Superior water repellency of water strider legs with hierarchical structures: experiments and analysis

Water striders are a type of insect with the remarkable ability to stand effortlessly and walk quickly on water. This article reports the water repellency mechanism of water strider legs. Scanning electron microscope (SEM) observations reveal the uniquely hierarchical structure on the legs, consisting of numerous oriented needle-shaped microsetae with elaborate nanogrooves. The maximal supporting force of a single leg against water surprisingly reaches up to 152 dynes, about 15 times the total body weight of this insect. We theoretically demonstrate that the cooperation of nanogroove structures on the oriented microsetae, in conjunction with the wax on the leg, renders such water repellency. This finding might be helpful in the design of innovative miniature aquatic devices and nonwetting materials.     

Dual high adhesion surface for water in air and for oil underwater

A new type of dual high surface adhesion both in an oil/water/solid system and in a water/air/solid system is reported. A walnutlike cuprous iodide (CuI) microcrystal surface, which is composed of numerous CuI nanocrystals, shows an amphiphobic, highly adhesive surface for water in air and for oil underwater. The maximum adhesive force is about 120.3 ± 1.6 μN in the air for a water droplet and about 23.8 ± 2.1 μN underwater for an oil droplet. These findings will help us to design novel high adhesive materials in two-phase or multiphase mediums.     

Direction-dependent circular dichroism spectra of single crystals of cubic sodium uranyl acetate

The contribution of the electric quadrupole mechanism to the chirality of optically active cubic sodium uranyl acetate single crystals has been established by measurements of the direction dependence of circular dichroism spectra. The CD bands for Σ_g⁺→Δ_g transitions are found to be direction dependent while those for Σ_g⁺→Π_g are direction-independent for light propagation k parallel to the (100) and (111) directions of the cubic cell. The contribution of the electric quadrupole mechanism to the chirality relative to that of the magnetic dipole mechanism is estimated to be about 20%.     

Investigating the adhesion of water droplets at low temperatures

Adhesion of droplets to solid surfaces at low temperatures is crucial for antifogging and antifreezing, etc. So far, most reports on adhesion measurements have been carried out in air-liquid-solid systems, but it remains difficult to precisely investigate the adhesion at low temperatures due to the uncontrollable condensation. On the basis of the liquid-liquid-solid system, a new method to measure the adhesion of water droplets at low temperatures was developed and employed. Moreover, the reported method could be viable in other liquid-liquid-solid systems with wider temperature window; thus, it will find applications in broad fields such as crude oil recovery, ore-dressing, and transfer printing.     

Tuning hydrophobicity and water adhesion by electrospinning and silanization

Electrospinning and silanization were synergistically employed to fabricate poly(vinyl alcohol) (PVA) and PVA/silica mixtures into flexible and chemically modifiable nanostructured surfaces with varying degrees of hydrophobicity and water adhesion. Surfaces possessing the greatest advancing water contact angle yet exhibiting a high level of water adhesion (θ(A)/θ(R) ≈ 168°/0°) were achieved by the reaction of PVA fiber mats with multiple cycles of SiCl(4)/H(2)O treatment, followed by silanization with (1H,1H,2H,2H-perfluorooctyl)trichlorosilane. It is postulated that the strong pinning effect and hence the water adhesion originated from the collapse of the underlying fibrous structures and the removal of air pockets. The addition of silica to the PVA matrix improved the rigidity and thus prevented the fibers from collapsing, allowing air to remain trapped within the fibrous structure and giving the surface greater water repellency. Throughout the investigation, the three wetting models--Wenzel's, Cassie-Baxter's, and the Cassie-impregnating--were regularly referred to as a conceptual framework. The hydrophobic surface that exhibited strong water adhesion, or the so-called "Petal effect", was elucidated in correlation with the fibrous structure of the film, as reviewed by microscopic analysis. In summary, electrospinning as a facile and cost-effective method provides promising opportunities for investigating the mechanistic character of nanowetting, nanoprinting, and nanocoating where the precise control of the dynamical three-phase contact line is of paramount importance.     

Nature's design of hierarchical superhydrophobic surfaces of a water strider for low adhesion and low-energy dissipation

The mechanics of wet adhesion between a water strider's legs and a water surface was studied. First, we showed that the nanoscale to microscale hierarchical surface structure on striders' legs is crucial to the stable water-repellent properties of the legs. The smallest structure is made for the sake of a stable Cassie state even under harsh environment conditions, which sets an upper limit for the dimension of the smallest structure. The maximum stress and the maximum deformation of the surface structures at the contact line are size-dependent because of the asymmetric surface tension, which sets a lower limit for the dimension of the smallest structure. The surface hierarchy can largely reduce the adhesion between the water and the legs by stabilizing the Cassie state, increasing the apparent contact angle, and reducing the contact area and the length of the contact line. Second, the processes of the legs pressing on and detaching from the water surface were analyzed with a 2D model. We found that the superhydrophobicity of the legs' surface is critically important to reducing the detaching force and detaching energy. Finally, the dynamic process of the legs striking the water surface, mimicking the maneuvering of water striders, was analyzed. We found that the large length of the legs not only reduces the energy dissipation in the quasi-static pressing and pulling processes but also enhances the efficiency of energy transfer from bioenergy to kinetic energy in the dynamic process during the maneuvering of the water striders. The mechanical principles found in this study may provide useful guidelines for the design of superior water-repellent surfaces and novel aquatic robots.     

Nanostructures increase water droplet adhesion on hierarchically rough superhydrophobic surfaces

Hierarchical roughness is known to effectively reduce the liquid-solid contact area and water droplet adhesion on superhydrophobic surfaces, which can be seen for example in the combination of submicrometer and micrometer scale structures on the lotus leaf. The submicrometer scale fine structures, which are often referred to as nanostructures in the literature, have an important role in the phenomenon of superhydrophobicity and low water droplet adhesion. Although the fine structures are generally termed as nanostructures, their actual dimensions are often at the submicrometer scale of hundreds of nanometers. Here we demonstrate that small nanometric structures can have very different effect on surface wetting compared to the large submicrometer scale structures. Hierarchically rough superhydrophobic TiO(2) nanoparticle surfaces generated by the liquid flame spray (LFS) on board and paper substrates revealed that the nanoscale surface structures have the opposite effect on the droplet adhesion compared to the larger submicrometer and micrometer scale structures. Variation in the hierarchical structure of the nanoparticle surfaces contributed to varying droplet adhesion between the high- and low-adhesive superhydrophobic states. Nanoscale structures did not contribute to superhydrophobicity, and there was no evidence of the formation of the liquid-solid-air composite interface around the nanostructures. Therefore, larger submicrometer and micrometer scale structures were needed to decrease the liquid-solid contact area and to cause the superhydrophobicity. Our study suggests that a drastic wetting transition occurs on superhydrophobic surfaces at the nanometre scale; i.e., the transition between the Cassie-Baxter and Wenzel wetting states will occur as the liquid-solid-air composite interface collapses around nanoscale structures. Consequently, water adheres tightly to the surface by penetrating into the nanostructure. The droplet adhesion mechanism presented in this paper gives valuable insight into a phenomenon of simultaneous superhydrophobicity and high water droplet adhesion and contributes to a more detailed comprehension of superhydrophobicity overall.     

Computer simulation of adhesion between hydrophilic and hydrophobic self-assembled monolayers in water

The grand canonical Monte Carlo technique and atomistic force fields are used to calculate the force-distance relations and free energies of adhesion between carboxyl and methyl terminated alkanethiolate self-assembled monolayers (SAMs) in water. Both symmetric and asymmetric confinements are considered, as formed by like and unlike SAMs, respectively. As the confinement is increased, water confined by the hydrophobic methyl terminated SAMs experiences capillary evaporation. As a consequence, the adhesion energy is determined by the direct interaction between bare SAMs. In the asymmetric system, an incomplete capillary evaporation is observed, with the number of water molecules dropped by more than an order of magnitude. The remaining water molecules are all adsorbed on the hydrophilic SAM, while the hydrophobic SAM is separated from the rest of the system by a thin vapor layer. The calculated free energies of adhesion are in acceptable agreement with experiment.     

Control of the water adhesion on hydrophobic micropillars by spray coating technique

We present an alternative approach for controlling the water adhesion on solid superhydrophobic surfaces by varying their coverage with a spray coating technique. In particular, micro-, submicro-, and nanorough surfaces were developed starting from photolithographically tailored SU-8 micropillars that were used as substrates for spraying first poly(tetrafluoroethylene) submicrometer particles and subsequently iron oxide nanoparticles. The sprayed particles serve to induce surface submicrometer and nanoscale roughness, rendering the SU-8 patterns superhydrophobic (apparent contact angle values of more than 150°), and also to tune the water adhesion between extreme states, turning the surfaces from “non-sticky” to “sticky” while preserving their superhydrophobicity. The influence of the chemical properties and of the geometrical characteristics of the functionalized surfaces on the wetting properties is discussed within the frame of the theory. This simple method can find various applications in the fabrication of microfluidic devices, smart surfaces, and biotechnological and antifouling materials.     

Staudinger–Vilarassa reaction versus Huisgen reaction for the control of surface hydrophobicity and water adhesion

Surface modifications are keys for a great number of applications. In order to perfectly control the surface properties, it is important to control the modification pathways. Two general pathways can be described in order to introduce modification on surfaces: the post‐strategies and the ante‐strategies. In this work, we focus on the comparison between the Huisgen and the Staudinger–Vilarrasa reaction for both post‐surface and ante‐surface modifications. Here, we focused on the possibility to use both two reactions to obtain superhydrophobic and oleophobic properties. This work includes monomer synthesis, surface modifications with alkyl, aryl or perfluoroalkyl chain. Copyright © 2016 John Wiley & Sons, Ltd.     

Production and characterization of stable superhydrophobic surfaces based on copper hydroxide nanoneedles mimicking the legs of water striders

The present work reports a simple and economic route for production and characterization of stable superhydrophobic surfaces from thin copper layers coated on arbitrary solid substrates. The thin copper layer was anodized in a 2 M aqueous solution of potassium hydroxide to form a thin film of copper hydroxide nanoneedles; then the film was reacted with n-dodecanethiol to form a thermally stable Cu(SC12H25)2 superhydrophobic coating. The contact angle of the modified nanoneedle surface was higher than 150 degrees , and its tilt angle was smaller than 2 degrees . Furthermore, the surface fabricated on copper foil kept its superhydrophobic property after heating at 160 degrees C in air for over 42 h. This technique has also been applied for fabrication of copper wire with superhydrophobic submicrofiber coating to mimic water strider legs. The maximal supporting force of the superhydrophobic copper column has also been investigated in comparison to real water striders.     

Effect of surface roughness and softness on water capillary adhesion in apolar media

The roughness and softness of interacting surfaces are both important parameters affecting the capillary condensation of water in apolar media, yet are poorly understood at present. We studied the water capillary adhesion between a cellulose surface and a silica colloidal probe in hexane by AFM force measurements. Nanomechanical measurements show that the Young's modulus of the cellulose layer in water is significantly less (~7 MPa) than in hexane (~7 GPa). In addition, the cellulose surface in both water and hexane is rather rough (6-10 nm) and the silica probe has a comparable roughness. The adhesion force between cellulose and silica in water-saturated hexane shows a time-dependent increase up to a waiting time of 200 s and is much (2 orders of magnitude) lower than that expected for a capillary bridge spanning the whole silica probe surface. This suggests the formation of one or more smaller bridges between asperities on both surfaces, which is confirmed by a theoretical analysis. The overall growth rate of the condensate cannot be explained from diffusion mediated capillary condensation alone; thin film flow due to the presence of a wetting layer of water at both the surfaces seems to be the dominant contribution. The logarithmic time dependence of the force can also be explained from the model of the formation of multiple capillary bridges with a distribution of activation times. Finally, the force-distance curves upon retraction show oscillations. Capillary condensation between an atomically smooth mica surface and the silica particle show less significant oscillations and the adhesion force is independent of waiting time. The oscillations in the force-distance curves between cellulose and silica may stem from multiple bridge formation between the asperities present on both surfaces. The softness of the cellulose surface can bring in additional complexities during retraction of the silica particle, also resulting in oscillations in the force-distance curves.     

Effects of rugged nanoprotrusions on the surface hydrophobicity and water adhesion of anisotropic micropatterns

A facile laser-etching method was used for the one-step creation of various controllable dimensions of anisotropic micropatterns consisting of an alternating arrangement of microgrooves and microstripes with rugged nanoprotrusions, which after modified with fluoroalkylsilane reagent, showed perfect isotropic superhydrophobicity without apparent CA hystereses, water adhesion, and drag resistance, other than the conventional view of anisotropic surface microstructures with anisotropic surface dewetting. The detailed experiments and analyses have indicated that the introduction of the rugged nanoprotrusions on the surface of microstripes provided ideal 3D roughness, which could not only enhance the apparent contact angles close to 180 degrees by the "point" contact fashion to maximally reduce the liquid-solid contact area but, most importantly, make droplets easily roll off the surface without apparent CA hysteresis by regulating the triple-phase contact line (TCL) to become extremely discrete. These findings would be helpful in understanding the role of complex micro- and nanostructures on natural superhydrophobic biosurfaces and guiding the design of perfect artificial superhydrophobic materials for technological innovations such as the raindrop easy-cleaning, aquatic super-floating, and drag-reducing coatings.     

Effects of the relative humidity and water droplet on adhesion of a bio-inspired nano-film

Inspired by geckos' adhesion, the effect of water membrane forming due to the environmental humidity, on the adhesion between a bio-inspired nano-film and a substrate is investigated first. The disjoining pressure is considered, which results in an enhancing adhesion between the nano-film and substrate. When the thickness of water membrane increases, water droplets will form and a repulsive capillary force between the nano-film and substrate is produced. The total adhesion force decreases with an increasing volume of water droplets. The two opposite results in the two different models are consistent well with two seemingly inconsistent experimental observations by Huber et al. (2005) [4] and Sun et al. (2005) [5], respectively, and may be significant for the development of artificial biomimetic attachment systems.     

An improved analytical method for determination of heterocyclic amines in chicken legs

Summary Several extraction, separation and detection methods for heterocyclic amines (HAs) in chicken legs were evaluated by liquid chromatography. Results showed that the most appropriate extraction method includes the removal of macrosubstances by centrifugation and subsequent purification using a PRS (propylsulfonic acid silica gel) and a C₁₈ cartridge, and the recovery obtained ranged between 51 and 89 %. For HPLC separation, a binary solvent system consisting of acetonitrile and 0.05 M ammonium acetate solution (pH 3.6) with gradient elution with flow rate of 1.0 mL min⁻¹ and detection at 258 nm was used to resolve 16 HAs. With fluorescence nine HAs could be detected by employing a programmable wavelength, and the sensitivity was 100–400 times higher than that by UV detection. The detection limits for UV and fluorescence detection were 0.02≈0.5 ng and 0.05≈3 pg respectively, with a signal-to-noise ratio 3. The presence of HAs in fried chicken legs was also determined.     

Roughness models for particle adhesion

The effects of different surface roughness models on a previously developed van der Waals adhesion model were examined. The van der Waals adhesion model represented surface roughness with a distribution of hemispherical asperities. It was found that the constraints used to define the asperity distribution on the surface, which were determined from AFM scans, varied with scan size and thus were not constant for all surfaces examined. The greatest variation in these parameters occurred with materials that had large asperities or with materials where a large fraction of the surface was covered by asperities. These rough surfaces were modeled with fractals and also with a fast Fourier transform algorithm. When the model surfaces generated using the Fourier transforms are used in the adhesion model, the model accurately predicts the experimentally observed adhesion forces measured with the AFM.     

Polymer microarrays for cellular adhesion

Microarray screening of polymer libraries for cellular adhesion was developed utilising a thin film of agarose to allow unsurpassed localisation of cell binding onto the array substrate and the discovery of cell specific polymers.     

Compliance switching for adhesion control

Climbing lizards display numerous advanced features in their locomotion, notably a method to quickly switch between a state of low and high adhesive force capacity. Inspired by the gecko's adhesive switching, a method of mechanically switching between low and high adhesive states is reported. In particular, the first switching of an adhesive system using only a change in system compliance is demonstrated. Mechanical clamping and a novel magnetic clamping system are used to switch an iron/PDMS composite adhesive between a soft and rigid state. The switch in compliance directly influences the maximum load of the adhesive as measured in lap‐shear. Notably, contact area and the contact chemistry remain unaltered despite significant changes in force capacity. The demonstration of a compliance‐only switching mechanism has broad implications for understanding natural adhesive systems—especially in organisms that can dynamically alter their rigidity (e.g. cells). © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 48–57     

Preparation of superhydrophobic Fe2O3 nanorod films with the tunable water adhesion

We describe a simple and inexpensive method to prepare α-Fe(2)O(3) nanorods films on glass slides fabricated via a simple hydrothermal procedure at 120 °C. Such films exhibit a hierarchical microtexture, and after the surface modification, they show extraordinary superhydrophobicity and the controllable water adhesion. Such superhydrophobic surfaces of the ferric oxides imply wide industrial applications.     

Quantitative method for determining the lateral strength of bacterial adhesion and application for characterizing adhesion kinetics

A quantitative method for measuring the shear force required to detach individual adhered bacteria using atomic force microscopy (AFM) was developed. By determining the total compression of the cantilever during cell detachment events, a more accurate means of calculating the applied lateral force necessary to remove individual cells was achieved compared to previous methods. In addition, a tunable assay for monitoring the dynamics of Pseudomonas aeruginosa and Staphylococcus aureus adhesion strength was employed. The accumulation of force measurements over time allowed for the characterization of adhesion strength kinetics. P. aeruginosa reinforced its adhesion to the surface at a rate 7-fold faster than for S. aureus; the average adhesion strength of P. aeruginosa was larger than that of S. aureus at corresponding time points. Adhered cells of the same species and strain demonstrated a range of adhesion forces that broadened with time, indicating that the change in adhesion strength does not proceed uniformly.