation shows that the straight wrinkles form perpendicularly to the line defects and the radial wrinkles form near spot-like defects. We observe that the extended radial wrinkles tend to split and evolve into branching patterns, this limits the deviation of the local wrinkle wavelength from the equilibrium wrinkle wavelength and causes the wrinkle wavelength to be always maintained in a narrow interval. Because the herringbone patterns have the minimum energy state, the straight and radial wrinkles evolve into herringbone wrinkles spontaneously. The morphological characteristic and evolution mechanism of the local wrinkles are described in detail. The observation may provide some clues to the formation and evolution of some localized wrinkling patterns in nature and multilayer materials.

Influence of graphene thickness and grain boundaries on MoS2 wrinkle nanostructures

In this work, the influence of the graphene grain structure and thickness on the MoS₂ wrinkle features were investigated.

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.

Self-Healing Superhydrophobic Materials Showing Quick Damage Recovery and Long-Term Durability

Superhydrophobic coatings/materials are important for a wide variety of applications, but the majority of these man-made coatings/materials still suffer from poor durability because of their lack of self-healing ability. Here, we report novel superhydrophobic materials which can quickly self-heal from various severe types of damage. In this study, we used poly(dimethylsiloxane) (PDMS) infused with two liquids: trichloropropylsilane, which reacts with ambient moisture to self-assemble into grass-like microfibers (named silicone micro/nanograss) on the surfaces and low-viscosity silicone oil (SO), which remains within the PDMS matrices and acts as a self-healing agent. Because of the silicone micro/nanograss structures on the PDMS surfaces and the effective preserve/protection system of a large quantity of SO within the PDMS matrices, our superhydrophobic materials showed quick superhydrophobic recovery under ambient conditions (within 1-2 h) even after exposure to plasma (24 h), boiling water, chemicals, and outside environments. Such an ability is superior to the best self-healing superhydrophobic coatings/materials reported so far.

Transition of surface-interface creasing in bilayer hydrogels

Controlling the morphologies and properties of the surface and/or interface of bimaterials consisting of soft polymers provides new opportunities in many engineering applications. Crease is a widely observed deformation mode in nature and engineering applications for soft polymers where the smooth surface folds into a region of self-contact with a sharp tip, usually induced by the instability from mechanical compression or swelling. In this work, we explore the competition mechanisms between surface and interface creases through numerical simulations and experimental studies on bilayer hydrogels. The surface or interface crease of the bilayer hydrogels under swelling is governed by both the modulus ratio (M₂/M₁) and the height ratio (H₂/H₁). Through extensive numerical simulations, we find that the interface crease of the bilayer hydrogels can only occur at a moderate modulus ratio (24 < M₂/M₁ < 96) and a large height ratio (H₂/H₁ ≥ 8). Guided by this phase diagram, our experiments confirm that both surface and interface creases can be generated by swelling triggered instability, and the transition of surface to interface creases occurs at the critical value of the height ratio (H₂/H₁) between 5 and 10. Such an observation is in good agreement with our numerical predictions. Fundamental understandings on the switching between the surface and interface creases provide new insights into the design of highly tunable soft materials and devices over a wide range of length scales.

Stress-induced surface instabilities and defects in thin films sputter deposited on compliant substrates

Existing analyses predict that thin metal films deposited on compliant substrates are subject to a variety of surface instabilities, such as wrinkles, folds, creases, etc., that become more prominent with increased compressive residual stress. Under compressive stress, cracks have been assumed to form only when the interfacial strength is weak, allowing the film to detach from the substrate. In this work, we demonstrate that cracks also form on surfaces under compressive mismatch strain when the interface is strong. In particular, we consider metal alloy films sputter deposited under bias on elastomers with different thicknesses, curing temperatures or surface treatments. The deposition parameters created residual compressive strains and strong adhesion in the bilayers. Samples without surface treatment formed wrinkles and through-thickness cracks at 0.25-0.4% mismatch strains. Only through-thickness cracks were observed in UV treated samples. The crack spacing was found to decrease by a factor of 4 when the surface was UV treated and by a factor of 3 as the elastomer thickness decreased from 30 to 6 μm. Cracks penetrated through the elastomer, 15-30 times deeper than the film thickness, and formed in all samples with a brittle coating. A numerical model was developed to explain the formation of through-thickness cracks and wrinkles under applied compressive mismatch strains. The model suggests that cracks can initiate from the peak of wrinkles when the critical fracture strength of the coating is exceeded. For the UV treated samples, through-thickness cracks are possibly impacted by the formation of an embrittled near surface PDMS layer.

Spontaneous formation of aligned DNA nanowires by capillarity-induced skin folding

Although DNA nanowires have proven useful as a template for fabricating functional nanomaterials and a platform for genetic analysis, their widespread use is still hindered because of limited control over the size, geometry, and alignment of the nanowires. Here, we document the capillarity-induced folding of an initially wrinkled surface and present an approach to the spontaneous formation of aligned DNA nanowires using a template whose surface morphology dynamically changes in response to liquid. In particular, we exploit the familiar wrinkling phenomenon that results from compression of a thin skin on a soft substrate. Once a droplet of liquid solution containing DNA molecules is placed on the wrinkled surface, the liquid from the droplet enters certain wrinkled channels. The capillary forces deform wrinkles containing liquid into sharp folds, whereas the neighboring empty wrinkles are stretched out. In this way, we obtain a periodic array of folded channels that contain liquid solution with DNA molecules. Such an approach serves as a template for the fabrication of arrays of straight or wrinkled DNA nanowires, where their characteristic scales are robustly tunable with the physical properties of liquid and the mechanical and geometrical properties of the elastic system.

Dynamics of Capillary-Driven Flow in 3D Printed Open Microchannels

Microchannels have applications in microfluidic devices, patterns for micromolding, and even flexible electronic devices. Three-dimensional (3D) printing presents a promising alternative manufacturing route for these microchannels due to the technology's relative speed and the design freedom it affords its users. However, the roughness of 3D printed surfaces can significantly influence flow dynamics inside of a microchannel. In this work, open microchannels are fabricated using four different 3D printing techniques: fused deposition modeling (FDM), stereolithography (SLA), selective laser sintering, and multi jet modeling. Microchannels printed with each technology are evaluated with respect to their surface roughness, morphology, and how conducive they are to spontaneous capillary filling. Based on this initial assessment, microchannels printed with FDM and SLA are chosen as models to study spontaneous, capillary-driven flow dynamics in 3D printed microchannels. Flow dynamics are investigated over short (∼10^-3 s), intermediate (∼1 s), and long (∼10² s) time scales. Surface roughness causes a start-stop motion down the channel due to contact line pinning, while the cross-sectional shape imparted onto the channels during the printing process is shown to reduce the expected filling velocity. A significant delay in the onset of Lucas-Washburn dynamics (a long-time equilibrium state where meniscus position advances proportionally to the square root of time) is also observed. Flow dynamics are assessed as a function of printing technology, print orientation, channel dimensions, and liquid properties. This study provides the first in-depth investigation of the effect of 3D printing on microchannel flow dynamics as well as a set of rules on how to account for these effects in practice. The extension of these effects to closed microchannels and microchannels fabricated with other 3D printing technologies is also discussed.

Enhanced light scattering effect of wrinkled transparent conductive ITO thin film

In this work, we fabricate uniform wrinkles on ITO and systematically study the properties of the wrinkled ITO in optics, electrics and mechanics. The wrinkled ITO shows a high optical transmittance and improved mechanical bending performance.

Controlling self-patterning of acrylate films by photopolymerization

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

Uniting Superhydrophobic, Superoleophobic and Lubricant Infused Slippery Behavior on Copper Oxide Nano-structured Substrates

Alloys, specifically steel, are considered as the workhorse of our society and are inimitable engineering materials in the field of infrastructure, industry and possesses significant applications in our daily life. However, creating a robust synthetic metallic surface that repels various liquids has remained extremely challenging. The wettability of a solid surface is known to be governed by its geometric nano-/micro structure and the chemical composition. Here, we are demonstrating a facile and economical way to generate copper oxide micro-nano structures with spherical (0D), needle (1D) and hierarchical cauliflower (3D) morphologies on galvanized steel substrates using a simple chemical bath deposition method. These nano/micro textured steel surfaces, on subsequent coating of a low surface energy material display excellent superhydrophobic, superoleophobic and slippery behavior. Polydimethylsiloxane coated textured surfaces illustrate superhydrophobicity with water contact angle about 160°(2) and critical sliding angle ~2°. When functionalized with low-surface energy perfluoroalkylsilane, these surfaces display high repellency for low surface tension oils as well as hydrocarbons. Among them, the hierarchical cauliflower morphology exhibits re-entrant structure thereby showing the best superoleophobicity with contact angle 149° for dodecane. Once infused with a lubricant like silicone oil, they show excellent slippery behavior with low contact angle hysteresis (~ 2°) for water drops.

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

Wrinkling of a soft elastomer surface capped by a relatively hard thin film or modified by some physical treatments to induce hardening has been widely studied for applications in fields such as low-cost micro-fabrication, optics and tribology. Here we show that a biaxial textile sheet embedded on the surface of an elastomer buckles and selectively forms anisotropic wrinkles when experiencing a compressive strain in the fibre axial direction. The wrinkles also possess a fine surface structure that originates from the periodic structure of the biaxial textile sheet. Depending on whether the surface is wrinkled or not, the unique frictional property due to which the friction on wrinkles significantly decreases by a factor of less than 0.1 because of the localized contact regions on the protrusions originating from the textile structure is shown.

Tailoring the Orientation and Periodicity of Wrinkles Using Ion-Beam Bombardment

The present study demonstrates that surface reformation in polydimethylsiloxane can be controlled using ion-beam (IB) irradiation. This can be done by simply varying the IB incidence angle and requires no change in the energy source. By controlling the incidence angle of IB irradiation, we were able to continuously control the pattern of the wrinkle structure, that is, a randomly formed pattern or an anisotropic one. Moreover, the directional characteristics of the wrinkle pattern control the alignment of liquid crystal molecules. This control is a function of the incidence angle of the IB. These simple methods can provide considerable flexibility in the fabrication of wrinkle structures.

Mechanically Tunable Slippery Behavior on Soft Poly(dimethylsiloxane)-Based Anisotropic Wrinkles Infused with Lubricating Fluid

We demonstrate a novel technique to fabricate mechanically tunable slippery surfaces using one-dimensional (anisotropic) elastic wrinkles. Such wrinkles show tunable topography (amplitude) on the application of mechanical strain. Following Nepenthes pitcher plants, lubricating fluid infused solid surfaces show excellent slippery behavior for test liquid drops. Therefore, combining the above two, that is, infusing suitable lubricating fluid on elastic wrinkles, would enable us to fabricate mechanically tunable slippery surfaces. Completely stretched (flat) wrinkles have uniform coating of lubricating fluid, whereas completely relaxed (full amplitude) wrinkles have most of the lubricating oil in the wrinkle grooves. Therefore, water drops on completely stretched surface show excellent slippery behavior, whereas on completely relaxed surface they show reduced slippery behavior. Therefore, continuous variation of wrinkle stretching provides reversibly tunable slippery behavior on such a system. Because the wrinkles are one-dimensional, they show anisotropic tunability of slippery behavior depending upon whether test liquid drops slip parallel or perpendicular to the wrinkles.

Nanotextured Shrink Wrap Superhydrophobic Surfaces by Argon Plasma Etching

We present a rapid, simple, and scalable approach to achieve superhydrophobic (SH) substrates directly in commodity shrink wrap film utilizing Argon (Ar) plasma. Ar plasma treatment creates a stiff skin layer on the surface of the shrink film. When the film shrinks, the mismatch in stiffness between the stiff skin layer and bulk shrink film causes the formation of multiscale hierarchical wrinkles with nano-textured features. Scanning electron microscopy (SEM) images confirm the presence of these biomimetic structures. Contact angle (CA) and contact angle hysteresis (CAH) measurements, respectively, defined as values greater than 150° and less than 10°, verified the SH nature of the substrates. Furthermore, we demonstrate the ability to reliably pattern hydrophilic regions onto the SH substrates, allowing precise capture and detection of proteins in urine. Finally, we achieved self-driven microfluidics via patterning contrasting superhydrophilic microchannels on the SH Ar substrates to induce flow for biosensing.

Single-Step Fluorocarbon Plasma Treatment-Induced Wrinkle Structure for High-Performance Triboelectric Nanogenerator

A triboelectric nanogenerator (TENG) has been thought to be a promising method to harvest energy from environment. To date, the utilization of surface structure and material modification has been considered the most effective way to increase its performance. In this work, a wrinkle structure based high-performance TENG is presented. Using the fluorocarbon plasma treatment method, material modification and surface structure are introduced in one step. The output ability of TENG is dramatically enhanced. After the optimization of plasma treatment, the maximum current and surface charge density are 182 μA about 165 μC m(-2). Compared with untreated TENG, the wrinkle structure makes the current and surface charge density increase by 810% and 528%, separately. X-ray photoelectron spectroscopy is employed to analyze the chemical modification mechanism of this fluorocarbon plasma treatment. Facilitated by its high output performance, this device could directly light 76 blue light emitting diodes under finger typing. The output electric energy could be stored then utilized to power a commercial calculator. As a result of the simple fabrication process and high output ability, devices fabricated using this method could bring forward practical applications using TENGs as power sources.

Controlled anisotropic wetting of scalloped silicon nanogroove

The anisotropic wetting characteristics of SNGs were investigated in dynamic and static regimes. The anisotropic wettability of the SNGs was successfully employed to control fluid flows in microfluidic channels.

Tunable Microfluidic Devices for Hydrodynamic Fractionation of Cells and Beads: A Review

The adjustable microfluidic devices that have been developed for hydrodynamic-based fractionation of beads and cells are important for fast performance tunability through interaction of mechanical properties of particles in fluid flow and mechanically flexible microstructures. In this review, the research works reported on fabrication and testing of the tunable elastomeric microfluidic devices for applications such as separation, filtration, isolation, and trapping of single or bulk of microbeads or cells are discussed. Such microfluidic systems for rapid performance alteration are classified in two groups of bulk deformation of microdevices using external mechanical forces, and local deformation of microstructures using flexible membrane by pneumatic pressure. The main advantage of membrane-based tunable systems has been addressed to be the high capability of integration with other microdevice components. The stretchable devices based on bulk deformation of microstructures have in common advantage of simplicity in design and fabrication process.