There are currently three main classes of liquid-repellent surfaces: micro-/nano-structured superhydrophobic surfaces, flat surfaces grafted with `liquid-like' polymer brushes, and lubricated surfaces. Despite recent progress, the mechanistic explanation for the differences in droplet behavior on such surfaces is still under debate. Here, we measured the dissipative force acting on a droplet moving on representatives of these surfaces at different velocities U = 0.01--1 mm/s using a cantilever force sensor with sub-μN accuracy, and correlated it to the contact line dynamics observed using optical interferometry at high spatial (micron) and temporal (lessthan 0.1s) resolutions. We find that the dissipative force---due to very different physical mechanisms at the contact line---is independent of velocity on superhydrophobic surfaces, but depends non-linearly on velocity for flat and lubricated surfaces. The techniques and insights presented here will inform future work on liquid-repellent surfaces and enable their rational design.
Microscale flows of fluids are mainly guided either by solid matrices or by liquid–liquid interfaces. However, the solid matrices are plagued with persistent fouling problems, while liquid–liquid interfaces are limited to low-pressure applications. Here we report a dynamic liquid/solid/gas material containing both air and liquid pockets, which are formed by partially infiltrating a porous matrix with a functional liquid. Using detailed theoretical and experimental data, we show that the distribution of the air- and liquid-filled pores is responsive to pressure and enables the formation and instantaneous recovery of stable liquid–liquid interfaces that sustain a wide range of pressures and prevent channel contamination. This adaptive design is demonstrated for polymeric materials and extended to metal-based systems that can achieve unmatched mechanical and thermal stability. Our platform with its unique adaptive pressure and antifouling capabilities may offer potential solutions to flow control in microfluidics, medical devices, microscale synthesis, and biological assays.
Inorganic microstructured materials are ubiquitous in nature. However, their formation in artificial self-assembly systems is challenging as it involves a complex interplay of competing forces during and after assembly. For example, colloidal assembly requires fine-tuning of factors such as the size and surface charge of the particles and electrolyte strength of the solvent to enable successful self-assembly and minimize crack formation. Co-assembly of templating colloidal particles together with a sol–gel matrix precursor material helps to release stresses that accumulate during drying and solidification, as previously shown for the formation of high-quality inverse opal (IO) films out of amorphous silica. Expanding this methodology to crystalline materials would result in microscale architectures with enhanced photonic, electronic, and catalytic properties. This work describes tailoring the crystallinity of metal oxide precursors that enable the formation of highly ordered, large-area (mm2) crack-free titania, zirconia, and alumina IO films. The same bioinspired approach can be applied to other crystalline materials as well as structures beyond IOs.
A liquid jet can stably bounce off a sufficiently soft gel by following the contour of the dimple created upon impact. This new phenomenon is insensitive to the wetting properties of the gels and was observed for different liquids over a wide range of surface tensions, γ = 24 − 72 mN/m. In contrast, other jet rebound phenomena are typically sensitive to γ: only a high γ jet rebounds off a hard solid (e.g. superhydrophobic surface) and only a low γ jet bounces off a liquid bath. This is because an air layer must be stabilized between the two interfaces. For a soft gel, no air layer is necessary and the jet rebound remains stable even when there is direct liquid-gel contact.
Structural color arises from the patterning of geometric features or refractive indices of the constituent materials on the length-scale of visible light. Many different organisms have developed structurally colored materials as a means of creating multifunctional structures or displaying colors for which pigments are unavailable. By studying such organisms, scientists have developed artificial structurally colored materials that take advantage of the hierarchical geometries, frequently employed for structural coloration in nature. These geometries can be combined with absorbers—a strategy also found in many natural organisms—to reduce the effects of fabrication imperfections. Furthermore, artificial structures can incorporate materials that are not available to nature—in the form of plasmonic nanoparticles or metal layers—leading to a host of novel color effects. Here, we explore recent research involving the combination of different geometries and materials to enhance the structural color effect or to create entirely new effects, which cannot be observed otherwise.
Catalysis is one of the most sophisticated areas of materials research that encompasses a diverse set of materials and phenomena occurring on multiple length and time scales. Designing catalysts that can be broadly applied toward global energy and environmental challenges requires the development of universal frameworks for complex catalytic systems through rational and independent (or quasi-independent) optimization of multiple structural and compositional features. Toward addressing this goal, a modular platform is presented in which sacrificial organic colloids bearing catalytic nanoparticles on their surfaces self-assemble with matrix precursors, simultaneously structuring the resulting porous networks and fine-tuning the locations of catalyst particles. This strategy allows combinatorial variations of the material building blocks and their organization, in turn providing numerous degrees of freedom for optimizing the material’s functional properties, from the nanoscale to the macroscale. The platform enables systematic studies and rational design of efficient and robust systems for a wide range of catalytic and photocatalytic reactions, as well as their integration into industrial and other real-life environments.
The developing field of active, stimuli-responsive materials is in need for new dynamic architectures that may offer unprecedented chemomechanical switching mechanisms. Toward this goal, syntheses of polymerizable bipyridine ligands, bis(4-vinylbenzyl)[2,2′-bipyridine]-4,4′-dicarboxylate and N4,N4′-bis(4-vinylphenyl)-2,2′-bipyridine-4,4′-dicarboxamide, and a number of redox-active Ruthenium(II) and Iron(II) complexes with them are reported. Detailed characterizations by NMR, Fourier transform infrared spectroscopy, high-resolution mass-spectrometry, X-ray, and cyclic voltammetry show that the topology of these molecules allows them to serve as both comonomers and crosslinkers in polymerization reactions. Electronic properties of the ligands are tunable by choosing carboxylate- or carboxamido-linkages between the core and the vinylaryl moieties, leading to a library of Ru and Fe complexes with the M(III)/M(II) standard redox potentials suitable for catalyzing self-oscillating Belousov–Zhabotinskii (BZ) reaction. New poly(N-isopropylacrylamide)-based redox-responsive functional gels containing hydrophilic comonomers, which have been prepared using representative Ru bpy complexes as both a crosslinker and redox-active catalyst, exhibit a unique feature: their swelling/contraction mode switches its dependence on the oxidation state of the Ru center, upon changing the ratio of comonomers in the hybrid gel network. The BZ self-oscillations of such crosslinked hydrogels have been observed and quantified for both supported film and free-standing gel samples, demonstrating their potential as chemomechanically active modules for new functional materials.
Recently, there has been much interest in using lubricated surfaces to achieve extreme liquid repellency: a foreign droplet immiscible with the underlying lubricant layer was shown to slide o at a small tilt angle <5◦ . This behaviour was hypothesized to arise from a thin lubricant overlayer film sandwiched between the droplet and solid substrate, but this has not been observed experimentally. Here, using thin-film interference, we are able to visualize the intercalated film under both static and dynamic conditions. We further demonstrate that for a moving droplet, the film thickness follows the Landau–Levich–Derjaguin law. The droplet is therefore oleoplaning—akin to tyres hydroplaning on a wet road—with minimal dissipative force and no contact line pinning. The techniques and insights presented in this study will inform future work on the fundamentals of wetting for lubricated surfaces and enable their rational design.
Structurally colored materials are often used for their resistance to photobleaching and their complex viewing-direction-dependent optical properties. Frequently, absorption has been added to these types of materials in order to improve the color saturation by mitigating the effects of nonspecific scattering that is present in most samples due to imperfect manufacturing procedures. The combination of absorbing elements and structural coloration often yields emergent optical properties. Here, a new hybrid architecture is introduced that leads to an interesting, highly directional optical effect. By localizing absorption in a thin layer within a transparent, structurally colored multilayer material, an optical Janus effect is created, wherein the observed reflected color is different on one side of the sample than on the other. A systematic characterization of the optical properties of these structures as a function of their geometry and composition is performed. The experimental studies are coupled with a theoretical analysis that enables a precise, rational design of various optical Janus structures with highly controlled color, pattern, and fabrication approaches. These asymmetrically colored materials will open applications in art, architecture, semitransparent solar cells, and security features in anticounterfeiting materials.
Mechanical forces in the cell’s natural environment have a crucial impact on growth, differentiation and behaviour. Few areas of biology can be understood without taking into account how both individual cells and cell networks sense and transduce physical stresses. However, the field is currently held back by the limitations of the available methods to apply physiologically relevant stress profiles on cells, particularly with sub-cellular resolution, in controlled in vitro experiments. Here we report a new type of active cell culture material that allows highly localized, directional and reversible deformation of the cell growth substrate, with control at scales ranging from the entire surface to the subcellular, and response times on the order of seconds. These capabilities are not matched by any other method, and this versatile material has the potential to bridge the performance gap between the existing single cell micro-manipulation and 2D cell sheet mechanical stimulation techniques.
We describe the behavior of a temperature-responsive hydrogel actuated integrated responsive structure (HAIRS). The structure is constructed by embedding a rigid high-aspect-ratio post in a layer of poly(Nisopropylacrylamide) (PNIPAM) hydrogel which is bonded to a rigid substrate. As the hydrogel contracts, the post abruptly tilts. The HAIRS has demonstrated its broad applications in generating reversible micropattern formation, active optics, tunable wettability, and artificial homeostasis. To quantitatively describe and predict the system behavior, we construct an analytical model combining the structural instability, i.e. buckling of the post, and the material instability, i.e. the volume phase transition of PNIPAM hydrogel. The two instabilities of the system result in a large hysteresis in response to heating and cooling processes. Experimental results validate the predicted phenomenon of the abrupt tilting as temperature and large hysteresis in a heating-and-cooling cycle in the PNIPAM actuated HAIRS. Based on this model, we further discuss the influence of the material properties on the actuation of the structure.
Tissue engineering using whole, intact cell sheets has shown promise in many cell-based therapies. However, current systems for the growth and release of these sheets can be expensive to purchase or difficult to fabricate, hindering their widespread use. Here, we describe a new approach to cell sheet release surfaces based on silicone oil-infused polydimethylsiloxane. By coating the surfaces with a layer of fibronectin (FN), we were able to grow mesenchymal stem cells to densities comparable to those of tissue culture polystyrene controls (TCPS). Simple introduction of oil underneath an edge of the sheet caused it to separate from the substrate. Characterization of sheets post-transfer showed that they retain their FN layer and morphology, remain highly viable, and are able to grow and proliferate normally after transfer. We expect that this method of cell sheet growth and detachment may be useful for low-cost, flexible, and customizable production of cellular layers for tissue engineering.
Surface topography has been introduced as a new tool to coordinate cell selection, growth, morphology, and differentiation. The materials explored so far for making such structural surfaces are mostly rigid and impermeable. Hydrogel, on the other hand, was proved a better synthetic media for cell culture because of its biocompatibility, softness, and high permeability. Herein, we fabricated a poly(2-hydroxyethyl methacrylate) (pHEMA) hydrogel substrate with high-aspectratio surface microfeatures. Such structural surface could effectively guide the orientation and shape of human mesenchymal stem cells (HMSCs). Notably, on the flat hydrogel surface, cells rounded up, whereas on the microplate patterned hydrogel surface, cells elongated and aligned along the direction parallel to the plates. The microplates were 2 μm thick, 20 μm tall, and 10−50 μm wide. The interplate spacing was 5−15 μm, and the intercolumn spacing was 5 μm. The elongation of cell body was more pronounced on the patterns with narrower interplate spacing and wider plates. The cells behaved like soft solid. The competition between surface energy and elastic energy defined the shape of the cells on the structured surfaces. The soft permeable hydrogel scaffold with surface structures was also demonstrated as being viable for longterm cell culture, and could be used to generate interconnected tissues with finely tuned cell morphology and alignment across a few centimeter sizes.
Inspection devices are frequently occluded by highly contaminating fluids that disrupt the visual field and their effective operation. These issues are particularly striking in endoscopes, where the diagnosis and treatment of diseases are compromised by the obscuring of the operative field by body fluids. Here we demonstrate that the application of a liquid-infused surface coating strongly repels sticky biological secretions and enables an uninterrupted field of view. Extensive bronchoscopy procedures performed in vivo on a porcine model shows significantly reduced fouling, resulting in either unnecessary or ∼10–15 times shorter and less intensive lens clearing procedures compared with an untreated endoscope.
Camera-guided instruments, such as endoscopes, have become an essential component of contemporary medicine. The 15–20 million endoscopies performed every year in the United States alone demonstrate the tremendous impact of this technology. However, doctors heavily rely on the visual feedback provided by the endoscope camera, which is routinely compromised when body fluids and fogging occlude the lens, requiring lengthy cleaning procedures that include irrigation, tissue rubbing, suction, and even temporary removal of the endoscope for external cleaning. Bronchoscopies are especially affected because they are performed on delicate tissue, in high-humidity environments with exposure to extremely adhesive biological fluids such as mucus and blood. Here, we present a repellent, liquid-infused coating on an endoscope lens capable of preventing vision loss after repeated submersions in blood and mucus. The material properties of the coating, including conformability, mechanical adhesion, transparency, oil type, and biocompatibility, were optimized in comprehensive in vitro and ex vivo studies. Extensive bronchoscopy procedures performed in vivo on porcine lungs showed significantly reduced fouling, resulting in either unnecessary or ∼10–15 times shorter and less intensive lens clearing procedures compared with an untreated endoscope. We believe that the material developed in this study opens up opportunities in the design of next-generation endoscopes that will improve visual field, display unprecedented antibacterial and antifouling properties, reduce the duration of the procedure, and enable visualization of currently unreachable parts of the body, thus offering enormous potential for disease diagnosis and treatment.
Many industries require irreversibly responsive materials for use as sensors or detectors of environmental exposure. We describe the synthesis and fabrication of a nontoxic surface coating that reports oxygen exposure of the substrate material through irreversible formation of colored spots. The coating consists of a selectively permeable rubber film that contains the colorless organic precursors to darkly pigmented synthetic melanin. Melanin synthesis within the film is triggered by exposure to molecular oxygen. The selectively permeable rubber film regulates the rate of oxygen diffusion, enabling independent control of the sensitivity and response time of the artificial melanosome, while preventing leaching of melanin or its precursors.
Nature evolved a variety of hierarchical structures that produce sophisticated functions. Inspired by these natural materials, colloidal self-assembly provides a convenient way to produce structures from simple building blocks with a variety of complex functions beyond those found in nature. In particular, colloid-based porous materials (CBPM) can be made from a wide variety of materials. The internal structure of CBPM also has several key attributes, namely porosity on a sub-micrometer length scale, interconnectivity of these pores, and a controllable degree of order. The combination of structure and composition allow CBPM to attain properties important for modern applications such as photonic inks, colorimetric sensors, self-cleaning surfaces, water purification systems, or batteries. This review summarizes recent developments in the field of CBPM, including principles for their design, fabrication, and applications, with a particular focus on structural features and materials' properties that enable these applications. We begin with a short introduction to the wide variety of patterns that can be generated by colloidal self-assembly and templating processes. We then discuss different applications of such structures, focusing on optics, wetting, sensing, catalysis, and electrodes. Different fields of applications require different properties, yet the modularity of the assembly process of CBPM provides a high degree of tunability and tailorability in composition and structure. We examine the significance of properties such as structure, composition, and degree of order on the materials' functions and use, as well as trends in and future directions for the development of CBPM.
Controlling dropwise condensation is fundamental to water-harvesting systems, desalination, thermal power generation, air conditioning, distillation towers, and numerous other applications. For any of these, it is essential to design surfaces that enable droplets to grow rapidly and to be shed as quickly as possible. However, approaches based on microscale, nanoscale or molecular-scale textures suffer from intrinsic trade-offs that make it difficult to optimize both growth and transport at once. Here we present a conceptually different design approach—based on principles derived from Namib desert beetles, cacti, and pitcher plants—that synergistically combines these aspects of condensation and substantially outperforms other synthetic surfaces. Inspired by an unconventional interpretation of the role of the beetle’s bumpy surface geometry in promoting condensation, and using theoretical modelling, we show how to maximize vapour diffusion flux at the apex of convex millimetric bumps by optimizing the radius of curvature and cross-sectional shape. Integrating this apex geometry with a widening slope, analogous to cactus spines, directly couples facilitated droplet growth with fast directional transport, by creating a free-energy profile that drives the droplet down the slope before its growth rate can decrease. This coupling is further enhanced by a slippery, pitcher-plant-inspired nanocoating that facilitates feedback between coalescence-driven growth and capillary-driven motion on the way down. Bumps that are rationally designed to integrate these mechanisms are able to grow and transport large droplets even against gravity and overcome the effect of an unfavourable temperature gradient. We further observe an unprecedented sixfold-higher exponent of growth rate, faster onset, higher steady-state turnover rate, and a greater volume of water collected compared to other surfaces. We envision that this fundamental understanding and rational design strategy can be applied to a wide range of water-harvesting and phase-change heat-transfer applications.
Passive anti-icing surfaces, or icephobic surfaces, are an area of great interest because of their significant economic, energy and safety implications in the prevention and easy removal of ice in many facets of society. The complex nature of icephobicity, which requires performance in a broad range of icing scenarios, creates many challenges when designing ice-repellent surfaces. Although superhydrophobic surfaces incorporating micro- or nanoscale roughness have been shown to prevent ice accumulation under certain conditions, the same roughness can be detrimental in other environments. Surfaces that present a smooth liquid interface can eliminate some of the drawbacks of textured superhydrophobic surfaces, but additional study is needed to fully realize their potential. As attention begins to shift towards alternative anti-icing strategies, it is important to consider and to understand the nature of ice repellency in all environments to identify the limitations of current solutions and to design new materials with robust icephobicity.
Although common in biological systems, synthetic self-assembly routes to complex 3D photonic structures with tailored degrees of disorder remain elusive. Here we show how liquids can be used to finely control disorder in porous 3D photonic crystals, leading to complex and hierarchical geometries. In these optofluidic crystals, dynamically tunable disorder is superimposed onto the periodic optical structure through partial wetting or evaporation. In both cases, macroscopic symmetry breaking is driven by subtle sub-wavelength variations in the pore geometry. These variations direct site-selective infiltration of liquids through capillary interactions. Incorporating cross-linkable resins into our liquids, we developed methods to freeze in place the filling patterns at arbitrary degrees of partial wetting and intermediate stages of drying. These percolation lithography techniques produced permanent photonic structures with adjustable disorder. By coupling strong changes in optical properties to subtle differences in fluid behavior, optofluidic crystals may also prove useful in rapid analysis of liquids.