Dewetting of conducting polymer inkjet droplets on patterned surfaces

Stepwise Aggregation Control of PEDOT:PSS Enabled High-Conductivity, High-Resolution Printing of Polymer Electrodes for Transparent Organic Phototransistors

Electrohydrodynamic (EHD) jet printing is a widely employed technology to create high-resolution patterns and thus has enormous potential for circuit production. However, achieving both high conductivity and high resolution in printed polymer electrodes is a challenging task. Here, by modulating the aggregation state of the conducting polymer in the solution and solid phases, a stable and continuous jetting of PEDOT:PSS is realized, and high-conductivity electrode arrays are prepared. The line width reaches less than 5 μm with a record-high conductivity of 1250 S/cm. Organic field-effect transistors (OFETs) are further developed by combining printed source/drain electrodes with ultrathin organic semiconductor crystals. These OFETs show great light sensitivity, with a specific detectivity (D*) value of 2.86 × 1014 Jones. In addition, a proof-of-concept fully transparent phototransistor is demonstrated, which opens up new pathways to multidimensional optical imaging.

Molecular Electronics: From Nanostructure Assembly to Device Integration

Integrated electronics and optoelectronics based on organic semiconductors have attracted considerable interest in displays, photovoltaics, and biosensing owing to their designable electronic properties, solution processability, and flexibility. Miniaturization and integration of devices are growing trends in molecular electronics and optoelectronics for practical applications, which requires large-scale and versatile assembly strategies for patterning organic micro/nano-structures with simultaneously long-range order, pure orientation, and high resolution. Although various integration methods have been developed in past decades, molecular electronics still needs a versatile platform to avoid defects and disorders due to weak intermolecular interactions in organic materials. In this perspective, a roadmap of organic integration technologies in recent three decades is provided to review the history of molecular electronics. First, we highlight the importance of long-range-ordered molecular packing for achieving exotic electronic and photophysical properties. Second, we classify the strategies for large-scale integration of molecular electronics through the control of nucleation and crystallographic orientation, and evaluate them based on factors of resolution, crystallinity, orientation, scalability, and versatility. Third, we discuss the multifunctional devices and integrated circuits based on organic field-effect transistors (OFETs) and photodetectors. Finally, we explore future research directions and outlines the need for further development of molecular electronics, including assembly of doped organic semiconductors and heterostructures, biological interfaces in molecular electronics and integrated organic logics based on complementary FETs.

Transfer-Printing of Insoluble Conducting Polymer for Soft 3D Conformal All-Organic Transistors

The development of high-precision insoluble conducting polymer patterns for soft electronics is extremely challenging, mainly because of the incompatibility of the synthesis process with the underlying layers. In this study, a novel transfer-printing method is designed that enables the fabrication of photolithographic insoluble conducting polypyrrole (PPy) electrode patterns on soft substrates with high precision, demonstrating compatibility with various soft organic functional layers. Excellent mechanical stability, good biocompatibility, ultra-smooth surface, and outstanding conformability are observed. The photolithographic PPy electrode patterns, combined with an elastic organic semiconductor and dielectric, produce conformal all-organic transistors with mobility of 1.8 cm2 V-1 s-1 . This study paves the way to use insoluble conducting polymers to develop complex, high-density flexible patterns and offers a promising organic electrode for the new-generation soft all-organic electronics.

Heterogenous Slippery Surfaces: Enabling Spontaneous and Rapid Transport of Viscous Liquids with Viscosities Exceeding 10 000 mPa s

Superhydrophobic and slippery lubricant-infused surfaces have garnered significant attention for their potential to passively transport low-viscosity liquids like water (1 mPa s). Despite exciting progress, these designs have proven ineffective for transporting high-viscosity liquids such as polydimethylsiloxane (5500 mPa s) due to their inherent limitations imposed by the homogenous surface design, resulting in high viscous drags and compromised capillary forces. Here, a heterogenous water-infused divergent surface (WIDS) is proposed that achieves spontaneous, rapid, and long-distance transport of viscous liquids. WIDS reduces viscous drag by spatially isolating the viscous liquids and surface roughness through its heterogenous, slippery topological design, and generates capillary forces through its heterogenous wetting distributions. The essential role of surface heterogeneity in viscous liquid transport is theoretically and experimentally verified. Remarkably, such a heterogenous paradigm enables transporting liquids with viscosities exceeding 12 500 mPa s, which is two orders of magnitude higher than state-of-the-art techniques. Furthermore, this heterogenous design is generic for various viscous liquids and can be made flexible, making it promising for various systems that require viscous liquid management, such as micropatterning.

Diameter-Optimum Spreading for the Impinging of Water Nanodroplets on Solid Surfaces

The impinging of water nanodroplets on solid surfaces is crucial to many nanotechnologies. Through large-scale molecular dynamics simulations, the size effect on the spreading of water nanodroplets after impinging on hydrophilic, graphite, and hydrophobic surfaces under low impinging velocities has been systematically studied. The spreading rates of nanodroplets first increase and then decrease and gradually become constant with the increase of nanodroplet diameter. The nanodroplets with the diameters of 17-19 nm possess the highest spreading rates because of the combined effect of the strongest interfacial interaction and the strongest surface interaction within water molecules. The highest water molecule densities, hydrogen bond numbers, and dielectric constants of interface and surface layers mainly contribute to the lowest interface work of adhesion and surface tension values at optimal diameters. These results unveil the nonmonotonic characteristics of spreading velocity, interface work of adhesion and surface tension with nanodroplet diameter for nanodroplets on solid surfaces.

Direct observation of spreading precursor liquids in a corner

Precursor liquid is a nanoscale liquid creeping ahead of the macroscopic edge of spreading liquids, whose behaviors tightly correlate with the three-phase reaction efficiency and patterning accuracy. However, the important spatial-temporal characteristic of the precursor liquid still remains obscure because its real-time spreading process has not been directly observed. Here, we report that the spreading ionic liquid precursors in a silicon corner can be directly captured on video using in situ scanning electron microscopy. In situ spreading videos show that the precursor liquid spreads linearly over time ([Formula: see text]) rather than obeying the classic Lucas-Washburn law ([Formula: see text]) and possesses a characteristic width of ∼250-310 nm. Theoretical analyses and molecular dynamics simulations demonstrate that the unique behaviors of precursor liquids originate from the competing effect of van der Waals force and surface energy. These findings provide avenues for directly observing liquid/solid interfacial phenomena on a microscopic level.

Spontaneous Transport Mechanics of Water Droplets under a Synergistic Action of Designed Pattern and Non-Wetting Gradient

The controllable spontaneous transport of water droplets on solid surfaces has a broad application background in daily life. Herein, a patterned surface with two different non-wetting characteristics was developed to control the droplet transport behavior. Consequently, the patterned surface exhibited great water-repellant properties in the superhydrophobic region, and the water contact angle reached 160° ± 0.2°. Meanwhile, the water contact angle on the wedge-shaped hydrophilic region dropped to 22° after UV irradiation treatment. On this basis, the maximum transport distance of water droplets could be observed on the sample surface with a small wedge angle of 5° (10.62 mm), and the maximum average transport velocity of droplets was obtained on the sample surface with a large wedge angle of 10° (218.01 mm/s). In terms of spontaneous droplet transport on an inclined surface (4°), both the 8 μL droplet and 50 μL droplet could move upward against gravity, which showed that the sample surface possessed an obvious driving force for droplet transport. Surface non-wetting gradient and the wedge-shaped pattern provided unbalanced surface tension to produce the driving forces in the process of droplet transport, and the Laplace pressure as well is produced inside the water droplet during this process. This work provides a new strategy to develop a patterned superhydrophobic surface for droplet transport.

Laser printed microelectronics

Printed organic and inorganic electronics continue to be of large interest for sensors, bioelectronics, and security applications. Many printing techniques have been investigated, albeit often with typical minimum feature sizes in the tens of micrometer range and requiring post-processing procedures at elevated temperatures to enhance the performance of functional materials. Herein, we introduce laser printing with three different inks, for the semiconductor ZnO and the metals Pt and Ag, as a facile process for fabricating printed functional electronic devices with minimum feature sizes below 1 µm. The ZnO printing is based on laser-induced hydrothermal synthesis. Importantly, no sintering of any sort needs to be performed after laser printing for any of the three materials. To demonstrate the versatility of our approach, we show functional diodes, memristors, and a physically unclonable function based on a 6 × 6 memristor crossbar architecture. In addition, we realize functional transistors by combining laser printing and inkjet printing.

Stretchable conductors for stretchable field-effect transistors and functional circuits

Stretchable electronics have received intense attention due to their broad application prospects in many areas, and can withstand large deformations and form close contact with curved surfaces. Stretchable conductors are vital components of stretchable electronic devices used in wearables, soft robots, and human-machine interactions. Recent advances in stretchable conductors have motivated basic scientific and technological research efforts. Here, we outline and analyse the development of stretchable conductors in transistors and circuits, and examine advances in materials, device engineering, and preparation technologies. We divide the existing approaches to constructing stretchable transistors with stretchable conductors into the following two types: geometric engineering and intrinsic stretchability engineering. Finally, we consider the challenges and outlook in this field for delivering stretchable electronics.

Influence of Surface Roughness on Droplet Evaporation and Absorption: Insights into Experiments from Lubrication-Theory-Based Models

While solid substrates are often idealized as being perfectly smooth, all real surfaces possess some level of topographical and chemical heterogeneity. This heterogeneity can greatly influence droplet dynamics. Mathematical models based on lubrication theory that account for surface roughness reveal how topographical defects induce contact-line pinning and affect the deposition patterns of colloidal particles suspended in the droplet. Contact-line pinning profoundly changes the behavior of droplet evaporation on horizontal and inclined impermeable substrates and droplet absorption on horizontal permeable substrates. Models accounting for surface roughness yield predictions that are qualitatively consistent with experimental observations and also provide insight into the underlying physical mechanisms. These models are a foundation for the exploration of a rich array of problems concerning droplet dynamics which are of both fundamental and practical interest.

Ultrastable Super-Hydrophobic Surface with an Ordered Scaly Structure for Decompression and Guiding Liquid Manipulation

Directional droplet manipulation is very crucial in microfluidics, intelligent liquid management, etc. However, excessive liquid pressure tends to destroy the solid-gas-liquid (SAL) composite interface, creating a highly adhesive surface, which is not conducive to liquid transport. Herein, we propose a strategy to enhance the surface durability, in which the surface cannot withstand liquid pressure only by "blocking" but must instead guide liquid transport for "decompression". Learning from the water resistance of water strider legs and the drag reduction of shark skin, we present a continuous integrated system to obtain an ultrastable super-hydrophobic surface with a highly ordered scaly structure via a liquid flow-induced alignment method for lossless unidirectional liquid transport. The nonwetting scaly structure can both buffer liquid pressure and drive droplet motion to further reduce the vertical pressure of the liquid. Moreover, droplets can be manipulated unidirectionally using a voice. This work could aid in manufacturing scalable anisotropic micro-nanostructure surfaces, which inspires efforts in realizing lossless continuous liquid control on demand and related microfluidic applications.

Self-assembly of highly ordered micro- and nanoparticle deposits

The evaporation of particle-laden sessile droplets is associated with capillary-driven outward flow and leaves nonuniform coffee-ring-like particle patterns due to far-from-equilibrium effects. Traditionally, the surface energies of the drop and solid phases are tuned, or external forces are applied to suppress the coffee-ring; however, achieving a uniform and repeatable particle deposition is extremely challenging. Here, we report a simple, scalable, and noninvasive technique that yields uniform and exceptionally ordered particle deposits on a microscale surface area by placing the droplet on a near neutral-wet shadow mold attached to a hydrophilic substrate. The simplicity of the method, no external forces, and no tuning materials' physiochemical properties make the present generic approach an excellent candidate for a wide range of sensitive applications. We demonstrate the utility of this method for fabricating ordered mono- and multilayer patternable coatings, producing nanofilters with controlled pore size, and creating reproducible functionalized nanosensors.

Droplet Bouncing: Fundamentals, Regulations, and Applications

Droplet impact is a ubiquitous phenomenon in nature, daily life, and industrial processes. It is thus crucial to tune the impact outcomes for various applications. As a special outcome of droplet impact, the bouncing of droplets keeps the form of the droplets after the impact and minimizes the energy loss during the impact, being beneficial in many applications. A unified understanding of droplet bouncing is in high demand for effective development of new techniques to serve applications. This review shows the fundamentals, regulations, and applications of millimeter-sized droplet bouncing on solid surfaces and same/miscible liquids (liquid pool and another droplet). Regulation methods and current applications are summarized, and potential directions are proposed.

Challenges, Prospects, and Emerging Applications of Inkjet-Printed Electronics: A Chemist's Point of View

Driven by the development of new functional inks, inkjet-printed electronics has achieved several milestones upon moving from the integration of simple electronic elements (e.g., temperature and pressure sensors, RFID antennas, etc.) to high-tech applications (e.g. in optoelectronics, energy storage and harvesting, medical diagnosis). Currently, inkjet printing techniques are limited by spatial resolution higher than several micrometers, which sets a redhibitorythreshold for miniaturization and for many applications that require the controlled organization of constituents at the nanometer scale. In this Review, we present the physico-chemical concepts and the equipment constraints underpinning the resolution limit of inkjet printing and describe the contributions from molecular, supramolecular, and nanomaterials-based approaches for their circumvention. Based on these considerations, we propose future trajectories for improving inkjet-printing resolution that will be driven and supported by breakthroughs coming from chemistry. Please check all text carefully as extensive language polishing was necessary. Title ok? Yes.

Electrolyte-Gated Organic Field-Effect Transistors for Quantitative Monitoring of the Molecular Dynamics of Crystallization at the Solid-Liquid Interface

Quantitative measurements of molecular dynamics at the solid-liquid interface are of crucial importance in a wide range of fields, such as heterogeneous catalysis, energy storage, nanofluidics, biosensing, and crystallization. In particular, the molecular dynamics associated with nucleation and crystal growth is very challenging to study because of the poor sensitivity or limited spatial/temporal resolution of the most widely used analytical techniques. We demonstrate that electrolyte-gated organic field-effect transistors (EGOFETs) are able to monitor in real-time the crystallization process in an evaporating droplet. The high sensitivity of these devices at the solid-liquid interface, through the electrical double layer and signal amplification, enables the quantification of changes in solute concentration over time and the transport rate of molecules at the solid-liquid interface during crystallization. Our results show that EGOFETs offer a highly sensitive and powerful, yet simple approach to investigate the molecular dynamics of compounds crystallizing from water.

Concentration gradients in evaporating binary droplets probed by spatially resolved Raman and NMR spectroscopy

Imagine you spill your drink and miss some spots when cleaning up. The next morning you notice that the stains look quite different on different surfaces. What has happened? In droplets of liquid mixtures, the components evaporate at different rates, which leads to gradients in concentration and surface tension. These gradients can cause, for example, so-called Marangoni flows, which in turn affect the evaporation process. To better understand evaporation-induced liquid flows, the concentration gradients have to be measured without disturbing the liquid. Marker molecules might be surface-active or even may affect the evaporation process. We report here on marker-free and contactless measurements of concentrations by spatially resolved Raman and NMR spectroscopy in evaporating binary droplets.

Understanding the evaporation process of binary sessile droplets is essential for optimizing various technical processes, such as inkjet printing or heat transfer. Liquid mixtures whose evaporation and wetting properties may differ significantly from those of pure liquids are particularly interesting. Concentration gradients may occur in these binary droplets. The challenge is to measure concentration gradients without affecting the evaporation process. Here, spectroscopic methods with spatial resolution can discriminate between the components of a liquid mixture. We show that confocal Raman microscopy and spatially resolved NMR spectroscopy can be used as complementary methods to measure concentration gradients in evaporating 1-butanol/1-hexanol droplets on a hydrophobic surface. Deuterating one of the liquids allows analysis of the local composition through the comparison of the intensities of the C–H and C–D stretching bands in Raman spectra. Thus, a concentration gradient in the evaporating droplet was established. Spatially resolved NMR spectroscopy revealed the composition at different positions of a droplet evaporating in the NMR tube, an environment in which air exchange is less pronounced. While not being perfectly comparable, both methods—confocal Raman and spatially resolved NMR experiments—show the presence of a vertical concentration gradient as 1-butanol/1-hexanol droplets evaporate.

Pattern formation in thin polymeric films via electrohydrodynamic patterning

The free surface of a thin polymeric film is often unstable and deforms into various micro-/nano-patterns under an externally applied electric field. This paper reviews a recent patterning technique, electrohydrodynamic patterning (EHDP), a straightforward, cost-effective and contactless bottom-up method. The theoretical and numerical studies of EHDP are shown. How the characteristic wavelength and the characteristic time depend on both the external conditions (such as voltage, film thickness, template-substrate spacing) and the initial polymer properties (such as rheological property, electrical property and surface tension) is theoretically and experimentally discussed. Various possible strategies for fabricating high-aspect-ratio or hierarchical patterns are theoretically and experimentally reviewed. Aligning and ordering of the anisotropic polymers by EHDP is emphasized. A perspective, including novelty and limitations of the methods, particularly in comparison to some conventional patterning techniques, and a possible future direction of research, is presented.

Inkjet Printing: A Viable Technology for Biosensor Fabrication

Printing technology promises a viable solution for the low-cost, rapid, flexible, and mass fabrication of biosensors. Among the vast number of printing techniques, screen printing and inkjet printing have been widely adopted for the fabrication of biosensors. Screen printing provides ease of operation and rapid processing; however, it is bound by the effects of viscous inks, high material waste, and the requirement for masks, to name a few. Inkjet printing, on the other hand, is well suited for mass fabrication that takes advantage of computer-aided design software for pattern modifications. Furthermore, being drop-on-demand, it prevents precious material waste and offers high-resolution patterning. To exploit the features of inkjet printing technology, scientists have been keen to use it for the development of biosensors since 1988. A vast number of fully and partially inkjet-printed biosensors have been developed ever since. This study presents a short introduction on the printing technology used for biosensor fabrication in general, and a brief review of the recent reports related to virus, enzymatic, and non-enzymatic biosensor fabrication, via inkjet printing technology in particular.

Long-Range-Ordered Assembly of Micro-/Nanostructures at Superwetting Interfaces

On-chip integration of solution-processable materials imposes stringent and simultaneous requirements of controlled nucleation and growth, tunable geometry and dimensions, and long-range-ordered assembly, which is challenging in solution process far from thermodynamic equilibrium. Superwetting interfaces, underpinned by programmable surface chemistry and topography, are promising for steering transport, dewetting, and microfluid dynamics of liquids, thus opening a new paradigm for micro-/nanostructure assembly in solution process. Herein, assembly methods on the basis of superwetting interfaces are reviewed for constructing long-range-ordered micro-/nanostructures. Confined capillary liquids, including capillary bridges and capillary corner menisci realized by controlling local wettability and surface topography, are highlighted for simultaneously attained deterministic patterning and long-range order. The versatility and robustness of confined capillary liquids are discussed with assembly of single-crystalline micro-/nanostructures of organic semiconductors, metal-halide perovskites, and colloidal-nanoparticle superlattices, which lead to enhanced device performances and exotic functionalities. Finally, a perspective for promising directions in this realm is provided.

Design of Organic-Free Superhydrophobic TiO2 with Ultraviolet Stability or Ultraviolet-Induced Switchable Wettability

Superhydrophobic TiO₂ with great application potential is mainly obtained by surface modification with low surface energy organics, which is easily degraded under sunlight irradiation, which results in the loss of superhydrophobic properties. Herein, we developed a room-temperature pulsed chemical vapor deposition (pulsed CVD) method to develop amorphous TiO₂-deposited TiO₂ nanoparticles. The ultraviolet stability/ultraviolet-induced reversible wettability switch had been simultaneously realized by different and controllable deposition cycles of amorphous TiO₂. The superhydrophobic properties of the organic-free TiO₂ were determined by the micrometer-nanometer-sub-nanometer multiscale structure, the multiscale pore structure, and the large Young's contact angle resulting from carboxylic acid adsorption. Also, we found that the adsorption rate and adsorption stability of oxygen and water at the surface oxygen vacancies were the key to facilitate the reversible switching between superhydrophilic and superhydrophobic states, which was well demonstrated by experimental characterization and theoretical simulation. In addition, we also found that the resistance of dense amorphous TiO₂ films on the TiO₂ surface to the migration of photogenerated electrons and holes was the key to maintain the stable superhydrophobic properties of superhydrophobic TiO₂ under ultraviolet illumination. The powders were strongly ground and the coating surface was rubbed on the surface of the sandpaper, which still maintained superhydrophobic properties, providing favorable conditions for the application of superhydrophobic TiO₂. This work modulates the ultraviolet stability and dark/ultraviolet-induced switchable superhydrophobicity/superhydrophilicity of coated TiO₂ by simply adjusting the number of deposition times in a pulsed CVD process for the first time, thus contributing to the development of organic-free superhydrophobic TiO₂.

Smart surfaces with reversibly switchable wettability: Concepts, synthesis and applications

As a growing hot research topic, manufacturing smart switchable surfaces has attracted much attention in the past a few years. The state-of-the-art study on reversibly switchable wettability of smart surfaces has been presented in this systematic review. External stimuli are brought about to render the alteration in chemical conformation and surface morphology to drive the wettability switch. Here, starting from the fundamental theories related to the surfaces wetting principles, highlights on different triggers for switchable wettability, such as pH, light, ions, temperature, electric field, gas, mechanical force, and multi-stimuli are discussed. Different applications that have various wettability requirement are targeted, including oil-water separation, droplets manipulation, patterning, liquid transport, and so on. This review aims to provide a deep insight into responsive interfacial science and offer guidance for smart surface engineering. It ends with a summary of current challenges, future opportunities, and potential solutions on smart switch of wettability on superwetting surfaces.

Three-dimensional capillary ratchet-induced liquid directional steering

[Figure: see text].

Directionally Guided Droplets on a Modular Bottom-Up Anisotropic Locally Ordered Nickel Nanocone Superhydrophobic Surface

The anisotropic surface prepared by the top-down etching technology shows unique advantages in terms of functional superhydrophobicity. However, it still has a shackle of the smallest etching size, which largely restricts the development of better superhydrophobicity. Therefore, it is still a huge challenge to realize the stepless size adjustment of an anisotropic surface in order to achieve better functionalization. In this work, a bottom-up approach inspired via the modular segmented preparation technology has been used to successfully build an anisotropic, locally ordered functionalized unique superhydrophobic structure, whose contact and rebound time of water droplets is extremely short. Furthermore, this structure with artfully arranged "tracks", which has a relatively large contact angle value, not only lasts more than 15 consecutive bounce cycles in the same direction, where the droplets after merging still bounce, but also exhibits a significant anisotropic sliding behavior, which is presented in different sliding angles, toward droplets rolling in different directions and has lower adhesion work and better self-cleaning and anti-fouling performance. Besides, some mechanisms such as the reduction-replacement-reduction cycle and repulsion-adhesion-switching have been proposed especially in modular preparation and anisotropic sliding behavior. More importantly, this sorted bottom-up structure has great potential for achieving higher efficiency of functionalized superhydrophobicity and other related applications.

Physically Unclonable Surfaces via Dewetting of Polymer Thin Films

From anti-counterfeiting to biotechnology applications, there is a strong demand for encoded surfaces with multiple security layers that are prepared by stochastic processes and are adaptable to deterministic fabrication approaches. Here, we present dewetting instabilities in nanoscopic (thickness <100 nm) polymer films as a form of physically unclonable function (PUF). The inherent randomness involved in the dewetting process presents a highly suitable platform for fabricating unclonable surfaces. The thermal annealing-induced dewetting of poly(2-vinyl pyridine) (P2VP) on polystyrene-grafted substrates enables fabrication of randomly positioned functional features that are separated at a microscopic length scale, a requirement set by optical authentication systems. At a first level, PUFs can be simply and readily verified via reflection of visible light. Area-specific electrostatic interactions between P2VP and citrate-stabilized gold nanoparticles allow for fabrication of plasmonic PUFs. The strong surface-enhanced Raman scattering by plasmonic nanoparticles together with incorporation of taggants facilitates a molecular vibration-based security layer. The patterning of P2VP films presents opportunities for fabricating hybrid security labels, which can be resolved through both stochastic and deterministic pathways. The adaptability to a broad range of nanoscale materials, simplicity, versatility, compatibility with conventional fabrication approaches, and high levels of stability offer key opportunities in encoding applications.

Role of soft-gel substrates on bouncing-merging transition in drop impact on a liquid film

Liquid droplets impacting on liquid films is common in many industrial and natural processes. It is crucial to understand the impact of droplets on a liquid film resting on soft deformable substrates in some of the applications including 3D printing of engineering structures, prosthetic implants and tissue engineering. By recognizing the practical relevance of soft-substrates, we present an experimental investigation to assess the role of deformable substrates on bouncing-to-merging transition in droplet impact on the liquid film. First, we prepared polyacrylamide (PAAm) soft-gel substrates with various "softness" (i.e., Young's modulus) by modulating the concentration of a crosslinker, N,N-methylene-bis-acrylamide (BIS). We found that the Young's modulus of PAAm initially increases with the concentration of crosslinker, and subsequently becomes almost constant due to inhomogeneity of crosslinking. Next, through the experiments of droplet impact on the liquid film resting on soft substrates with different Young's moduli, we observe that the early merging and corresponding bouncing-to-merging transitional boundaries remain unaffected by the "softness" since such merging occurs further away from the substrate. However, the late merging, which appears during the retraction process of the deformed droplet, occurs relatively close to the substrate, and hence is found to be significantly affected by its "softness". A scaling analysis is presented to quantify the role of change in Young's modulus of the substrate on late merging, which is supported by the experimental data.

Height of a liquid drop on a wetting stripe

Adsorption of liquid on a planar wall decorated by a hydrophilic stripe of width L is considered. Under the condition that the wall is only partially wet (or dry) while the stripe tends to be wet completely, a liquid drop is formed above the stripe. The maximum height ℓ_{m}(δμ) of the drop depends on the stripe width L and the chemical potential departure from saturation δμ where it adopts the value ℓ_{0}=ℓ_{m}(0). Assuming a long-range potential of van der Waals type exerted by the stripe, the interfacial Hamiltonian model is used to show that ℓ_{0} is approached linearly with δμ with a slope which scales as L^{2} over the region satisfying L≲ξ_{∥}, where ξ_{∥} is the parallel correlation function pertinent to the stripe. This suggests that near the saturation there exists a universal curve ℓ_{m}(δμ) to which the adsorption isotherms corresponding to different values of L all collapse when appropriately rescaled. Although the series expansion based on the interfacial Hamiltonian model can be formed by considering higher order terms, a more appropriate approximation in the form of a rational function based on scaling arguments is proposed. The approximation is based on exact asymptotic results, namely, that ℓ_{m}∼δμ^{-1/3} for L→∞ and that ℓ_{m} obeys the correct δμ→0 behavior in line with the results of the interfacial Hamiltonian model. All the predictions are verified by the comparison with a microscopic density functional theory (DFT) and, in particular, the rational function approximation-even in its simplest form-is shown to be in a very reasonable agreement with DFT for a broad range of both δμ and L.

Physicochemical parameters that underlie inkjet printing for medical applications

One of the most common types of 3D printing technologies is inkjet printing due to its numerous advantages, including low cost, programmability, high resolution, throughput, and speed. Inkjet printers are also capable of fabricating artificial tissues with physiological characteristics similar to those of living tissues. These artificial tissues are used for disease modeling, drug discovery, drug screening, and replacements for diseased or damaged tissues. This paper reviews recent advancements in one of the most common 3D printing technologies, inkjet dispensing. We briefly consider common printing techniques, including fused deposition modeling (FDM), stereolithography (STL), and inkjet printing. We briefly discuss various steps in inkjet printing, including droplet generation, droplet ejection, interaction of droplets on substrates, drying, and solidification. We also discuss various parameters that affect the printing process, including ink properties (e.g., viscosity and surface tension), physical parameters (e.g., internal diameter of printheads), and actuation mechanisms (e.g., piezoelectric actuation and thermal actuation). Through better understanding of common 3D printing technologies and the parameters that influence the printing processes, new types of artificial tissues, disease models, and structures for drug discovery and drug screening may be prepared. This review considers future directions in inkjet printing research that are focused on enhancing the resolution, printability, and uniformity of printed structures.

Anisotropy-induced directional self-transportation of low surface tension liquids: a review

Inspired by natural surfaces such as butterfly wings, cactus leaves, or the Nepenthes alata plant, synthetic materials may be engineered to directionally transport liquids on their surface without external energy input. This advantageous feature has been adopted for various mechanical and chemical processes, e.g. fog harvesting, lubrication, lossless chemical reactions, etc. Many studies have focused on the manipulation and transport of water or aqueous droplets, but significantly fewer have extended their work to low surface tension (LST) liquids, although these fluids are involved in numerous industrial and everyday processes. LST liquids completely wet most surfaces which makes spontaneous transportation an active challenge. This review focuses on recently developed strategies for passively and directionally transporting LST liquids.

Numerical Analysis of Droplets from Multinozzle Inkjet Printing

Multinozzle printing processing with the fabrication of a functional material film lays the foundation for the development of efficient scale production of a photoelectric device. However, a prominent challenge is how to realize the volume uniformity of the droplets. Here, a classical analysis method is introduced first by printing poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) to analyze the behavior of droplets. It relies on a variance calculation for the clarification of the law of implicit behavior of droplets in terms of digitizing. This method reveals the effect of printing parameters on the uniformity of the volume of droplets in multinozzle printing. Overall, by combining both ink formulations and printing parameter optimization, it is concluded that the minimum volume variance of nozzles with different numbers is less than 0.5% and the influence of various parameters in multinozzle printing is found to be ranked. The feasibility of this analysis method is presented and is of great significance to achieving a very stable, large-scale multinozzle printing device.

How do chemical patterns affect equilibrium droplet shapes?

By utilizing a proposed analytical model in combination with the phase-field method, we present a comprehensive study on the effect of chemical patterns on equilibrium droplet morphologies. Here, three influencing factors, the droplet sizes, contact angles, and the ratios of the hydrophilic area to the hydrophobic area, are contemplated. In the analytical model, chemical heterogeneities are described by different non-linear functions. By tuning these functions and the related parameters, the analytical model is capable of calculating the energy landscapes of the system. The chemically patterned surfaces display complex energy landscapes with chemical-heterogeneity-induced local minima, which correspond to the equilibrium morphologies of the droplets. Phase-field (PF) simulations are accordingly conducted and compared with the predicted equilibrium morphologies. In addition, we propose a modified Cassie-Baxter (CB) model to delineate the equilibrium droplet shapes. In contrast to the classic CB model, our extension is not only restricted to the shape with a spherical cap. Both the energy landscape method and the modified CB model are demonstrated to have a good agreement with the PF simulations.

Wettability-Guided Screen Printing of Perovskite Microlaser Arrays for Current-Driven Displays

Halide perovskites have shown tremendous potential for next-generation flat-panel laser displays due to their remarkable optoelectronic properties and outstanding material processability; however, the lack of a general approach for the fast growth of perovskite laser arrays capable of electrical operations impedes actualization of their display applications. Herein, a universal and robust wettability-guided screen-printing technique is reported for the rapid growth of large-scale multicolor perovskite microdisk laser arrays, which can serve as laser display panels and further be used to realize current-driven displays. The perovskite microlasers are precisely defined with controlled physical dimensions and spatial locations by such a printing strategy, and each perovskite microlaser serves as a pixel of a display panel. Moreover, the screen-printing procedure is highly compatible with light-emitting diode (LED) device architectures, which is favorable for the mass production of micro-LED arrays. On this basis, a prototype of a current-driven display is demonstrated with desired functionalities. The outstanding performance and feasible fabrication of screen-printed perovskite microlaser arrays embedded in LEDs provide deep insights into the concepts and device architectures of electrically driven laser display technology.

Switchable Direction of Liquid Transport via an Anisotropic Microarray Surface and Thermal Stimuli

Design and construction of special surface microstructures has made many amazing breakthroughs in directional liquid transport. Despite much progress in this field, challenges still remain in on-demand switchable direction transport of liquid in situ and real-time via transforming the arrangement of the surface microstructure and external stimuli. Herein, we demonstrate a strategy to achieve switchable direction transport of liquid via a tunable anisotropic microarray surface, that is, assembling a V-shaped prism microarray (VPM) surface, which can also be intelligently manipulated by thermal stimuli. By transforming the parallel and staggered prism microstructure arrangement of the VPM, switchable direction transport of a liquid can be successfully achieved on the VPM surface. Flow direction switching among unidirectional transport, bidirectional transport, and reverse unidirectional transport is also achieved on the temperature-adaptive VPM surface by thermal stimuli, which can be used for on-demand liquid transport according to the paths of the microfluidic channels. The work provides a way for precise liquid manipulation in desired liquid transport, which may be utilized in nonpower conveying systems, autolubrication, life fluid medical instruments, and other microfluidic devices.

Droplet Directional Movement on the Homogeneously Structured Superhydrophobic Surface with the Gradient Non-Wettability

The surface with the gradient non-wettability intensely appeals to researchers because of its academic significance and applications for directional droplet movement. Herein, we developed a homogeneous structure superhydrophobic surface with the gradient non-wettability by a combination strategy of chemical etching and vapor diffusion modification. As a consequence, the as-prepared surface exhibits a remarkable gradient characteristic of water repellency, and the water contact angle is mainly located within the range of 162 ± 0.5 to 149 ± 0.4°. Meanwhile, the sliding angle also exhibits a corresponding change from 3 to 11°. On this basis, the gradient characteristic of non-wettability induces the distinguishing droplet adhesion on the surface, that is, from 19 μN for the most hydrophobic end to 57 μN for the opposite one. Because of the difference of the water adhesion force, droplets on the as-prepared surface can well roll alongside a specific direction (i.e., gradient direction of non-wettability). In terms of dynamic impact droplets, they can rapidly rebound off the sample surface with the short contact time of 12.8 ms, and the finally fallen droplets mainly deviate toward weaker regions because of water repellency. To analyze this phenomenon, it is found that the asymmetric mechanic behavior is mainly caused by the unbalanced retraction force between the both ends of the impact droplet. This work provides a novel strategy to construct the homogeneous structure superhydrophobic surface with the gradient non-wettability for the applications in the droplet movement control or transport.

In Situ Droplet Microgoniometry Using Optical Microscopy

Solid-liquid interactions are ubiquitous phenomena in nature and industry. Wettability of a liquid on a solid is governed by the chemical heterogeneity and physical roughness of the solid surface and can be characterized by measuring the advancing and receding contact angles of the liquid droplet residing on the solid. To characterize contact angle, goniometry and the Wilhelmy plate method have been widely used. Although powerful, these methods have difficulty characterizing microdroplets, can be cumbersome and expensive, and have trouble handling surfaces with local wetting heterogeneity and deformed noncircular contact lines. Furthermore, past methods are incapable of measuring contact angle in situ during experiments (e.g., condensation). Here, we develop simple yet powerful contact angle measurement techniques using conventional optical microscopy that utilizes focal plane shift imaging, ray optics, and wave interference. We used our techniques to study the wetting characteristics for a wide range of water droplet diameters (10 μm < D < 600 μm) and apparent contact angles (0° ≤ θ^app ≤ 180°). The outcomes of this work establish a powerful tool to more easily and rapidly characterize microscale droplet advancing and receding contact angles.

Micro-to-nanometer patterning of solution-based materials for electronics and optoelectronics

Technologies for micro-to-nanometer patterns of solution-based materials (SBMs) contribute to a wide range of practical applications in the fields of electronics and optoelectronics. Here, state-of-the-art micro-to-nanometer scale patterning technologies of SBMs are disseminated. The utilisation of patterning for a wide-range of SBMs leads to a high level of control over conventional solution-based film fabrication processes that are not easily accessible for the control and fabrication of ordered micro-to-nanometer patterns. In this review, various patterning procedures of SBMs, including modified photolithography, direct-contact patterning, and inkjet printing, are briefly introduced with several strategies for reducing their pattern size to enhance the electronic and optoelectronic properties of SBMs explained. We then conclude with comments on future research directions in the field.

Control of Direct Written Ink Droplets Using Electrowetting

Here, we investigate the feasibility and effectiveness of electrowetting in the motion control of droplets of different liquids, which are widely used as inks in direct writing (DW)-based three-dimensional (3D) printing processes for various applications. To control the movement of DW ink droplets on dielectric substrates, the electrodes were embedded in the substrate. It is demonstrated that droplets of pure liquid inks, aqueous polymer solution inks, and carbon fiber suspension inks can be moved on multi-angled surfaces. Also, experimental results reveal that droplets of a commercial hydrogel, agar-agar, alginate, xanthan gum, and gum arabic can be moved by electrowetting. Droplets of sizes 200 μm-3 mm were manipulated and moved by the electric field on different dielectric substrates accurately and repeatedly. Effective electrowetting-based control and movement of droplets were observed on horizontal, vertical, and even inverted substrates. These findings imply the feasibility and potential application of electrowetting as a flexible, rapid, and new method for ink droplet control in 3D printing processes.

Electronic performance of printed PEDOT:PSS lines correlated to the physical and chemical properties of coated inkjet papers

PEDOT:PSS organic printed electronics chemical interactions with the ink-receiving layer (IRL) of monopolar inkjet paper substrates and coating color composition were evaluated through Raman spectroscopy mapping in Z (depth) and (XY) direction, Fourier transform infrared spectroscopy (FTIR) and energy dispersive X-ray spectroscopy (EDS). Other evaluated properties of the IRLs were pore size distribution (PSD), surface roughness, ink de-wetting, surface energy and the impact of such characteristics on the electronics performance of the printed layers. Resin-coated inkjet papers were compared to a multilayer coated paper substrate that also contained an IRL but did not contain the plastic polyethylene (PE) resin layer. This substrate showed better electronic performance (i.e., lower sheet resistance), which we attributed to the inert coating composition, higher surface roughness and higher polarity of the surface which influenced the de-wetting of the ink. The novelty is that this substrate was rougher and with somewhat lower printing quality but with better electronic performance and the advantage of not having PE in their composite structure, which favors recycling.

PEDOT:PSS ink chemical interactions with the coated surface of inkjet papers and their composition were evaluated through Raman, FTIR and EDS. Morphology of the pores and surface energy were also evaluated and how these impact sheet resistance.

Solvodynamic Printing As A High Resolution Printing Method

Printing techniques are becoming increasingly prevalent in modern manufacturing. However, its biggest drawback is the limit in printing resolution. In this paper, we present solvodynamic printing as a novel printing system which aims to improve print resolution by incorporating an additional immiscible carrier solvent into the ink delivery system. The resolution is improved due to the solvent-solvent interactions between the ink and the carrier solvent which alter the contact angle of the ink on the substrate and limit the printed feature size. We demonstrate the proof of concept of solvodynamic printing by printing silver nanoparticle inks on a polyethylene naphthalate substrate. Silver nanoparticle tracks with widths of 35.2 ± 7.0 μm were achieved using a 300 μm nozzle. This is equivalent to 11.7 ± 2.3% of the nozzle diameter. The result shows great potential in solvodynamic printing as not many modern printing techniques can achieve such nozzle to feature size ratios.

Iridium(III)-Complexed Polydendrimers for Inkjet-Printing OLEDs: The Influence of Solubilizing Steric Hindrance Groups

With the great success of organic light-emitting diodes (OLEDs) based on thermal evaporation techniques, the development of printable materials for inkjet-printing high-performance OLEDs is particularly attractive yet challenging. In this paper, a set of printable Ir(III)-complexed polydendrimers, poly[bis[2-(2,4-difluorophenyl)-4-(4-((2-ethylhexyl)oxy)phenyl)pyridine][1-ethyl-5-phenyl-3-propyl-1H-1,2,4-triazole] iridium(III)] (PIr-D1) and poly[bis[2-(2,4-difluorophenyl)-4-(4-((2-ethylhexyl)oxy)-2,6-dimethylphenyl)pyridine][1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazole] iridium(III)] (PIr-D2), were designed and synthesized via ring-opening metathesis polymerization (ROMP). As a comparison, the iridium precursor complexes bis[2-(2,4-difluorophenyl)-4-(4-((2-ethylhexyl)oxy)phenyl)pyridine][1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazole]iridium(III) (Ir-D1) and bis[2-(2,4-difluorophenyl)-4-(4-((2-ethylhexyl)oxy)-2,6-dimethylphenyl)pyridine][1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazole] iridium(III) (Ir-D2) and the core structure bis[2-(2,4-difluorophenyl)pyridine] [1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazole] iridium(III) (Ir-D0) were also synthesized and the corresponding OLEDs were fabricated. Compared with the dendritic iridium complexes Ir-D1 and Ir-D2, the resulting polydendrimers PIr-D2 and PIr-D2 showed enhanced film-forming properties, good thermal stability, and attractive ink rheological characteristics with a suitable viscosity for inkjet-printing. Promising device performance has been achieved for the resulting polydendrimers by both spin-coating and inkjet-printing, showing low driving voltages and relatively high current efficiencies and brightnesses. The results suggest that the construction of polydendritic Ir(III) complexes is an attractive design strategy for exploring efficient printable light-emitting materials for inkjet-printing high-performance OLEDs.

Precise control of nanoscale spacing between electrodes using different natured self-assembled monolayers

Herein, we introduce an interdigitated horizontal electrode (IHE) structure with a metal-based electron-collecting (or -injecting) electrode and a hole-collecting (or -injecting) electrode composed of a conductive polymeric material that has a nanoscale distance and is horizontally separated. In the IHE, a metal electrode is fabricated on a silicon-oxide substrate, and a self-assembled monolayer (SAM) is selectively bonded to the metal and the oxide to form a conductive polymer electrode by dip coating. Each of the SAM materials is composed of a head part bonded to the substrate surface and a tail part that is hydrophilic or hydrophobic. This inherent property makes the metal electrode hydrophobic and the oxide substrate hydrophilic. Ag is used as a metal electrode material and is combined with alkanethiol SAMs. The alkylsilane SAMs are combined with the silicon oxide substrate to make them hydrophilic, using poly (3, 4-ethylenedioxythiophene)-poly (PEDOT: PSS) as the conductive polymer material. In this study, we have found that there is a difference in the spacing between the two electrodes that depends on the combination of SAM materials. Each interval was spaced from a minimum of 140 nm to a maximum of 385 nm.

Bioinspired inner microstructured tube controlled capillary rise

Effective, long-range, and self-propelled water elevation and transport are important in industrial, medical, and agricultural applications. Although research has grown rapidly, existing methods for water film elevation are still limited. Scaling up for practical applications in an energy-efficient way remains a challenge. Inspired by the continuous water cross-boundary transport on the peristome surface of Nepenthes alata, here we demonstrate the use of peristome-mimetic structures for controlled water elevation by bending biomimetic plates into tubes. The fabricated structures have unique advantages beyond those of natural pitcher plants: bulk water diode transport behavior is achieved with a high-speed passing state (several centimeters per second on a milliliter scale) and a gating state as a result of the synergistic effect between peristome-mimetic structures and tube curvature without external energy input. Significantly, on further bending the peristome-mimetic tube into a "candy cane"-shaped pipe, a self-siphon with liquid diode behavior is achieved. Such a transport mechanism should inspire the design of next generation water transport devices.

Enhancing Printing Resolution on Hydrophobic Polymer Surfaces Using Patterned Coatings of Cellulose Nanocrystals

High-resolution inkjet printing of a hydrophobic polymer surface (polystyrene, PS) was accomplished using a patterned coating of cellulose nanocrystals (CNCs) that prevents the ink from bleeding. A periodically crack-free, wrinkled (wavelength of around 850 nm) stamp was prepared by surface oxidation of an uniaxially stretched poly(dimethylsiloxane) elastomeric substrate and was used as a template to transfer aligned patterns of cellulose nanocrystals (CNCs) onto PS surfaces by wet stamping. The morphology of the aligned CNC coatings on PS was then compared with randomly deposited CNCs on PS using atomic force microscopy. The wettability of the CNCs and polymer surfaces with water and ink was measured and analyzed in the context of inkjet printing. This biomaterial coating technique enables high-resolution printing of modern water-based inks onto hydrophobic surfaces for applications in renewable packaging and printing of biomolecules for high throughput diagnostics. Further, with suitable modifications, the technology is scalable to roll-to-roll manufacturing for industrial flexo printing.

Microstructure and Resistivity Analysis of Silver Nanoparticle-Based Crystalline Conductive Films Synthesized using PEG Surfactant

Silver nanoparticle-based crystalline conductive films were synthesized using a simple and environmentally friendly method centered on chemical reduction. A stoichiometric balance of three different molecular weights of polyethylene glycol (PEG) was used as a capping agent. Resistivity, and its correlation with temperature and the particle size of nanoparticle films, was probed. The silver nanoparticles were characterized using thermogravimetric analysis (TGA) and field emission scanning electron microscopy (FESEM). Further silver films deposited on a glass substrate were characterized by FESEM, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and resistivity measurements. Particle size distribution and room temperature electrical conductivity were also investigated. The high conductivity of sintered films suggested applications for the ink-jet printing of electronic circuitry on thermally sensitive substrates.