Polarity-Induced Reactive Wetting: Spreading and Retracting Sessile Water Drops.

Wetting is typically defined by the relative liquid to solid surface tension/energy, which are composed of polar and nonpolar subcontributions. Current studies often assume that they remain invariant, that is, surfaces are wetting-inert. Complex wetting scenarios, such as adaptive or reactive wetting processes, may involve time-dependent variations in interfacial energies. To maximize differences in energetic states, we employ low-energy perfluoroalkyls integrated with high-energy silica-based polar moieties grown on low-energy polydimethylsiloxane. To this end, we tune the hydrophilic-like wettability on these perfluoroalkyl-silica-polydimethylsiloxane surfaces. Drop contact behaviors range from invariantly hydrophobic at ca. 110° to rapidly spreading at ca. 0° within 5 s. Unintuitively, these vapor-grown surfaces transit toward greater hydrophilicity with increasing perfluoroalkyl deposition. Notably, this occurs as sequential silica-and-perfluoroalkyl deposition also leaves behind embedded polar moieties. We highlight how surfaces having such chemical heterogeneity are inherently wetting-reactive. By creating an abrupt wetting transition composed of reactive and inert domains, we introduce spatial dependency. Drops contacting the transition spread before retracting, occurring over the time scale of a few seconds. This phenomenon contradicts current understanding, exhibiting a uniquely (1) decreasing advancing contact angle and (2) increasing receding contact angle. To explain the behavior, we model such time- and space- dependent reactive wetting using first order kinetics. In doing so, we explore how reactive and recovery mechanisms govern the characteristic time scales of spreading and retracting sessile drops.

Wetting on silicone surfaces.

Silicone is frequently used as a model system to investigate and tune wetting on soft materials. Silicone is biocompatible and shows excellent thermal, chemical, and UV stability. Moreover, the mechanical properties of the surface can be easily varied by several orders of magnitude in a controlled manner. Polydimethylsiloxane (PDMS) is a popular choice for coating applications such as lubrication, self-cleaning, and drag reduction, facilitated by low surface energy. Aiming to understand the underlying interactions and forces, motivated numerous and detailed investigations of the static and dynamic wetting behavior of drops on PDMS-based surfaces. Here, we recognize the three most prevalent PDMS surface variants, namely liquid-infused (SLIPS/LIS), elastomeric, and liquid-like (SOCAL) surfaces. To understand, optimize, and tune the wetting properties of these PDMS surfaces, we review and compare their similarities and differences by discussing (i) the chemical and molecular structure, and (ii) the static and dynamic wetting behavior. We also provide (iii) an overview of methods and techniques to characterize PDMS-based surfaces and their wetting behavior. The static and dynamic wetting ridge is given particular attention, as it dominates energy dissipation, adhesion, and friction of sliding drops and influences the durability of the surfaces. We also discuss special features such as cloaking and wetting-induced phase separation. Key challenges and opportunities of these three surface variants are outlined.

Tuning the Charge of Sliding Water Drops.

When a water drop slides over a hydrophobic surface, it usually acquires a positive charge and deposits the negative countercharge on the surface. Although the electrification of solid surfaces induced after contact with a liquid is intensively studied, the actual mechanisms of charge separation, so-termed slide electrification, are still unclear. Here, slide electrification is studied by measuring the charge of a series of water drops sliding down inclined glass plates. The glass was coated with hydrophobic (hydrocarbon/fluorocarbon) and amine-terminated silanes. On hydrophobic surfaces, drops charge positively while the surfaces charge negatively. Hydrophobic surfaces coated with a mono-amine (3-aminopropyltriethyoxysilane) lead to negatively charged drops and positively charged surfaces. When coated with a multiamine (N-(3-trimethoxysilylpropyl)diethylenetriamine), a gradual transition from positively to negatively charged drops is observed. We attribute this tunable drop charging to surface-directed ion transfer. Some of the protons accepted by the amine-functionalized surfaces (-NH2 with H+ acceptor) remain on the surface even after drop departure. These findings demonstrate the facile tunability of surface-controlled slide electrification.

Wetting-regulated gas-involving (photo)electrocatalysis: biomimetics in energy conversion.

(Photo)electrolysis of water or gases with water to species serving as industrial feedstocks and energy carriers, such as hydrogen, ammonia, ethylene, propanol, etc., has drawn tremendous attention. Moreover, these processes can often be driven by renewable energy under ambient conditions as a sustainable alternative to traditional high-temperature and high-pressure synthesis methods. In addition to the extensive studies on catalyst development, increasing attention has been paid to the regulation of gas transport/diffusion behaviors during gas-involving (photo)electrocatalytic reactions towards the goal of creating industrially viable catalytic systems with high reaction rates, excellent long-term stabilities and near-unity selectivities. Biomimetic surfaces and systems with special wetting capabilities and structural advantages can shed light on the future design of (photo)electrodes and address long-standing challenges. This article is dedicated to bridging the fields of wetting and catalysis by reviewing the cutting-edge design methodologies of both gas-evolving and gas-consuming (photo)electrocatalytic systems. We first introduce the fundamentals of various in-air/underwater wetting states and their corresponding bioinspired structural properties. The relationship amongst the bubble transport behavior, wettability, and porosity/tortuosity is also discussed. Next, the latest implementations of wetting-related design principles for gas-evolving reactions (i.e. the hydrogen evolution reaction and oxygen evolution reaction) and gas-consuming reactions (i.e. the oxygen reduction reaction and CO2 reduction reaction) are summarized. For photoelectrode designs, additional factors are taken into account, such as light absorption and the separation, transport and recombination of photoinduced electrons and holes. The influences of wettability and 3D structuring of (photo)electrodes on the catalytic activity, stability and selectivity are analyzed to reveal the underlying mechanisms. Finally, remaining questions and related future perspectives are outlined.

The Effect of Surface Entropy on the Heat of Non-Wetting Liquid Intrusion into Nanopores.

On-demand access to renewable and environmentally friendly energy sources is critical to address current and future energy needs. To achieve this, the development of new mechanisms of efficient thermal energy storage (TES) is important to improve the overall energy storage capacity. Demonstrated here is the ideal concept that the thermal effect of developing a solid-liquid interface between a non-wetting liquid and hydrophobic nanoporous material can store heat to supplement current TES technologies. The fundamental macroscopic property of a liquid's surface entropy and its relationship to its solid surface are one of the keys to predict the magnitude of the thermal effect by the development of the liquid-solid interface in a nanoscale environment-driven through applied pressure. Demonstrated here is this correlation of these properties with the direct measurement of the thermal effect of non-wetting liquids intruding into hydrophobic nanoporous materials. It is shown that the model can resonably predict the heat of intrusion into rigid mesoporous silica and some microporous zeolite when the temperature dependence of the contact angle is applied. Conversely, intrusion into flexible microporous metal-organic frameworks requires further improvement. The reported results with further development have the potential to lead to the development of a new supplementary method and mechanim for TES.

How a water drop removes a particle from a hydrophobic surface.

To understand the removal of particles from surfaces by water drops, we used an inverted laser scanning confocal microscope to image the collision between a water drop and a particle on a flat polydimethylsiloxane (PDMS) surface. The dynamic drop-particle contact line was monitored by fixing the drop directly above the objective lens while moving the sample stage at well-defined speeds (10-500 µm s-1). The lateral force acting on the drop during the collision was measured as a function of speed, using a force sensor mounted on the microscope. Depending on the collision speed, the particle either stays attached at the rear of the drop or detaches from it. We propose a criterion to determine whether the particle remains attached to the drop based on the capillary and resistive forces acting on the particle during the collision. The forces measured when the particle crosses the air-water interface are compared to existing models. We adapted these to account for rolling of the particle. By comparing our experimental measurements with an analytical model for the capillary torque acting on a particle rolling at an interface, we provide detailed insights on the origins of the resistive force acting on the particle when it is pushed or pulled by the drop. A low friction force between the surface and the particle increases the likelihood of particle removal.

Capillary Balancing: Designing Frost-Resistant Lubricant-Infused Surfaces.

Slippery lubricant-infused surfaces (SLIPS) have shown great promise for anti-frosting and anti-icing. However, small length scales associated with frost dendrites exert immense capillary suction pressure on the lubricant. This pressure depletes the lubricant film and is detrimental to the functionality of SLIPS. To prevent lubricant depletion, we demonstrate that interstitial spacing in SLIPS needs to be kept below those found in frost dendrites. Densely packed nanoparticles create the optimally sized nanointerstitial features in SLIPS (Nano-SLIPS). The capillary pressure stabilizing the lubricant in Nano-SLIPS balances or exceeds the capillary suction pressure by frost dendrites. We term this concept capillary balancing. Three-dimensional spatial analysis via confocal microscopy reveals that lubricants in optimally structured Nano-SLIPS are not affected throughout condensation (0 °C), extreme frosting (-20 °C to -100 °C), and traverse ice-shearing (-10 °C) tests. These surfaces preserve low ice adhesion (10-30 kPa) over 50 icing cycles, demonstrating a design principle for next-generation anti-icing surfaces.

Adaptive Wetting of Polydimethylsiloxane.

To better understand the wetting of cross-linked polydimethylsiloxane (PDMS), we measured advancing and receding contact angles of sessile water drops on cross-linked PDMS as a function of contact line velocity (up to 100 µm/s). Three types of samples were investigated: pristine PDMS, PDMS where oligomers were removed by toluene treatment, and PDMS with an enriched concentration of oligomers. Depending on the velocity of advancing contact lines and the contact time with water, different modes of wetting were observed: one with a relatively low contact angle hysteresis (Δθ ≈ 10°) and one with a larger hysteresis. We attribute the low hysteresis state, called the lubricated state, to the enrichment of free oligomers at the water-PDMS interface. The enrichment of oligomers is induced by drop contact. The kinetics of the transition to the lubricated state can be described by adaptation theory. PDMS adapts to the presence of water by an enrichment of free oligomers at the interface and a correlated reduction in interfacial tension.

Three-dimensional radial-junction ZnO nanowire/a-Si:H core-shell infrared photodiodes.

Hydrogenated amorphous silicon (a-Si:H)-based infrared photodiodes were fabricated by coating a-Si:H thin-film p-i-n layers over hydrothermally-synthesized disordered zinc oxide (ZnO) nanowire (NW) networks. Due to enhanced light scattering, the reversed biased three dimensional (3-D) radial-junction NW diodes showed an ∼10× increase in photocurrent under a broad spectrum (800-2000 nm) infrared (IR) illumination compared to planar devices. The diodes were optimized by using InGaZnO (IGZO) transparent top contacts that had 20% higher optical transmission in the IR compared to Al-doped ZnO electrodes. Reverse-bias dark current was minimized by optimizing the area of the NW sidewalls and the a-Si:H shell layer thickness. The former reduces the effects of carrier recombination along the NW core-shell interface and the latter minimizes the tunnelling current across the radial-junction device. An enhancement of ∼100× was achieved for these devices compared to non-optimized diodes.

Surface Chemistry Enhancements for the Tunable Super-Liquid Repellency of Low-Surface-Tension Liquids.

Super-hydrophobic, super-oleo(amphi)phobic, and super-omniphobic materials are universally important in the fields of science and engineering. Despite rapid advancements, gaps of understanding still exist between each distinctive wetting state. The transition of super-hydrophobicity to super-(oleo-, amphi-, and omni-)phobicity typically requires the use of re-entrant features. Today, re-entrant geometry induced super-(amphi- and omni-)phobicity is well-supported by both experiments and theory. However, owing to geometrical complexities, the concept of re-entrant geometry forms a dogma that limits the industrial progress of these unique states of wettability. Moreover, a key fundamental question remains unanswered: are extreme surface chemistry enhancements able to influence super-liquid repellency? Here, this was rigorously tested via an alternative pathway that does not require explicit designer re-entrant features. Highly controllable and tunable vertical network polymerization and functionalization were used to achieve fluoroalkyl densification on nanoparticles. For the first time, relative fluoro-functionalization densities are quantitatively tuned and correlated to super-liquid repellency performance. Step-wise tunable super-amphiphobic nanoparticle films with a Cassie-Baxter state (contact angle of >150° and sliding angle of <10°) against various liquids is demonstrated. This was tested down to very low surface tension liquids to a minimum of ca. 23.8 mN/m. Such findings could eventually lead to the future development of super-(amphi)omniphobic materials that transcend the sole use of re-entrant geometry.

Size-dependent optoelectrical properties of 365 nm ultraviolet light-emitting diodes.

In this paper, the size-dependent optical and electrical properties of 365 nm InGaN/AlGaN ultraviolet micron-size light-emitting diodes (µLEDs) on c-plane sapphire substrates is investigated. The series resistance of the µLED increased from 20 Ω to 15 kΩ when the diameter of the device decreased from 150 to 3 µm. The ideality factor increased from 4 to 4.6 over the same range of diameters due to the increase in the defect density for the smaller µLEDs. Moreover, electroluminescence characterization showed a fixed and red-spectral-shift emission for the µLEDs with diameters smaller than 10 µm and larger than 15 µm, respectively. The red-shift was due to band-gap narrowing in InGaN/AlGaN multi-quantum wells as a result of self-heating at higher current densities in the larger diameter µLEDs. Due to an increase in the heat dissipation of devices with a high surface to volume ratio, the smaller diameter devices were found to have higher light extraction efficiency and no measurable emission spectrum shift.

Integration of GaN light-emitting diodes with a-Si:H thin-film transistors for flexible displays.

In this work, the successful integration of a-Si:H thin-film transistors (TFTs) and high-efficiency µ-iLEDs on large-area flexible substrates has been demonstrated. A conventional low-temperature a-Si:H TFT fabrication process combined with a laser lift-off transfer procedure was used to integrate µ-iLEDs with flexible TFT pixel circuits. Electrical and optical characterization showed the current-voltage and electroluminescence characteristics of the TFTs and LEDs did not change after integration onto the flexible platforms. This approach provides a potential methodology for creating flexible optoelectronic systems for wearable and large-area display applications.

Superamphiphobic Bionic Proboscis for Contamination-Free Manipulation of Nano and Core-Shell Droplets.

Manipulation of nanoliter droplets is a key step for many emerging technologies including ultracompact microfluidics devices, 3D and flexible electronic printing. Despite progress, contamination-free generation and release of nanoliter droplets by compact low-cost devices remains elusive. In the present study, inspired by butterflies' minute manipulation of fluids, the authors have engineered a superamphiphobic bionic proboscis (SAP) layout that surpasses synthetic and natural designs. The authors demonstrate the scalable fabrication of SAPs with tunable inner diameters down to 50 µm by the rapid gas-phase nanotexturing of the outer and inner surfaces of readily available hypodermic needles. Optimized SAPs achieve contamination-free manipulation of water and oil droplets down to a liquid surface tension of 26.56 mN m-1 and a volume of 10 nL. The unique potential of this layout is showcased by the rapid and carefully controlled in-air synthesis of core-shell droplets with well-controlled compositions. These findings provide a new low-cost tool for high-precision manipulation of nanoliter droplets, offering a powerful alternative to established thermal- and electrodynamic-based devices.

Optical and Electrical Characteristics of Hybrid ZnO Nanowire/a-Si:H Solar Cells on Flexible Substrates under Mechanical Bending.

Disordered 3-D hybrid ZnO nanowire/a-Si:H thin-film radial-junction solar cells are directly fabricated onto flexible substrates. A 41% reduction in optical reflectivity resulted in a 15% increase in the current density when the substrate is mechanically bent concave-up toward the incoming light. The light scattering of the nanowire devices was enhanced by decreasing the spacing between the nanowire solar cell by bending the substrate.

Adjustable optical response of amorphous silicon nanowires integrated with thin films.

We experimentally demonstrate a new optical platform by integrating hydrogenated amorphous silicon nanowire arrays with thin films deposited on transparent substrates like glass. A 535 nm thick thin film is anisotropically etched to fabricate vertical nanowire arrays of 100 nm diameter arranged in a square lattice. Adjusting the nanowire length, and consequently the thin film thickness permits the optical properties of this configuration to be tuned for either transmission filter response or enhanced broadband absorption. Vivid structural colors are also achieved in reflection and transmission. The optical properties of the platform are investigated for three different etch depths. Transmission filter response is achieved for a configuration with nanowires on glass without any thin film. Alternatively, integrating thin film with nanowires increases the absorption efficiency by ∼97% compared to the thin film starting layer and by ∼78% over nanowires on glass. The ability to tune the optical response of this material in this fashion makes it a promising platform for high performance photovoltaics, photodetectors and sensors.

Zinc oxide nanowire arrays for silicon core/shell solar cells.

The optics of core / shell nanowire solar cells was investigated. The optical wave propagation was studied by finite difference time domain simulations using realistic interface morphologies. The interface morphologies were determined by a 3D surface coverage algorithm, which provides a realistic film formation of amorphous silicon films on zinc oxide nanowire arrays. The influence of the nanowire dimensions on the interface morphology and light trapping was investigated and optimal dimensions of the zinc oxide nanowire were derived.

Designing Plastrons for Underwater Bubble Capture: From Model Microstructures to Stochastic Nanostructures.

Bubbles and foams are often removed via chemical defoamers and/or mechanical agitation. Designing surfaces that promote chemical-free and energy-passive bubble capture is desirable for numerous industrial processes, including mineral flotation, wastewater treatment, and electrolysis. When immersed, super-liquid-repellent surfaces form plastrons, which are textured solid topographies with interconnected gas domains. Plastrons exhibit the remarkable ability of capturing bubbles through coalescence. However, the two-step mechanics of plastron-induced bubble coalescence, namely, rupture (initiation and location) and subsequent absorption (propagation and drainage) are not well understood. Here, the influence of 1) topographical feature size and 2) gas fraction on bubble capture dynamics is investigated. Smaller feature sizes accelerate rupture while larger gas fractions markedly improve absorption. Rupture is initiated solely on solid domains and is more probable near the edges of solid features. Yet, rupture time becomes longer as solid fraction increases. This counterintuitive behavior represents unexpected complexities. Upon rupture, the bubble's moving liquid-solid contact line influences its absorption rate and equilibrium state. These findings show the importance of rationally minimizing surface feature sizes and contact line interactions for rapid bubble rupture and absorption. This work provides key design principles for plastron-induced bubble coalescence, inspiring future development of industrially-relevant surfaces for underwater bubble capture.

Ultrasonic Healing of Plastrons.

Superhydrophobic surfaces (SHS) exhibit a pronounced ability to resist wetting. When immersed in water, water does not penetrate between the microstructures of the SHS. Instead, a thin layer of trapped gas remains, i.e., plastron. This fractional wetting is also known as the Cassie-Baxter state (CB). Impairment of superhydrophobicity occurs when water penetrates the plastron and, when complete wetting is achieved, a Wenzel state (W) results. Subsequent recovery back to CB state is one of the main challenges in the field of SHS wetting. Current methods for plastron recovery require complex mechanical or chemical integration, are time-consuming or lack spatial control. Here an on-demand, contact-less approach for performing facile transitions between these wetting states at micrometer length scales is proposed. This is achieved by the use of acoustic radiation force (ARF) produced by high-intensity focused ultrasound (HIFU). Switching from CB to W state takes <100 µs, while the local recovery back to CB state takes <45 s. To the best of authors knowledge, this is the first demonstration of ARF-induced manipulation of the plastron enabling facile two-way controlled switching of wetting states.

Design of Fluoro-Free Surfaces Super-Repellent to Low-Surface-Tension Liquids.

Super-liquid-repellent surfaces feature high liquid contact angles and low sliding angles find key applications in anti-fouling and self-cleaning. While repellency for water is easily achieved with hydrocarbon functionalities, repellency for many low-surface-tension liquids (down to 30 mN m-1 ) still requires perfluoroalkyls (a persistent environmental pollutant and bioaccumulation hazard). Here, the scalable room-temperature synthesis of stochastic nanoparticle surfaces with fluoro-free moieties is investigated. Silicone (dimethyl and monomethyl) and hydrocarbon surface chemistries are benchmarked against perfluoroalkyls, assessed using model low-surface-tension liquids (ethanol-water mixtures). It is discovered that both hydrocarbon- and dimethyl-silicone-based functionalization can achieve super-liquid-repellency down to 40-41 mN m-1 and 32-33 mN m-1 , respectively (vs 27-32 mN m-1 for perfluoroalkyls). The dimethyl silicone variant demonstrates superior fluoro-free liquid repellency likely due to its denser dimethyl molecular configuration. It is shown that perfluoroalkyls are not necessary for many real-world scenarios requiring super-liquid-repellency. Effective super-repellency of different surface chemistries against different liquids can be adequately predicted using empirically verified phase diagrams. These findings encourage a liquid-centric design, i.e., tailoring surfaces for target liquid properties. Herein, key guidelines are provided for achieving functional yet sustainably designed super-liquid-repellency.

Hybrid Si nanowire/amorphous silicon FETs for large-area image sensor arrays.

Silicon nanowire (SiNW) field-effect transistors (FETs) were fabricated from nanowire mats mechanically transferred from a donor growth wafer. Top- and bottom-gate FET structures were fabricated using a doped a-Si:H thin film as the source/drain (s/d) contact. With a graded doping profile for the a-Si:H s/d contacts, the off-current for the hybrid nanowire/thin-film devices was found to decrease by 3 orders of magnitude. Devices with the graded contacts had on/off ratios of ∼10(5), field-effect mobility of ∼50 cm(2)/(V s), and subthreshold swing of 2.5 V/decade. A 2 in. diagonal 160 × 180 pixel image sensor array was fabricated by integrating the SiNW backplane with an a-Si:H p-i-n photodiode.