Contact self-cleaning of synthetic gecko adhesive from polymer microfibers

Natural gecko toes covered by nanomicro structures can repeatedly adhere to surfaces without collecting dirt. Inspired by geckos, we fabricated a high-aspect-ratio fibrillar adhesive from a stiff polymer and demonstrated self-cleaning of the adhesive during contact with a surface. In contrast to a conventional pressure-sensitive adhesive (PSA), the contaminated synthetic fibrillar adhesive recovered about 33% of the shear adhesion of clean samples after multiple contacts with a clean, dry surface.     

Friction and Adhesion of Gecko-Inspired PDMS Flaps on Rough Surfaces

Geckos have developed a unique hierarchical structure to maintain climbing ability on surfaces with different roughness, one of the extremely important parameters that affect the friction and adhesion forces between two surfaces. Although much attention has been paid on fabricating various structures that mimic the hierarchical structure of a gecko foot, yet no systematic effort, in experiment or theory, has been made to quantify the effect of surface roughness on the performance of the fabricated structures that mimic the hierarchical structure of geckos. Using a modified surface forces apparatus (SFA), we measured the adhesion and friction forces between microfabricated tilted PDMS flaps and optically smooth SiO(2) and rough SiO(2) surfaces created by plasma etching. Anisotropic adhesion and friction forces were measured when sliding the top glass surface along (+y) and against (-y) the tilted direction of the flaps. Increasing the surface roughness first increased the adhesion and friction forces measured between the flaps and the rough surface due to topological matching of the two surfaces but then led to a rapid decrease in both of these forces. Our results demonstrate that the surface roughness significantly affects the performance of gecko mimetic adhesives and that different surface textures can either increase or decrease the adhesion and friction forces of the fabricated adhesives.     

Design and fabrication of gecko-inspired adhesives

Recently, there has been significant interest in developing dry adhesives mimicking the gecko adhesive system, which offers several advantages compared to conventional pressure-sensitive adhesives. Specifically, gecko adhesive pads have anisotropic adhesion properties; the adhesive pads (spatulae) stick strongly when sheared in one direction but are non-adherent when sheared in the opposite direction. This anisotropy property is attributed to the complex topography of the array of fine tilted and curved columnar structures (setae) that bear the spatulae. In this study, we present an easy, scalable method, relying on conventional and unconventional techniques, to incorporate tilt in the fabrication of synthetic polymer-based dry adhesives mimicking the gecko adhesive system, which provides anisotropic adhesion properties. We measured the anisotropic adhesion and friction properties of samples with various tilt angles to test the validity of a nanoscale tape-peeling model of spatular function. Consistent with the peel zone model, samples with lower tilt angles yielded larger adhesion forces. The tribological properties of the synthetic arrays were highly anisotropic, reminiscent of the frictional adhesion behavior of gecko setal arrays. When a 60° tilt sample was actuated in the gripping direction, a static adhesion strength of ~1.4 N/cm(2) and a static friction strength of ~5.4 N/cm(2) were obtained. In contrast, when the dry adhesive was actuated in the releasing direction, we measured an initial repulsive normal force and negligible friction.     

Facile creation of hierarchical PDMS microstructures with extreme underwater superoleophobicity for anti-oil application in microfluidic channels

Composition modification and surface microstructures have been widely utilized in interface science to improve the surface performance. In this paper, we observed a significant improvement of oil contact angle (CA) from 66 ± 2° to 120 ± 4° by introducing a radical silanol group on a flat PDMS surface through oxygen plasma pretreatment. By combining surface microstructures and plasma modification, we produced three kinds of superoleophobic surfaces: 20 μm pitch micropillar arrays, 2.5 μm pitch micropillar arrays and gecko foot-like hierarchical microstructures. Among them, the hierarchical surface with high surface roughness showed extreme underwater superoleophobicity, which featured ultrahigh CA (175 ± 3°) and ultrasmall sliding angle (<1°). Quantitative measurements demonstrated that these superoleophobic surfaces exhibited distinct adhesive behaviors, by which they were interpreted as Wenzel's, Cassie's and the Lotus state, respectively. A microfluidic channel with superoleophobic microstructures was further created by novel curve-assisted imprint lithography, and the characterization based on anti-oil contamination applications was carried out and discussed. We believe that the superoleophobic surfaces will power broad applications in oil microdroplet transportation, anti-oil channels and droplet microfluidic systems.     

Contact shape controls adhesion of bioinspired fibrillar surfaces

Following a recent bioinspired paradigm, patterned surfaces can exhibit better adhesion than flat contacts. Previous studies have verified that finer contact structures give rise to higher adhesion forces. In this study, we report on the effect of the tip shape, which was varied systematically in fibrillar PDMS surfaces, produced by lithographic and soft-molding methods. For fiber radii between 2.5 and 25 microm, it is found that shape exerts a stronger effect on adhesion than size. The highest adhesion is measured for mushroom-like and spatular terminals, which attain adhesion values 30 times in excess of the flat controls and similar to a gecko toe. These results explain the shapes commonly found in biological systems, and help in the exploration of the parameter space for artificial attachment systems.     

Shear adhesion strength of thermoplastic gecko-inspired synthetic adhesive exceeds material limits

Natural gecko array wearless dynamic friction has recently been reported for 30,000 cycles on a smooth substrate. Following these findings, stiff polymer gecko-inspired synthetic adhesives have been proposed for high-cycle applications such as robot feet. Here we examine the behavior of high-density polyethylene (HDPE) and polypropylene (PP) microfiber arrays during repeated cycles of engagement on a glass surface, with a normal preload of less than 40 kPa. We find that fiber arrays maintained 54% of the original shear stress of 300 kPa after 10,000 cycles, despite showing a marked plastic deformation of fiber tips. This deformation could be due to shear-induced plastic creep of the fiber tips from high adhesion forces, adhesive wear, or thermal effects. We hypothesize that a fundamental material limit has been reached for these fiber arrays and that future gecko synthetic adhesive designs must take into account the high adhesive forces generated to avoid damage. Although the synthetic material and natural gecko arrays have a similar elastic modulus, the synthetic material does not show the same wear-free dynamic friction as the gecko.     

Superhydrophobicity of biological and technical surfaces under moisture condensation: stability in relation to surface structure

The stability of superhydrophobic properties of eight plants and four technical surfaces in respect to water condensation has been compared. Contact and sliding angles were measured after application of water drops of ambient temperature (20 degrees C) onto cooled surfaces. Water evaporating from the drops condensed, due to the temperature difference between the drops and the surface, on the cooled samples, forming "satellite droplets" in the vicinity of the drops. Surface cooling to 15, 10, and 5 degrees C showed a gradual decrease of superhydrophobicity. The decrease was dependent on the specific surface architecture of the sample. The least decrease was found on hierarchically structured surfaces with a combination of a coarse microstructure and submicrometer-sized structures, similar to that of the Lotus leaf. Control experiments with glycerol droplets, which show no evaporation, and thus no condensation, were carried out to verify that the effects with water were caused by condensation from the drop (secondary condensation). Furthermore, the superhydrophobic properties after condensation on cooled surfaces from a humid environment for 10 min were examined. After this period, the surfaces were covered with spherical water droplets, but most samples retained their superhydrophobicity. Again, the best stability of the water-repellent properties was found on hierarchically structured surfaces similar to that of the Lotus leaf.     

Durable Lotus-effect surfaces with hierarchical structure using micro- and nanosized hydrophobic silica particles

Surfaces with a very high apparent water contact angle (CA) and low water contact angle hysteresis (CAH) exhibit many useful characteristics, among them extreme water repellency, low drag for fluid flow, and a self-cleaning effect. The leaf of the Lotus plant (Nelumbo nucifera) achieves these properties using a hierarchical structure with roughness on both the micro- and nanoscale. It is of great interest to create durable surfaces with the so-called "Lotus effect" for many important applications. In this study, hierarchically structured surfaces with Lotus-effect properties were fabricated using micro- and nanosized hydrophobic silica particles and a simple spray method. In addition, hierarchically structured surfaces were prepared by spraying a nanoparticulate coating over a micropatterned surface. To examine the similarities between surfaces using microparticles versus a uniform micropattern as the microstructure, CA and CAH were compared across a range of pitch values for the two types of microstructures. Wear experiments were performed using an atomic force microscope (AFM), a ball-on-flat tribometer, and a water jet apparatus to verify multiscale wear resistance. These surfaces have potential uses in engineering applications requiring Lotus-effect properties and high durability.     

花生叶表面的高黏附超疏水特性研究及其仿生制备

花生是一种常见的豆科作物.与低黏附超疏水的荷叶不同,花生叶表面同时具有超疏水和高黏附特性.水滴在花生叶表面的接触角为151±2°,显示出超疏水特性.此外,水滴可以牢固地附着在花生叶表面,将花生叶翻转90°甚至180°,水滴均不会从表面滚落,显示了良好的黏附性(黏附力超过80μN).研究发现,花生叶表面呈现微纳米多级结构,丘陵状微米结构表面具有无规则排列的纳米结构.花生叶表面特殊的微纳米多尺度结构是其表面呈现高黏附超疏水特性的关键因素.结合实验数据,对花生叶表面特殊浸润性机理进行了简要阐述.受此启发,利用聚二甲基硅氧烷复形得到了与花生叶表面微结构类似的高黏附疏水表面.本文以期为仿生制备高黏附超疏水表面提供新思路.     

Teflon hierarchical nanopillars with dry and wet adhesive properties

The superior material properties of β‐keratin along with the hierarchical high‐aspect‐ratio structure of geckos' foot pad have enabled geckos to stick readily and rapidly to almost any surfaces in both dry and wet conditions. In this research, nonsticky fluoropolymer (Teflon AF) resembling β‐keratin rigidity and having an extremely low surface energy and dielectric constant was applied to fabricate a novel dry adhesive consisting of high‐aspect‐ratio nanopillars terminated with a “fluffy” top layer. Both the nanopillars and the terminating layer are fabricated concurrently by replica molding using a nanoporous anodic aluminum oxide membrane as the mold. These Teflon AF hierarchical nanostructures are shown to have an exceptional capacity to generate strong adhesion in both dry conditions and under water because of combined actions of van der Waals forces, electrostatic attractions, and hydrophobic effects. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

利用胶体探针技术研究多巴与纳米、微米及微纳复合结构表面之间的相互作用

通过在硅片表面有机蒸镀不同厚度的二十九烷制备了不同晶体密度的仿生旱金莲叶面蜡质纳米结构表面,采用端基修饰多巴的原子力显微镜胶体探针,对各纳米结构表面进行了粘附性能测试,发现蒸镀200 nm厚度二十九烷结晶的纳米结构表面具有较低粘附力。采用反应离子刻蚀方法制备了不同高度的硅材质仿生鲨鱼皮微米结构表面,并选择了200 nm厚度二十九烷在仿生鲨鱼皮表面进行有机蒸镀制备了微纳复合结构表面,通过胶体探针的研究发现多巴与高度为1、3、5μm微纳复合结构表面的粘附力均小于与200 nm厚度二十九烷结晶的纳米结构表面之间的粘附力,说明微纳复合结构表面具有很强的抗多巴粘附能力,并且这种复合结构表面相对于硅材质的仿生鲨鱼皮微米结构表面还兼有旱金莲叶面的强疏水性和极佳的抗水粘附能力。     

Conformal adhesion enhancement on biomimetic microstructured surfaces

Inspired by the superior adhesive ability of the gecko foot pad, we report an experimental study of conformal adhesion of a soft elastomer thin film on biomimetic micropatterned surfaces (micropillars), showing a remarkable adhesion enhancement due to the surface patterning. The adhesion of a low-surface-energy poly(dimethylsiloxane) tape to a SU-8 micropatterned surface was found be able to increase by 550-fold as the aspect ratio increases from 0 to 6. The dependency of the adhesion enhancement on the aspect ratio is highly nonlinear. A series of peeling experiment coupled with optical interference imaging were performed to investigate the adhesion enhancement as a function of the height of the micropillars and the associated delamination mechanisms. Local elastic energy dissipation, side-wall friction, and plastic deformations were analyzed and discussed in terms of their contributions to the adhesion enhancement. We conclude that the local adhesion and friction events of pulling micropillars out of the embedded polymer film play a primary role in the observed adhesion enhancement. The technical implications of this local friction-based adhesion enhancement mechanism were discussed for the effective assembly of similar or dissimilar material components at small scales. The combined use of the micro/nanostructured surfaces with the van der Waals interactions seem to be a potentially more universal solution than the conventional adhesive bonding technology, which depends on the chemical and viscoelastic properties of the materials.     

An improved meniscus surface model for contacting rough surfaces

Based on the Extended-Maugis-Dugdale (EMD) elastic theory, a single asperity capillary meniscus model considering asperity deformation due to both contact and adhesive forces was developed. Specifically included in the single asperity meniscus model was the solid surface interaction inside the contact area. Subsequently, the single asperity model was coupled with a statistical roughness surface model to develop an improved meniscus surface model applicable to a wide-range of humidity levels and adhesion parameter values. Simulations were performed using typical surfaces found in microelectromechanical systems (MEMS) and magnetic storage hard disk drives to examine the effects of surface roughness and relative humidity. It was found that smoother surfaces give rise to higher adhesive and pull-off forces, and at higher relative humidity levels, the capillary force governs the adhesive behavior. As the humidity decreases, the solid surface interaction increases and needs to be included in the total meniscus adhesion. By integrating the adhesive force-displacement curves, the adhesion energy per unit area was calculated for MEMS surfaces and favorably compared with reported experimental data.     

Frictional behaviors of three kinds of nanotextured surfaces

Nanodot‐textured surface, nanorod‐textured surface and nanocomposite‐textured surface were prepared by the hydrothermal technique and successive ion layer absorption and reaction technique. The adhesion and friction properties of the three kinds of nanotextured surfaces were investigated using an atomic force microscope colloidal probe. Experimental results revealed that the nanorod‐textured surface and nanocomposite‐textured surface can significantly reduce adhesive and friction forces compared with a nanodot‐textured surface. The main reason for this phenomenon was that the nanorod and nanocomposite textures can reduce contact area between the sample surface and the colloidal probe. The effects of surface root mean square roughness, applied load and sliding velocity on the adhesion and friction behaviors of the nanotextured surfaces were investigated. The adhesive and friction forces of the nanotextured surfaces decreased with the increasing surface root mean square roughness. Compared with the nanocomposite‐textured surface, the nanorod‐textured surface has better adhesion and frictional performance. Copyright © 2016 John Wiley & Sons, Ltd.     

Atomic force microscopy measurement of heterogeneity in bacterial surface hydrophobicity

The structure and physicochemical properties of microbial surfaces at the molecular level determine their adhesion to surfaces and interfaces. Here, we report the use of atomic force microscopy (AFM) to explore the morphology of soft, living cells in aqueous buffer, to map bacterial surface heterogeneities, and to directly correlate the results in the AFM force-distance curves to the macroscopic properties of the microbial surfaces. The surfaces of two bacterial species, Acinetobacter venetianus RAG-1 and Rhodococcus erythropolis 20S-E1-c, showing different macroscopic surface hydrophobicity were probed with chemically functionalized AFM tips, terminating in hydrophobic and hydrophilic groups. All force measurements were obtained in contact mode and made on a location of the bacterium selected from the alternating current mode image. AFM imaging revealed morphological details of the microbial-surface ultrastructures with about 20 nm resolution. The heterogeneous surface morphology was directly correlated with differences in adhesion forces as revealed by retraction force curves and also with the presence of external structures, either pili or capsules, as confirmed by transmission electron microscopy. The AFM force curves for both bacterial species showed differences in the interactions of extracellular structures with hydrophilic and hydrophobic tips. A. venetianus RAG-1 showed an irregular pattern with multiple adhesion peaks suggesting the presence of biopolymers with different lengths on its surface. R. erythropolis 20S-E1-c exhibited long-range attraction forces and single rupture events suggesting a more hydrophobic and smoother surface. The adhesion force measurements indicated a patchy surface distribution of interaction forces for both bacterial species, with the highest forces grouped at one pole of the cell for R. erythropolis 20S-E1-c and a random distribution of adhesion forces in the case of A. venetianus RAG-1. The magnitude of the adhesion forces was proportional to the three-phase contact angle between hexadecane and water on the bacterial surfaces.     

Nanostructure on taro leaves resists fouling by colloids and bacteria under submerged conditions

The antifouling and self-cleaning properties of plants such as Nelumbo nucifera (lotus) and Colocasia esculenta (taro) have been attributed to the superhydrophobicity resulting from the hierarchical surface structure of the leaf and the air trapped between the nanosized epicuticular wax crystals. The reported study showed that the nanostructures on the taro leaf surfaces were also highly resistant to particle and bacterial adhesion under completely wetted conditions. Adhesion force measurements using atomic force microscopy revealed that the adhesion force on top of the papilla as well as the area around it was markedly lower than that on the edge of an epidermal cell. The decreased adhesion force and the resistance to particle and bacterial adhesion were attributed to the dense nanostructures found on the epidermal papilla and the area surrounding it. These results suggest that engineered surfaces with properly designed nanoscale topographic structures could potentially reduce or prevent particle/bacterial fouling under submerged conditions.     

Major influence of the hydrophobic chain length in the formation of poly(3,4-propylenedioxypyrrole) (PProDOP) nanofibers with special wetting properties

Several species in nature have special wetting properties such as Lotus leaves or rose petals. Both the surface morphology and surface energy play a fundamental role. In particular, nanofibers were found to be exceptional surface structures due to a possible control in both water hydrophobicity and water adhesion as a function of their length, diameter, their orientation to the substrate or the spacing between them. Here, in the aim to prepare nanofibers with high liquid-repellent properties using conducting polymers, we have synthesized 3,4-propylenedioxypyrrole (ProDOP) derivatives with hydrocarbon and fluorocarbon chains in the 3-position, keeping the NH group free (important condition to lead to nanofibers thanks to hydrogen bonds). Different hydrocarbon and fluorocarbon chain lengths are studied. We obtain, for example, nanofibers of different size with octyl, decyl and C₄F₉ chains (intermediate hydrophobicity) with different liquid-repellent properties and liquid adhesion properties. More precisely, PProDOP-H₈ is close to superhydrophobic properties (low water adhesion) while PProDOP-H₁₀ is parahydrophobic (high water adhesion). This works could find many potential applications in the nanotechnology field as water harvesting surfaces, liquid separation membrane, and in anti-bioadhesion. Due to the presence of free NH groups, these materials could also be used as pH-sensitive materials while the nitrogen could also be easily functionalized.     

Durable ultra‐hydrophobic surfaces for self‐cleaning applications

Self‐cleaning surfaces have received a great deal of attention, both in research studies and commercial applications. Both transparent and non‐transparent self‐cleaning surfaces are highly desirable as they offer many advantages, and their potential applications are endless. The self‐cleaning mechanism can be seen in nature. The Lotus flower, a symbol of purity in Asian cultures, grows in muddy waters, but it stays clean and untouched by dirt, organisms, and pollutants. The Lotus leaf self‐cleaning surface is hydrophobic and rough, showing a multi‐layer morphology of nanoscaled roughness. While hydrophobicity produces a high contact angle, the surface morphology reduces the adhesion of water drops to the surface, which slides easily across the leaf surface carrying the dirt particles with them. Different ultra‐hydrophobic, non‐transparent, and transparent coatings, for potential self‐cleaning applications, were produced on polycarbonate (PC) substrates, using hydrophobic chemistry and different configurations of roughening micro‐ and nano‐particles. However, in most cases, these coatings present low adhesion and durability. The stability and durability of the ultra‐hydrophobic surfaces is of key importance for potential, commercially viable, self‐cleaning applications thus durability and stability enhancement of such coatings was attempted by different methods, evaluated, and eventually improved using a solvent‐bonding technique. Copyright © 2008 John Wiley & Sons, Ltd.     

Pattern-dependent tunable adhesion of superhydrophobic MnO2 nanostructured film

Tuning the adhesive force on a superhydrophobic MnO(2) nanostructured film was achieved by fabricating different patterns including meshlike, ball cactus-like, and tilted nanorod structures. The marvelous modulation range of the adhesive forces from 130 to nearly 0 μN endows these superhydrophobic surfaces with extraordinarily different dynamic properties of water droplets. This pattern-dependent adhesive property is attributed to the kinetic barrier difference resulting from the different continuity of the three-interface contact line. This finding will provide the general strategies for the adhesion adjustment on superhydrophobic surfaces.