Qianbin Wang

Self-removal of condensed water on the legs of water striders

Significance Condensing water droplets can be self-removed from the legs of water striders in a three-step mechanism. First, droplets migrate inside the texture and grow. Then, drops are suddenly expelled out of the hairs, owing to the elastic deformation of the network of setae by growing drops. Ultimately, condensed drops are expelled from the leg. Self-removal of condensing water prevents a major source of risk for the creature, whose hydrophobicity would reverse if setae were impregnated by water. The design of the legs therefore provides a remarkable example where the conjunction of hydrophobicity, geometry, and flexibility yields water repellency both at large and very small scales. The ability to control drops and their movements on phobic surfaces is important in printing or patterning, microfluidic devices, and water-repellent materials. These materials are always micro-/nanotextured, and a natural limitation of repellency occurs when drops are small enough (as in a dew) to get trapped in the texture. This leads to sticky Wenzel states and destroys the superhydrophobicity of the material. Here, we show that droplets of volume ranging from femtoliter (fL) to microliter (μL) can be self-removed from the legs of water striders. These legs consist of arrays of inclined tapered setae decorated by quasi-helical nanogrooves. The different characteristics of this unique texture are successively exploited as water condenses, starting from self-penetration and sweeping effect along individual cones, to elastic expulsion between flexible setae, followed by removal at the anisotropic leg surface. We envision that this antifogging effect at a very small scale could inspire the design of novel applicable robust water-repellent materials for many practical applications.

Chinese Brushes: Controllable Liquid Transfer in Ratchet Conical Hairs

The controllable liquid transfer of a Chinese brush is attributable to the unique anisotropic multi-scale structures of the freshly emergent hairs. A large mass of liquid an be dynamically balanced within the brush as a cooperative effect of the Laplace pressure difference, the asymmetrical retention force, and gravity. Inspired by this, a device is developed with parallel hairs that allows for direct writing of micro-lines.

Bio-Inspired Multistructured Conical Copper Wires for Highly Efficient Liquid Manipulation

Animal hairs are typical structured conical fibers ubiquitous in natural system that enable the manipulation of low viscosity liquid in a well-controlled manner, which serves as the fundamental structure in Chinese brush for ink delivery in a controllable manner. Here, drawing inspiration from these structure, we developed a dynamic electrochemical method that enables fabricating the anisotropic multiscale structured conical copper wire (SCCW) with controllable conicity and surface morphology. The as-prepared SCCW exhibits a unique ability for manipulating liquid with significantly high efficiency, and over 428 times greater than its own volume of liquid could be therefore operated. We propose that the boundary condition of the dynamic liquid balance behavior on conical fibers, namely, steady holding of liquid droplet at the tip region of the SCCW, makes it an excellent fibrous medium to manipulate liquid. Moreover, we demonstrate that the titling angle of the SCCW can also affect its efficiency of liquid manipulation by virtue of its mechanical rigidity, which is hardly realized by flexible natural hairs. We envision that the bio-inspired SCCW could give inspiration in designing materials and devices to manipulate liquid in a more controllable way and with high efficiency.

Highly Boosted Oxygen Reduction Reaction Activity by Tuning the Underwater Wetting State of the Superhydrophobic Electrode.

By rationally designing superhydrophobic electrodes with different underwater wetting states, it is revealed that only the underwater Wenzel-Cassie coexistent state shows the clearly enhanced ability in catalyzing the oxygen reduction reaction, a typical underwater gas-consuming reaction at electrode. It is proposed that the maximizing and stabilizing the liquid/gas/solid triphase interface, endowed by the underwater Wenzel-Cassie coexistent state, plays a rather crucial role.

Directional Solution Coating by the Chinese Brush: A Facile Approach to Improving Molecular Alignment for High-Performance Polymer TFTs

Directional solution coating by the Chinese brush provides a facile approach to fabricate highly oriented polymer thin films by finely controlling the wetting and dewetting processes under directional stress. The biggest advantage of the Chinese brush over the normal western brush is the freshly emergent hairs used, whose unique tapered structure renders a dynamic balance of the liquid within the brush by multiple forces when interacting with the liquid. Consequently, the liquid is steadily held within the brush without any unexpected leakage, making the liquid transfer proceed in a well-controllable manner. It is demonstrated that the Chinese brush coating enables the crystallization of the polymer and the self-assembly of conjugated backbones to proceed in a quasi-steady state via a certain direction, which is attributed to the controllable receding of the three-phase contact line during the dewetting process by the multiple parallel freshly emergent hairs. The as-prepared polymer thin films exhibit over six times higher charge-carrier mobility compared to the spin-coated films, which therefore provides a general approach for high-performance organic thin-film transistors.

Bio-Inspired Direct Patterning Functional Nanothin Microlines: Controllable Liquid Transfer

Developing a general and low-cost strategy that enables direct patterning of microlines with nanometer thickness from versatile liquid-phase functional materials and precise positioning of them on various substrates remains a challenge. Herein, with inspiration from the oriental wisdom to control ink transfer by Chinese brushes, we developed a facile and general writing strategy to directly pattern various functional microlines with homogeneous distribution and nanometer-scale thickness. It is demonstrated that the width and thickness of the microlines could be well-controlled by tuning the writing method, providing guidance for the adaptation of this technique to various systems. It is also shown that various functional liquid-phase materials, such as quantum dots, small molecules, polymers, and suspensions of nanoparticles, could directly write on the substrates with intrinsic physicochemical properties well-preserved. Moreover, this technique enabled direct patterning of liquid-phase materials on certain microdomains, even in multiple layered style, thus a microdomain localized chemical reaction and the patterned surface chemical modification were enabled. This bio-inspired direct writing device will shed light on the template-free printing of various functional micropatterns, as well as the integrated functional microdevices.

Chinese brushes: From controllable liquid manipulation to template-free printing microlines

AbstractAs a traditional writing instrument for calligraphy and painting, the Chinese brush has enjoyed a high reputation over the last 5,000 years due to its ability to controllably handle liquid ink, and has been widely used to deposit ink into certain characters or figures as a means of cultural communication. In this mini-review, we first show how the key to the controllable liquid transfer in a Chinese brush lies in the anisotropic multi-scale structural features of the freshly emergent hairs. Then, drawing inspiration from this, applications in controllable liquid pumping, highly efficient liquid transfer and template-free printing microlines are addressed. We envision that the fundamentals of Chinese brushes and their applications in liquid manipulation mentioned in this review may also be extended to other liquid phase functional material systems.

Self-assembly of alumina nanowires into controllable micro-patterns by laser-assisted solvent spreading: towards superwetting surfaces

Self-assembly of nanowires into micro-scale patterns, especially in a controlled manner, has received increasing research interest because of the wide variety of potential applications, including micro-optics and electronic devices, as well as nanomaterials-based energy conversion systems. In this contribution, a novel laser-assisted solution spreading method was developed to fabricate and self-assemble alumina nanowires (ANWs) into large-scale 3-dimensional (3D) micro-patterned surfaces in one step. Here, sodium hydroxide (NaOH) solution played a dual role, both chemically etching the anodic aluminum oxide template (AAO) into ANWs and self-assembling the as-obtained ANWs into micro-patterns under capillary force. It is notable that the micro-scale patterns can be artificially controlled by introducing laser points before solution spreading on the AAO template, and thus the laser-etched area will act as the fixation point during the ANW assembly process. Moreover, the as-prepared micro-patterned ANW film exhibits typical micro-/nano-hierarchical surface topology and shows superhydrophilicity. The film can be transformed into a superhydrophobic surface by chemical modification with 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FAS). Here, by taking advantage of wetting and dewetting processes of a solution on an AAO template, we propose a facile method that enables the fabrication of 3D micro-patterned ANW surfaces, which have superwetting properties. We envisage that this method could shed new light on the fabrication of functional micro-patterned devices where a one-dimensional nano-material and solution phase are involved.