Anisotropic wetting on tunable micro-wrinkled surfaces

Dry and wet wrinkling of a silk fibroin biopolymer by a shape-memory material with insight into mechanical effects on secondary structures in the silk network

Surface wrinkling provides an approach to modify the surfaces of biomedical devices to better mimic features of the extracellular matrix and guide cell attachment, proliferation, and differentiation. Biopolymer wrinkling on active materials holds promise but is poorly explored. Here we report a mechanically actuated assembly process to generate uniaxial micro-and nanosized silk fibroin (SF) wrinkles on a thermo-responsive shape-memory polymer (SMP) substrate, with wrinkling demonstrated under both dry and hydrated (cell compatible) conditions. By systematically investigating the influence of SMP programmed strain magnitude, film thickness, and aqueous media on wrinkle stability and morphology, we reveal how to control the wrinkle sizes on the micron and sub-micron length scale. Furthermore, as a parameter fundamental to SMPs, we demonstrate that the temperature during the recovery process can also affect the wrinkle characteristics and the secondary structures in the silk network. We find that with increasing SMP programmed strain magnitude, silk wrinkled topographies with increasing wavelengths and amplitudes are achieved. Furthermore, silk wrinkling is found to increase β-sheet content, with spectroscopic analysis suggesting that the effect may be due primarily to tensile (e.g., Poisson effect and high-curvature wrinkle) loading modes in the SF, despite the compressive bulk deformation (uniaxial contraction) used to produce wrinkles. Silk wrinkles fabricated from sufficiently thick films (roughly 250 nm) persist after 24 h in cell culture medium. Using a fibroblast cell line, analysis of cellular response to the wrinkled topographies reveals high viability and attachment. These findings demonstrate use of wrinkled SF films under physiologically relevant conditions and suggest the potential for biopolymer wrinkles on biomaterials surfaces to find application in cell mechanobiology, wound healing, and tissue engineering.

Resonance Modes of Water Drops Pinned to a Vibrating Rectangular Post

We studied the effects of vertical vibrations on a water drop that was pinned to the sharp edges of a rectangular post. By varying the frequency and amplitude of the vertical displacement, distinct resonance peaks were observed using a simple optical technique. The vibrational spectra of the first two modes exhibited two closely spaced peaks, which corresponded to standing waves that exist along the major and minor contour lengths of the drops. The values of the resonance frequencies can be explained rather well by a simple model, which was originally proposed for axially symmetric drops.

Controlled Preparation of Highly Stretchable, Crack-Free Wrinkled Surfaces with Tunable Wetting and Optical Properties

Constructing wrinkles by utilizing strain-driven surface instability in film-substrate systems is a general method to prepare micronano structures, which have a wide range of applications in smart surfaces and devices such as flexible electronics, reversible wetting, friction, and optics. However, cracks generated during the preparation and use process significantly affect the uniformity of wrinkled surfaces and degrade the functional properties of the film devices. The realization of crack-free wrinkles with high stretchability in hard film systems is still a great challenge. Here, we report on a facile technique for controllable preparation of large-area, highly stretchable, crack-free wrinkled surfaces by ultraviolet ozone (UVO) treatment of Ecoflex. The thickness dependence of the wrinkles and the in situ wrinkling process during mechanical loading are investigated. The wrinkles including striped, labyrinth-like, herringbone, and transitional structures are controllable by changing strain mode (uniaxial or biaxial), loading history (simultaneous or sequential), strain anisotropy, and gradient loading. The wrinkled surfaces obtained using UVO-treated Ecoflex have tunable wetting and optical properties and can maintain excellent mechanical stability under large strains. This study provides a facile method for the preparation of large-area, crack-free wrinkles, which is simple, fast, low-cost, and robust. The resulting wrinkled surfaces remain stable under high stretching, which is beneficial for many practical applications, especially in the cases of large strains.

Dynamic wetting of Newtonian and viscoelastic fluids on microstructured surfaces

Hypothesis Although extensive research has been conducted on the dynamic wetting of Newtonian fluids, limited insights have been gained for viscoelastic fluids, particularly on engineered surfaces. We hypothesize that differences in dynamic wetting on microstructured surfaces exist between such fluids, which may be attributed to variations in viscosity and elasticity as well as changes in the microscopic morphology of the moving contact line. Experiments To systematically investigate the wetting differences between Newtonian and viscoelastic fluids on microstructured surfaces, we conducted forced wetting experiments of glycerol-water and carboxymethyl cellulose aqueous solutions on microstructured polytetrafluoroethylene surfaces through a modified Wilhelmy plate method. Findings Results demonstrated an apparent difference in the relationship between the dynamic contact angle and moving velocity with different microstructured surfaces for Newtonian and viscoelastic fluids. The power-law exponent between the capillary number and cubic of the dynamic contact angle increases with the strengthening of shear thinning and elastic effects. In contrast, this exponent is rarely influenced by the scale of microstructured surfaces, particularly in highly viscous regions where viscous force dominates. In addition, viscosity affects the viscous bending and distance that liquid molecules jump at the contact line. These findings have potential applications in coating complex fluids on engineered surfaces.

Ultrasensitive Ionic Liquid Polymer Composites with a Convex and Wrinkled Microstructure and Their Application as Wearable Pressure Sensors

The development of pressure sensors with high sensitivity and effectiveness that exhibit linearity over a wide pressure range is crucial for wearable devices. In this study, we fabricated a novel ionic liquid (IL)/polymer composite with a convex and randomly wrinkled microstructure in a cost-effective and facile manner using an opaque glass and stretched polydimethylsiloxane template. The fabricated IL/polymer composite was used as the dielectric layer in a capacitive pressure sensor. The sensor exhibited a high linear sensitivity of 56.91 kPa^-1 owing to the high interfacial capacitance formed by the electrical double layer of the IL/polymer composite over a relatively wide range (0-80 kPa). We also demonstrated the sensor performance for various applications such as a glove-attached sensor, sensor array, respiration monitoring mask, human pulse, blood pressure measurement, human motion detection, and a wide range of pressure sensing. It would be expected that the proposed pressure sensor has sufficient potential for use in wearable devices.

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.

A one-step, tunable method of selective reactive sputter deposition as a wrinkling approach for silver/polydimethylsiloxane for electrically conductive pliable surfaces

The wrinkle period and morphology of a metal thin film on an elastic substrate is typically controlled by modifying the substrate before carrying out additional metal deposition steps. Herein, we show that a simultaneously selective and reactive sputtering plasma that modifies the surface of a polydimethylsiloxane (PDMS) substrate while not reacting with the metal during the deposition process decreases the wrinkle wavelength and induces additional wrinkling components and features such as ripples or folds. The selective reaction of the nitrogen plasma with PDMS functionalizes the siloxane surface into silicon oxynitride. This hardens the immediate surface of PDMS, with a quadratic increase in the Young's modulus as a function of the sputtering flow ratio. The increase in the critical strain mismatch and the corresponding presence of folds in the nitrogen-modified wrinkled silver film form a suitable plasmonic platform for surface-enhanced Raman spectroscopy (SERS), yielding an enhancement factor of 4.8 × 10⁵ for detecting lipids. This enhancement is linked to the emergence of electromagnetic hotspots from surface plasmon polariton coupling between the folds/wrinkles, which in turn enables the detection of low concentrations of organics using SERS. Furthermore, when strained, the nitrogen-modified wrinkles enhance electrical conductivity by a factor of 12 compared with unmodified films. Finally, the optical properties of the substrate can be tuned by altering the N₂ content. The simple addition of nonreactive nitrogen to silver sputtering enables simultaneous PDMS hardening and growth of the silver film and together provide a new avenue for tuning wrinkling parameters and enhancing the electrical conductivity of pliable surfaces.

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.

Reconfigurable Micro- and Nano-Structured Camouflage Surfaces Inspired by Cephalopods

Wrinkled surfaces and materials are found throughout the natural world in various plants and animals and are known to improve the performance of emerging optical and electrical technologies. Despite much progress, the reversible post-fabrication tuning of wrinkle sizes and geometries across multiple length scales has remained relatively challenging for some materials, and the development of comprehensive structure-function relationships for optically active wrinkled surfaces has often proven difficult. Herein, by drawing inspiration from natural cephalopod skin and leveraging methodologies established for artificial adaptive infrared platforms, we engineer systems with hierarchically reconfigurable wrinkled surface morphologies and dynamically tunable visible-to-infrared spectroscopic properties. Specifically, we demonstrate architectures for which mechanical actuation changes the surface morphological characteristics; modulates the reflectance, transmittance, and absorptance across a broad spectral window; controls the specular-to-diffuse reflectance ratios; and alters the visible and thermal appearances. Moreover, we demonstrate the incorporation of these architectures into analogous electrically actuated appearance-changing devices that feature competitive figures of merit, such as reasonable maximum areal strains, rapid response times, and good stabilities upon repeated actuation. Overall, our findings constitute another step forward in the continued development of cephalopod-inspired light- and heat-manipulating systems and may facilitate advanced applications in the areas of sensing, electronics, optics, soft robotics, and thermal management.

Mechanically Switchable Wetting Petal Effect in Self-Patterned Nanocolumnar Films on Poly(dimethylsiloxane)

Switchable mechanically induced changes in the wetting behavior of surfaces are of paramount importance for advanced microfluidic, self-cleaning and biomedical applications. In this work we show that the well-known polydimethylsiloxane (PDMS) elastomer develops self-patterning when it is coated with nanostructured TiO₂ films prepared by physical vapor deposition at glancing angles and subsequently subjected to a mechanical deformation. Thus, unlike the disordered wrinkled surfaces typically created by deformation of the bare elastomer, well-ordered and aligned micro-scaled grooves form on TiO₂/PDMS after the first post-deposition bending or stretching event. These regularly patterned surfaces can be reversibly modified by mechanical deformation, thereby inducing a switchable and reversible wetting petal effect and the sliding of liquid droplets. When performed in a dynamic way, this mechanical actuation produces a unique capacity of liquid droplets (water and diiodomethane) transport and tweezing, this latter through their selective capture and release depending on their volume and chemical characteristics. Scanning electron and atomic force microscopy studies of the strained samples showed that a dual-scale roughness, a parallel alignment of patterned grooves and their reversible widening upon deformation, are critical factors controlling this singular sliding behavior and the possibility to tailor their response by the appropriate manufacturing of surface structures.

Dynamic contact line lithography: Template-less complex Meso-patterning with polystyrene and poly(methyl methacrylate)

HYPOTHESIS

Micro/nanopatterning on a 2D surface is apt for cutting-edge miniaturization technology, which directly or indirectly requires high-end expensive lithographic tools. The evaporative deposition at the receding contact-line of a polymer solution, termed as Dynamic Contact Line Lithography (DCLL), can be a potential inexpensive technique for template-less meso-patterning if the deposition patterns from DCLL can be predicted a priori.

EXPERIMENTS

A deposition map (morphological phase diagram) from the myriads of patterns is constructed in terms of contact-line velocity and the polymer concentration. Specifically, two combinations: polystyrene (PS)/cyclohexane and poly (methyl methacrylate) (PMMA)/toluene are used to show the generic nature of the phase diagrams. The surface wettability of Si (water contact angle, CA ~15°) is tuned from CA ~35° to ~98° by patterning with DCLL.

FINDINGS

Directed by the phase diagrams, fabrication of a complex rectangular cross-pattern of PS and PMMA micro-threads with a periodicity of ~65 μm and ~50 μm respectively on a Si surface is demonstrated to establish the robustness and potential of the DCLL and predictive phase diagram.

Dynamic manipulation of droplets using mechanically tunable microtextured chemical gradients

Materials and strategies applicable to the dynamic transport of microdroplets are relevant to surface fluidics, self-cleaning materials, thermal management systems, and analytical devices. Techniques based on electrowetting, topographic micropatterns, and thermal/chemical gradients have advanced considerably, but dynamic microdroplet transport remains a challenge. This manuscript reports the fabrication of mechano-tunable, microtextured chemical gradients on elastomer films and their use in controlled microdroplet transport. Specifically, discreet mechanical deformations of these films enabled dynamic tuning of the microtextures and thus transport along surface-chemical gradients. The interplay between the driving force of the chemical gradient and the microtopography was characterized, facilitating accurate prediction of the conditions (droplet radius and roughness) which supported transport. In this work, the use of microtextured surface chemical gradients in mechano-adaptive materials with microdroplet manipulation functionality was highlighted.

Harnessing Wrinkling Patterns Using Shape Memory Polymer Microparticles

Shape memory polymers (SMPs) are the simplest and most attractive alternatives for soft substrates of typical bilayer wrinkle systems because of shape fixity and recovery capabilities. Herein, we have successfully programmed large compressive strains in chemical cross-linking shape memory polystyrene (PS) microparticles via nanoimprint lithography, which acted as the substrate of a wrinkle system using a gold nanoparticle (Au NP) film as the top layer. When triggered by two different stimuli (direct heating and toluene vapors), the thin Au NP film could transform into various wrinkle structures atop the recovered PS particles. In addition, we also investigated the evolution mechanisms of wrinkling by heating and toluene vapors and tuned the wrinkled surfaces through altering the Au NP thickness and stimulation methods (direct heating and toluene vapors), which utilized the structural adjustability of Au NPs to program the amplitude, wavelength, and morphology of the wrinkles. The concept presented here provides a cost-effective approach to realize the surface wrinkling and can be extended to other available SMPs.

Ordered surface crack patterns in situ formed under confinement on fluidic microchannel boundaries in polydimethylsiloxane

We present ordered surface crack patterns discovered in microfluidic channels/chambers in polydimethylsiloxane (PDMS). The cracks are formed in situ under confinement due to compression applied following an oxygen plasma step in a soft lithography process. The crack patterns are noticeable only after fluorescent labeling and vary with fluidic layout as well as material compliance.

Buckling transitions and soft-phase invasion of two-component icosahedral shells

What is the optimal distribution of two types of crystalline phases on the surface of icosahedral shells, such as of many viral capsids? We here investigate the distribution of a thin layer of soft material on a crystalline convex icosahedral shell. We demonstrate how the shapes of spherical viruses can be understood from the perspective of elasticity theory of thin two-component shells. We develop a theory of shape transformations of an icosahedral shell upon addition of a softer, but still crystalline, material onto its surface. We show how the soft component "invades" the regions with the highest elastic energy and stress imposed by the 12 topological defects on the surface. We explore the phase diagram as a function of the surface fraction of the soft material, the shell size, and the incommensurability of the elastic moduli of the rigid and soft phases. We find that, as expected, progressive filling of the rigid shell by the soft phase starts from the most deformed regions of the icosahedron. With a progressively increasing soft-phase coverage, the spherical segments of domes are filled first (12 vertices of the shell), then the cylindrical segments connecting the domes (30 edges) are invaded, and, ultimately, the 20 flat faces of the icosahedral shell tend to be occupied by the soft material. We present a detailed theoretical investigation of the first two stages of this invasion process and develop a model of morphological changes of the cone structure that permits noncircular cross sections. In conclusion, we discuss the biological relevance of some structures predicted from our calculations, in particular for the shape of viral capsids.

Preclinical In Vitro Assessment of Submicron-Scale Laser Surface Texturing on Ti6Al4V

Loosening of orthodontic and orthopedic implants is a critical and common clinical problem. To minimize the numbers of revision surgeries due to peri-implant inflammation or insufficient osseointegration, developments of new implant manufacturing strategies are indicated. Ultrafast laser surface texturing is a promising contact-free technology to modify the physicochemical properties of surfaces toward an anti-infectious functionalization. This work aims to texture Ti6Al4V surfaces with ultraviolet (UV) and green (GR) radiation for the manufacturing of laser-induced periodic surface structures (LIPSS). The assessment of these surface modifications addresses key aspects of topography, morphology and chemical composition. Human primary mesenchymal stromal cells (hMSCs) were cultured on laser-textured and polished Ti6Al4V to characterize the surfaces in terms of their in vitro biocompatibility, cytotoxicity, and metal release. The outcomes of the in vitro experiment show the successful culture of hMSCs on textured Ti6Al4V surfaces developed within this work. Cells cultured on LIPSS surfaces were not compromised in terms of their viability if compared to polished surfaces. Yet, the hMSC culture on UV-LIPSS show significantly lower lactate dehydrogenase and titanium release into the supernatant compared to polished. Thus, the presented surface modification can be a promising approach for future applications in orthodontics and orthopedics.

Natural Architectures for Tissue Engineering and Regenerative Medicine

The ability to control the interactions between functional biomaterials and biological systems is of great importance for tissue engineering and regenerative medicine. However, the underlying mechanisms defining the interplay between biomaterial properties and the human body are complex. Therefore, a key challenge is to design biomaterials that mimic the in vivo microenvironment. Over millions of years, nature has produced a wide variety of biological materials optimised for distinct functions, ranging from the extracellular matrix (ECM) for structural and biochemical support of cells to the holy lotus with special wettability for self-cleaning effects. Many of these systems found in biology possess unique surface properties recognised to regulate cell behaviour. Integration of such natural surface properties in biomaterials can bring about novel cell responses in vitro and provide greater insights into the processes occurring at the cell-biomaterial interface. Using natural surfaces as templates for bioinspired design can stimulate progress in the field of regenerative medicine, tissue engineering and biomaterials science. This literature review aims to combine the state-of-the-art knowledge in natural and nature-inspired surfaces, with an emphasis on material properties known to affect cell behaviour.

Harnessing Dynamic Wrinkling Surfaces for Smart Displays

Reversible and switchable wrinkling surfaces in response to various external stimuli have extensive potential applications. In this Letter, we prepared the reversible wrinkling on poly(dimethylsiloxane) (PDMS) surfaces, responsive to the solvents, by ultraviolet-ozone (UVO) treatment with/without mechanical prestrain. Based on the solvent-responsive wrinkling, three types of optical transparency dynamics were achieved easily in a single and simple film-substrate system, including (I) completely reversible transparency with controlled relaxation time and isotropic light scattering; (II) completely reversible transparency with anisotropic light scattering and tunable diffusion degree; and (III) incompletely reversible transparency. The reversibility and stability of wrinkles can be controlled by tailoring the solvent type, UVO exposure time, and mechanical prestrain. The underlying mechanisms for the three wrinkling dynamics have been clearly elucidated. The extremely simple material system and the facile but efficient technique pave a novel way for realizing versatile optical dynamics for smart displays.

Spontaneous wrinkle emergence in nascent eumelanin thin films

Self-patterning processes originated by physical stimuli have been extensively documented in thin films, whereas spontaneous wrinkling phenomena due to chemical transformation processes are, to the best of our knowledge, unprecedented. Herein we report a case of spontaneous polymerization-driven surface nano-patterning (∼500 nm) that develops in smooth thin solid films of 5,6-dihydroxyindole (DHI), a major precursor of eumelanin polymers, over a time scale of 30 to 60 days in air at room temperature. The phenomenon can be observed only above a critical film thickness of ∼250 nm and it is affected by exposure to ammonia vapors causing acceleration of the oxidation process. The thickness-dependent onset of wrinkling can be attributed to non-homogeneous rates of oxidation through the film causing slow swelling/expansion of the inner layers followed by fast stiffening and cross-linking in the outer layer exposed to higher oxygen levels.

Modeling and Measurement on the Sliding Behavior of Microgrooved Surfaces

The sliding behavior of anisotropic surfaces is a crucial property to their applications from fundamental research to practical fields. Herein, we propose a theoretical model for analyzing the sliding behavior based on the concept of adhesion energy. Surface Evolver simulation is conducted to determine the adhesion energy per unit area. The microgrooved surfaces are fabricated and characterized to validate the proposed theory. It is found that the apparent contact angle measured along the direction parallel to the strips increases with the increase of microgroove width, while the corresponding sliding angles exhibit an opposite trend. The adhesion energy per unit area has a constant value regardless of the droplet volume. The different sliding behaviors of anisotropic surfaces along the perpendicular and parallel directions are attributed to the difference in the corresponding adhesion energies per unit area. The proposed model is expected to be used for predicting the sliding behavior of anisotropic surfaces.

Dynamics of wrinkling in ultrathin elastic sheets

The wrinkling of thin elastic objects provides a means of generating regular patterning at small scales in applications ranging from photovoltaics to microfluidic devices. Static wrinkle patterns are known to be governed by an energetic balance between the object's bending stiffness and an effective substrate stiffness, which may originate from a true substrate stiffness or from tension and curvature along the wrinkles. Here, we investigate dynamic wrinkling induced by the impact of a solid sphere onto an ultrathin polymer sheet floating on water. The vertical deflection of the sheet's center induced by impact draws material radially inward, resulting in an azimuthal compression that is relieved by the wrinkling of the entire sheet. We show that this wrinkling is truly dynamic, exhibiting features that are qualitatively different to those seen in quasistatic wrinkling experiments. Moreover, we show that the wrinkles coarsen dynamically because of the inhibiting effect of the fluid inertia. This dynamic coarsening can be understood heuristically as the result of a dynamic stiffness, which dominates the static stiffnesses reported thus far, and allows control of wrinkle wavelength.

Time-Dependent Anisotropic Wetting Behavior of Deterministic Structures of Different Strut Widths on Ti6Al4V

This study investigated the wetting behavior of Ti6Al4V surfaces that were groove-structured by means of femtosecond laser irradiation. The material was treated under ambient air conditions by use of a laser wavelength of 1030 nm and a pulse duration of 300 fs. Highly accurate structures with a gap width of 20 µm, a gap depth of 10 µm, and varying strut widths (1–300 µm) were generated and the contact angles in parallel and perpendicular direction were determined using sessile drop method with ultrapure water 1, 8, and 15 days after irradiation. All deterministic surfaces exhibited a pronounced contact angle change over time. The structures showed a strong anisotropic wetting behavior with a maximum contact angle aspect ratio of 2.47 at a strut width of 40 µm and a maximum difference between the parallel and perpendicular contact angle of 47.9° after 1 day.

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.

Polymer Spreading on Unidirectionally Nanotextured Substrates Using Molecular Dynamics

A unidirectional nanotexture alters the wettability of a substrate and can be used to create patterned polymer films, tailored polymer coverage/reflow, or aligned polymer molecules. However, the physical mechanisms underlying polymer spreading on nanoscale textures are not well-understood, and competing theories exist to explain how texture peaks and grooves alter the wettability of a substrate. We use molecular dynamics to simulate polymer spreading on substrates with unidirectional nanoscale textures as a function of texture shape and size and compare to polymer spreading on a flat substrate. We show that the texture groove shape is the primary factor that modifies polymer spreading on unidirectionally nanotextured substrates because the texture groove shape determines the minimum potential energy of a substrate. At the texture groove, the energy potentials of several surfaces combine, which increases polymer attraction and drives spreading along the texture groove. A texture groove also acts as a sink that inhibits polymer spreading perpendicular to the texture. Texture peaks create energy barriers that inhibit polymer spreading perpendicular to the texture, but this is a secondary mechanism that does not significantly affect anisotropic spreading. This research unifies competing theories of anisotropic liquid spreading documented in the literature and aims to aid in the design of nanoscale textures and ultrathin liquid film systems.

Tunable, Ultrasensitive, and Flexible Pressure Sensors Based on Wrinkled Microstructures for Electronic Skins

Flexible pressure sensors play an important role in electronic skins (E-Skins), which mimic the mechanical forces sensing properties of human skin. A rational design for a pressure sensor with adjustable characteristics is in high demand for different application scenarios. Here, we present tunable, ultrasensitive, and flexible pressure sensors based on compressible wrinkled microstructures. Modifying the morphology of polydimethylsiloxane (PDMS) microstructure enables the device to obtain different sensitivities and pressure ranges for different requirements. Furthermore, by intentionally introducing hollow structures in the PDMS wrinkles, our pressure sensor exhibits an ultrahigh sensitivity of 14.268 kPa^-1. The elastic microstructure-based capacitive sensor also possesses a very low detectable pressure limit (1.5 Pa), a fast response time (<50 ms), a wide pressure range, and excellent cycling stability. Implementing respiratory monitoring and vocalization recognition is realized by attaching the flexible pressure sensor onto the chest and throat, respectively, showing its great application potential for disease diagnosis, monitoring, and other advanced clinical/biological wearable technologies.

Anisotropic Sliding of Underwater Bubbles On Microgrooved Slippery Surfaces by One-Step Femtosecond Laser Scanning

Slippery liquid-infused surfaces (SLIPS) with excellent liquid sliding abilities have attracted great attention due to their multifunctions in broad fields. However, current research is mainly concentrated on the isotropic SLIPS, and there are few studies about the fabrication of anisotropic SLIPS and the investigation of anisotropic bubble sliding. Herein, we reported a kind of distinct periodic microgrooved slippery surface (MGSS) by one-step femtosecond laser scanning and realized bubble anisotropic sliding in a liquid system. The MGSS enables the bubble to slide along the direction of grooves but prevents the bubble from sliding along the perpendicular direction to the groove. The mechanism is mainly related to the energy barrier difference caused by the spin-coating oil film thickness and the groove height along the parallel and perpendicular directions. The relationship between the driven force of buoyancy and the resistance of contact angle hysteresis was investigated by theoretical analysis, and the theoretical prediction showed a great adherence with the experimental results. We also studied the influence of laser power and groove period on the degree of anisotropy, and it was found that the groove space has little effect on the degree of anisotropy and the strongest bubble anisotropy can reach nearly 80°. Finally, the MGSS was successfully used in anisotropic bubble transportation on flat and curved surfaces. We believe that such functional surfaces will be promising candidates for manipulating bubble directional sliding behavior and further underwater gas collection.

Measuring Liquid Drop Properties on Nanoscale 1D Patterned Photoresist Structures

This communication reports liquid wetting properties of DI-water on one-dimensional nano-patterned photoresist lines atop a silicon substrate as the pattern period is varied from 0.3- to 1.0-µm. Both constant photoresist height and constant width/height ratios are investigated. The line/period ratio was fixed at 0.3 (0.4) for different measurement sequences. The surface of the photoresist was treated with a short CHF3 reactive ion etch to ensure consistent hydrophobic photoresist: water surface energies. Average parallel contact angle (θ||), average perpendicular contact angle (θ⊥), drop width (W), and drop length (L) at constant volume were measured on nano-patterned surfaces fabricated with interferometric lithography. Both θ|| and θ⊥ contact angles increase as the period (0.3- to 1-μm) increases; the θ|| spreading rate is faster than θ⊥ due to pinning on the grooves resulting in an elongated drop shape. The traditional Wenzel and Cassie-Baxter models of drop contact angles were developed for isotropic random 2D roughness and do not account for the anisotropy induced by the 1D line patterns. The observed angular variations with period are not consistent with either model. Understanding liquid wetting properties and hydrophobicity on 1D silicon surfaces has many applications in lab-on-a-chip, micro/nano-fluidic devices, roll-to-roll nano-imprint fabrication, self-cleaning surfaces, and micro-reactors.

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.

Biomimetic Self-Cleaning Anisotropic Solid Slippery Surface with Excellent Stability and Restoration

Anisotropic slippery surfaces are widely used in anti-fouling, smart control of liquid movement and directional liquid transportation. However, anisotropic slippery liquid-infused porous surfaces (SLIPS) cannot meet the need of practical applications owing to loss and contamination of liquid lubricants. Inspired by solid epicuticular wax on the surface of land plant leaves, we herein report a type of biomimetic anisotropic solid slippery surface (ASSS) based on paraffin wax-incorporated paper with directional micro-grooves. This ASSS material shows anisotropic sliding behavior for liquid droplets with different surface tensions. It is demonstrated to be of excellent stability compared with SLIPS as the solid lubricant cannot be lost and stain the contacting surfaces. It also exhibits outstanding acid and alkali corrosion resistance and restoration capability upon physical damage. Both hydrophilic and hydrophobic contaminants on our ASSS can be self-cleaned by using only water droplets. Our ASSS extends the fabrication of new slippery materials and overcomes some drawbacks of SLIPS.

The influence of plasma treatment on the elasticity of the in situ oxidized gradient layer in PDMS: towards crack-free wrinkling

Controlled surface wrinkling is widely applied for structuring surfaces in the micro- and nano-range. The formation of cracks in the wrinkling process is however limiting applications, and developing approaches towards crack-free wrinkles is therefore vital. To understand crack-formation, we systematically characterized the thickness and mechanics of thin layers formed by O2-plasma-oxidation of polydimethyl siloxane (PDMS) as a function of plasma power and pressure using Atomic Force Microscopy Quantitative Nano-mechanical Mapping (AFM-QNM). We found a nearly constant layer thickness with simultaneously changing Young's moduli for both power and pressure screenings. We determined the respective crack densities, revealing conditions for crack-free wrinkling. Thus we could identify correlations between the intensity of plasma treatment and the cracking behavior. The primary cause for crack-suppression is a continuous elasticity gradient starting within the soft bulk PDMS, and rising up to several hundred MPa at the oxidized layer's surface. With mechanical simulations via the Finite Elements Method (FEM) we were able to demonstrate a noticeable difference in maximal stress intensity σmax between a comparable, but theoretical single layer and a gradient interface. A threshold in tensile stress of σcrit = 14 MPa distinguishes between intact and cracked layers.

Formation of high aspect ratio wrinkles and ridges on elastic bilayers with small thickness contrast

An elastic bilayer composed of a stiff film bonded to a soft substrate forms wrinkles under compression. While these uniform and periodic wrinkles initially grow in amplitude with applied strain, the onset of secondary bifurcations such as period doubling typically limit the aspect ratio (i.e., amplitude divided by wavelength) of wrinkles that can be achieved. Here, we present a simple strategy that employs a supported bilayer with comparable thicknesses of the film and substrate to achieve wrinkles with higher aspect ratio. We use both experiments and finite element simulations to reveal that at small thickness contrast, period doubling can be delayed, allowing the wrinkles to grow uniformly to high aspect ratio. In addition, we show that the periodic wrinkles can evolve through symmetry breaking and transition to a periodic pattern of ridges with even higher aspect ratio.

The effect of the orientation and the height of periodic sub-micrometric texturing on dropwise condensation

Controlling condensation conditions by surface topography is of prime interest. The aim of this work is to investigate the behavior of water droplets condensing on oriented sub-micrometric structures representing ripples with wavelengths around 800 nm. Droplet behavior was studied on different ripples heights and on untextured surfaces. It was specifically looked at how the presence of ripples creates geometrical confinement, and how that influences the deformation and the orientation of single droplets. Results show that the condensed droplets follow the orientation of textured features, especially with high structures height (150 nm). This is shown by the decreasing of droplet roundness with ripples height. The relative number of circular droplets (roundness near to 1) is around 0.6 for 70 nm high ripples and decrease to around 0.2 for 150 nm high ripples. The corresponding relative number on untextured surface is around 0.5. A mechanism, based on droplets pinning and hysteresis, is proposed to explain the influence of the ripples orientation in a vertical plane, onto the droplet deformation during coalescence step. Finally, the presence of ripples is shown to barely impact breath figure dynamics. Number of droplets and mean droplet radius for the textured and untextured surfaces present a comparable evolution.

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.

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.

Liquid crystal polymers with motile surfaces

In analogy with developments in soft robotics it is anticipated that soft robotic functions at surfaces of objects may have a large impact on human life with respect to comfort, health, medical care and energy. In this review, we demonstrate the possibilities and versatilities of liquid crystal networks and elastomers being explored for soft robotics, with an emphasis on motile surface properties, such as topographical dynamics. Typically the surfaces reversibly transfer from a flat state to a pre-designed corrugated state under various stimuli. But also reversible conversion between different corrugated states is feasible. Generally, the driving triggers are heat, light, electricity or contact with pH changing media. Also, the macroscopic effects of those dynamic topographies, such as altering the friction, wettability and/or performing work are illustrated. The review concludes with the existing challenges as well as outlook opportunities.