Yongmei Zheng

Directional water collection on wetted spider silk.

Many biological surfaces in both the plant and animal kingdom possess unusual structural features at the micro- and nanometre-scale that control their interaction with water and hence wettability. An intriguing example is provided by desert beetles, which use micrometre-sized patterns of hydrophobic and hydrophilic regions on their backs to capture water from humid air. As anyone who has admired spider webs adorned with dew drops will appreciate, spider silk is also capable of efficiently collecting water from air. Here we show that the water-collecting ability of the capture silk of the cribellate spider Uloborus walckenaerius is the result of a unique fibre structure that forms after wetting, with the ‘wet-rebuilt’ fibres characterized by periodic spindle-knots made of random nanofibrils and separated by joints made of aligned nanofibrils. These structural features result in a surface energy gradient between the spindle-knots and the joints and also in a difference in Laplace pressure, with both factors acting together to achieve continuous condensation and directional collection of water drops around spindle-knots. Submillimetre-sized liquid drops have been driven by surface energy gradients or a difference in Laplace pressure, but until now neither force on its own has been used to overcome the larger hysteresis effects that make the movement of micrometre-sized drops more difficult. By tapping into both driving forces, spider silk achieves this task. Inspired by this finding, we designed artificial fibres that mimic the structural features of silk and exhibit its directional water-collecting ability.

Directional adhesion of superhydrophobic butterfly wings

We showed directional adhesion on the superhydrophobic wings of the butterfly . A droplet easily rolls off the surface of the wings along the radial outward (RO) direction of the central axis of the body, but is pinned tightly against the RO direction. Interestingly, these two distinct states can be tuned by controlling the posture of the wings (downward or upward) and the direction of airflow across the surface (along or against the RO direction), respectively. Research indicated that these special abilities resulted from the direction-dependent arrangement of nano-tips on ridging nano-stripes and micro-scales overlapped on the wings at the level, where two distinct contact modes of a droplet with orientation-tuneable microstructures occur and thus produce different adhesive forces. We believe that this finding will help the design of smart, fluid-controllable interfaces that may be applied in novel microfluidic devices and directional, easy-cleaning coatings.

Bioinspired super-antiwetting interfaces with special liquid-solid adhesion.

Super-antiwetting interfaces, such as superhydrophobic and superamphiphobic surfaces in air and superoleophobic interfaces in water, with special liquid-solid adhesion have recently attracted worldwide attention. Through tuning surface microstructures and compositions to achieve certain solid/liquid contact modes, we can effectively control the liquid-solid adhesion in a super-antiwetting state. In this Account, we review our recent progress in the design and fabrication of these bioinspired super-antiwetting interfaces with special liquid-solid adhesion. Low-adhesion superhydrophobic surfaces are biologically inspired, typically by the lotus leaf. Wettability investigated at micro- and nanoscale reveals that the low adhesion of the lotus surface originates from the composite contact mode, a microdroplet bridging several contacts, within the hierarchical structures. Recently high-adhesion superhydrophobic surfaces have also attracted research attention. These surfaces are inspired by the surfaces of gecko feet and rose petals. Accordingly, we propose two biomimetic approaches for the fabrication of high-adhesion superhydrophobic surfaces. First, to mimic a sticky geckos foot, we designed structures with nanoscale pores that could trap air isolated from the atmosphere. In this case, the negative pressure induced by the volume change of sealed air as the droplet is pulled away from surface can produce a normal adhesive force. Second, we constructed microstructures with size and topography similar to that of a rose petal. The resulting materials hold air gaps in their nanoscale folds, controlling the superhydrophobicity in a Wenzel state on the microscale. Furthermore, we can tune the liquid-solid adhesion on the same superhydrophobic surface by dynamically controlling the orientations of microstructures without altering the surface composition. The superhydrophobic wings of the butterfly (Morpho aega) show directional adhesion: a droplet easily rolls off the surface of wings along one direction but is pinned tightly against rolling in the opposite direction. Through coordinating the stimuli-responsive materials and appropriate surface-geometry structures, we developed materials with reversible transitions between a low-adhesive rolling state and a high-adhesive pinning state for water droplets on the superhydrophobic surfaces, which were controlled by temperature and magnetic and electric fields. In addition to the experiments done in air, we also demonstrated bioinspired superoleophobic water/solid interfaces with special adhesion to underwater oil droplets and platelets. In these experiments, the high content of water trapped in the micro- and nanostructures played a key role in reducing the adhesion of the oil droplets and platelets. These findings will offer innovative insights into the design of novel antibioadhesion materials.

A multi-structural and multi-functional integrated fog collection system in cactus

Multiple biological structures have demonstrated fog collection abilities, such as beetle backs with bumps and spider silks with periodic spindle-knots and joints. Many Cactaceae species live in arid environments and are extremely drought-tolerant. Here we report that one of the survival systems of the cactus Opuntia microdasys lies in its efficient fog collection system. This unique system is composed of well-distributed clusters of conical spines and trichomes on the cactus stem; each spine contains three integrated parts that have different roles in the fog collection process according to their surface structural features. The gradient of the Laplace pressure, the gradient of the surface-free energy and multi-function integration endow the cactus with an efficient fog collection system. Investigations of the structure–function relationship in this system may help us to design novel materials and devices to collect water from fog with high efficiencies.

Icephobic/anti-icing properties of micro/nanostructured surfaces.

using materials such as fl uorocarbons, organic materials, and inorganic materials, [ 6 ] which demonstrate signifi cant hydrophobic properties. Whereas the robust superhydrophobicity of these materials exists at around room temperature, the overwhelming majority of them will fail when put into a subzero degree environment. [ 7 ] It is well known that hydrophobicity and icephobicity properties are extremely important to favor cold environment devices, [ 8 ] such as aerofoils, power towers, ships, radars, and even pipes of airconditioners or refrigerators. Once ice forms on this equipment, they may fail to work normally or may even be damaged. Anti-icing surfaces have been studied since the 1950s. [ 9 ] However, it is still a great challenge to design and fabricate more effi cient ice repellent surfaces, which has aroused the interest of many researchers. [ 7 , 10–25 ]

Colorful humidity sensitive photonic crystal hydrogel

A colorful humidity sensitive photonic crystal (PC) hydrogel was facilely fabricated by infiltrating acrylamide (AAm) solution into a P(St–MMA–AA) PC template and subsequently photo-polymerizing. The color of the samples was sensitive to humidity; it could reversibly vary from transparent to violet, blue, cyan, green and red under various humidity conditions, covering the whole visible range. This could be attributed to the humidity sensitivity of the samples stopband, with a maximum change of 240 nm resulting from the varying humidity. Furthermore, the color response showed good stability under cyclic humidity experiments. As-prepared PAAm–P(St–MMA–AA) PC hydrogel successfully combined the humidity sensitivity of PAAm and structure color of the PC template, which suggested a promising composite material as an economical alternative to traditional humidity sensors, and also provided a new insight into the design and development of novel composite functional materials based on a PC template.

Direction Controlled Driving of Tiny Water Drops on Bioinspired Artificial Spider Silks

www.MaterialsViews.com C O M M Direction Controlled Driving of Tiny Water Drops on Bioinspired Artifi cial Spider Silks U N IC A By Hao Bai , Xuelin Tian , Yongmei Zheng ,* Jie Ju , Yong Zhao , and Lei Jiang * IO N Directional driving of liquid drops is of signifi cant interest in many applications, such as microfl uidic devices, [ 1–6 ] fog harvesting, [ 7 ] fi ltration, [ 8 ] and condensers. [ 9 ] For this purpose, great progress has been made in driving drops larger than hundreds of micrometers [ 9–23 ] by introducing chemical, [ 1 , 10 , 12–15 , 23 ] thermal, [ 16–19 ] or shape [ 20–22 ] gradients on surfaces. However, driving micrometer-sized drops is much more diffi cult because they encounter a larger contact angle (CA) hysteresis effect. [ 7 , 24 ] In nature, the wetted silk of cribellate spider offers new insights into solving this problem by combining different gradients together. [ 7 ] Here, inspired by the spider silk, we fabricated a series of artifi cial spider silks with spindle-knots in which the chemical compositions and surface nanostructures were subtly designed. Our investigations demonstrated that tiny water drops (tens of picoliters) could be driven with controllable direction (“toward” or “away from” the knot) by optimizing the cooperation of curvature, chemical, and roughness gradients on the fi ber surfaces. The study will pave the way for designing smart materials and devices to drive tiny water drops in a controllable manner. When a nylon fi ber was immersed into polymer solution and drawn out horizontally, a string of polymer drops, which became spindle-knots after being dried, formed on the fi ber due to the Rayleigh instability [ 25 ] of the polymer solution. The surface energy of the spindle-knots was tailored by choosing different polymers including poly(vinyl acetate) (PVAc), poly(methyl methacrylate) (PMMA), polystyrene (PS), and poly(vinylidene fl uoride) (PVDF), which have intrinsic water contact angles of 56.7 ° , 68.4 ° , 93.3 ° , and 92.7 ° , respectively (see Supporting Information, Figure S1). On the other hand, the surface roughness (porous nanostructures) of the spindle-knots was also designed through phase separation

Efficient Water Collection on Integrative Bioinspired Surfaces with Star-Shaped Wettability Patterns

Inspired by the water-collecting strategies of desert beetles and spider silk, a novel kind of surface with star-shaped wettablity patterns has been developed. By combining both wettability and shape gradients, the as-prepared surface has gained higher efficiency in water collection compared to circle-shaped wettability patterns and uniformly superhydrophilic or superhydrophobic surfaces.

Janus interface materials: superhydrophobic air/solid interface and superoleophobic water/solid interface inspired by a lotus leaf

We discovered underwater superoleophobicity on the lower side of a lotus leaf, and fabricated Janus interface materials with in-air superhydrophobicity on one side and underwater superoleophobicity on the other side inspired by the Janus feature of the lotus leaf. The ingenious design on lotus leaf surfaces, superhydrophobicity on its upper side and underwater superoleophobicity on its lower side, not only helps us thoroughly understand the special surface wettability of the lotus leaf, but also gives a typical example of multi-functionality in biological systems. This study supplies us with an intelligent strategy to design and create bionic multi-functional interface materials.

Wetting properties on nanostructured surfaces of cicada wings

SUMMARY This study has investigated the wettability of forewings of 15 species of cicadas, with distinctly different wetting properties related to their nanostructures. The wing surfaces exhibited hydrophilic or weak to strong hydrophobic properties with contact angles ranging from 76.8 deg. to 146.0 deg. The nanostructures (protrusions), observed using environmental scanning electron microscopy (ESEM), were classified into four types according to the patterning, diameter (82–148 nm), spacing (44–117 nm) and height (159–446 nm). Surface analysis by X-ray photoelectron spectroscopy (XPS) showed significant differences in wing membrane chemistry. Thus, wetting properties at the macroscopic scale were dependent on slight differences in nanoscale architecture and composition of the wax layer. This investigation offers insights into the diversity of nanostructuring and how subtle small-scale changes may facilitate large changes in wettability.