Biomimetic mushroom-shaped fibrillar adhesive microstructure

To improve the adhesive properties of artificial fibrillar contact structures, the attachment systems of beetles from the family Chrysomelidae were chosen to serve as a model. Biomimetic mushroom-shaped fibrillar adhesive microstructure inspired by these systems was characterized using a variety of measurement techniques and compared with a control flat surface made of the same material. Results revealed that pull-off force and peel strength of the structured specimens are more than twice those of the flat specimens. In contrast to the control system, the structured one is found to be very tolerant to contamination and able to recover its adhesive properties after being washed in a soap solution. Based on the combination of several geometrical principles found in biological attachment devices, the presented microstructure exhibits a considerable step towards the development of an industrial dry adhesive.     

Suction component in adhesion of mushroom-shaped microstructure

To shed light on the role of suction in adhesion of microstructure with mushroom-shaped terminal elements, we compared pull-off forces measured at different retraction velocities on structured and smooth surfaces under different pressure conditions. The results obtained allow us to suggest that suction may contribute up to 10 per cent of the pull-off force measured on the structured surfaces at high velocities. We therefore conclude that the attachment ability of this biomimetic adhesive must not be solely based on van der Waals forces. Our experiments also suggest a change in visco-elastic properties of the structured surfaces compared with the bulk material. Based on the results obtained, it is assumed that this adhesive may be suitable in dynamic pick-and-drop processes even under vacuum conditions at which sufficiently high adhesive capability is maintained.     

Close-up of mushroom-shaped fibrillar adhesive microstructure: contact element behaviour

To analyse the performance of mushroom-shaped fibrillar adhesive microstructure, its behaviour was studied during different stages of attachment-loading-detachment cycle. Visualizing the evolutions of real contact area of single microfibres, it is shown that the mushroom-shaped geometry of contact elements promotes fast and simple generation of reliable adhesion. The mushroom-shaped geometry seems to transform fibrillar contact elements into passive suction devices and makes them tolerant to overload, thus enhancing their robustness and stability. These findings may also be extrapolated to biological fibrillar attachment devices sharing the same geometry.     

Tuning the geometrical parameters of biomimetic fibrillar structures to enhance adhesion.

Fibrillar structures are common features on the feet of many animals, such as geckos, spiders and flies. Theoretical analyses often use periodical array to simulate the assembly, and each fibril is assumed to be of equal load sharing (ELS). On the other hand, studies on a single fibril show that the adhesive interface is flaw insensitive when the size of the fibril is not larger than a critical one. In this paper, the Dugdale-Barenblatt model has been used to study the conditions of ELS and how to enhance adhesion by tuning the geometrical parameters in fibrillar structures. Different configurations in an array of fibres are considered, such as line array, square and hexagonal patterns. It is found that in order to satisfy flaw-insensitivity and ELS conditions, the number of fibrils and the pull-off force of the fibrillar interface depend significantly on the fibre separation, the interface interacting energy, the effective range of cohesive interaction and the radius of fibrils. Proper tuning of the geometrical parameters will enhance the pull-off force of the fibrillar structures. This study may suggest possible methods to design strong adhesion devices for engineering applications.     

Modelling the combined effect of surface roughness and topography on bacterial attachment

Bacterial attachment is a complex process affected by flow conditions,imparted stresses,and the sur-face properties and structure of both the supporting material and the cell.Experiments on the initial attachment of cells of the bacterium Streptococcus gordonii (S.gordonii),an important early coloniser of dental plaque,to samples of stainless steel (SS) have been reported in this work.The primary aim motivating this study was to establish what affect,if any,the surface roughness and topology of sam-ples of SS would have on the initial attachment of cells of the bacterium S.gordonii.This material and bacterium were chosen by virtue of their relevance to dental implants and dental implant infections.Prior to bacterial attachment,surfaces become conditioned by the interfacing environment (salivary pellicle from the oral cavity for instance).For this reason,cell attachment to samples of SS pre-coated with saliva was also studied.By implementing the Extended Derjaguin Landau Verwey and Overbeek(XDLVO) theory coupled with convection-diffusion-reaction equations and the surface roughness infor-mation,a computational model was developed to help better understand the physics of cell adhesion.Surface roughness was modelled by reconstructing the surface topography using statistical parame-ters derived from atomic force microscopy (AFM) measurements.Using this computational model,the effects of roughness and surface patterns on bacterial attachment were examined quantitatively in both static and flowing fluid environments.The results have shown that rougher surfaces (within the sub-microscale) generally increase bacterial attachment in static fluid conditions which quantitatively agrees with experimental measurements.Under flow conditions,computational fluid dynamics (CFD) simula-tions predicted reduced convection-diffusion inside the channel which would act to decrease bacterial attachment.When combined with surface roughness effects,the computational model also predicted that the surface topographies discussed within this work produced a slight decrease in overall bacterial attachment.This would suggest that the attachment-preventing effects of surface patterns dominate over the adhesion-favourable sub-microscale surface roughness;hence,producing a net reduction in adhered cells.This qualitatively agreed with experimental observations reported here and quantitatively matched experimental observations for low flow rates within measurement error.     

Functional Adhesive Surfaces with “Gecko” Effect: The Concept of Contact Splitting

Nature has developed reversibly adhesive surfaces whose stickiness has attracted much research attention over the last decade. The central lesson from nature is that “patterned” or “fibrillar” surfaces can produce higher adhesion forces to flat and rough substrates than smooth surfaces. This paper critically examines the principles behind fibrillar adhesion from a contact mechanics perspective, where much progress has been made in recent years. The benefits derived from “contact splitting” into fibrils are separated into extrinsic/intrinsic contributions from fibril deformation, adaptability to rough surfaces, size effects due to surface‐to‐volume ratio, uniformity of stress distribution, and defect‐controlled adhesion. Another section covers essential considerations for reliable and reproducible adhesion testing, where better standardization is still required. It is argued that, in view of the large number of parameters, a thorough understanding of adhesion effects is required to enable the fabrication of reliable adhesive surfaces based on biological examples.     

对偶件表面粗糙度对UHMWPE摩擦磨损性能的影响

人工关节在服役过程中的磨损主要发生在超高分子量聚乙烯(UHMWPE)关节臼的表面。UH-MWPE的磨擦磨损性能受多因素影响,其中与UHM-WPE对磨的硬质材料(对偶件)表面粗糙度是关键因素之一。综述了近年来在对偶件表面粗糙度对UH-MWPE摩擦磨损性能影响方面的研究工作,结合UHMWPE的磨损机制,重点阐述对偶件表面粗糙度对UHMWPE磨损率、UHMWPE磨屑尺寸和人工关节润滑机制的影响。     

表面粗糙度对TiAl/AgCu/TiAl钎焊质量的影响

为探究表面粗糙度对钎焊质量的影响,在优选的钎焊温度870 ℃和保温时间5 min条件下,以AgCu合金为钎料,进行TiAl合金的真空钎焊,研究了表面粗糙度与磨痕方向对钎料润湿铺展、钎焊接头界面组织、力学性能的影响.采用扫描电镜、能谱分析、光学显微镜和万能材料试验机等分析测试手段,对钎焊接头的组织与力学性能进行分析.结果表明：AgCu钎料在各向异性的TiAl基板表面润湿铺展时会受表面特征影响,沿着特定的方向优先铺展,当基板表面不具各向异性特征时,钎料润湿铺展近似为圆形;表面粗糙度S_a=0.103 μm的基板,钎焊接头的平均剪切强度最优,达到323 MPa,而表面粗糙度S_a=0.133 μm的基板,接头平均剪切强度为245 MPa,钎焊接头强度均方差最大.     

Shearing of fibrillar adhesive microstructure: friction and shear-related changes in pull-off force.

To characterize the effect of shearing on function of fibrillar adhesive microstructure, friction and shear-related changes in pull-off force of a biomimetic polyvinylsiloxane mushroom-shaped fibrillar adhesive microstructure were studied. In contrast to a control flat surface, which exhibited pronounced stick-slip motion accompanied with high friction, the fibrillar microstructure demonstrated a stable and smooth sliding with a friction coefficient approximately four times lower. The structured contact also manifested zero pull-off force in a sheared state, while the flat surface exhibited highly scattered and unreliable pull-off force when affected by contact shearing. It appears that the fibrillar microstructure can be used in applications where a total attachment force should be generated in a binary on/off state and, most surprisingly, is suitable to stabilize and minimize elastomer friction.     

金膜表面聚丙烯酰吡咯膜的结构和表面形貌研究

研究了一种拟用于SPR生物传感器的生物材料.为此,首先在金膜表面制备聚丙烯酰吡咯膜,然后对膜的结构和表面形貌进行了研究.其中,制备聚丙烯酰吡咯(PAP)的方法是以丙烯酰氯和吡咯钾盐合成丙烯酰吡咯,然后以偶氮二异丁腈(AIBN)为引发剂,进行自由基聚合;金膜表面制备PAP膜的方法是:PAP溶于N,N-二甲基甲酰胺(DMF),然后旋涂到金膜上;最后,涂膜表面与吡咯在三氯化铁溶液中化学法聚合,形成聚合吡咯层.红外光谱可以看出,所得聚合物物质就是需要的产品;AFM非原位表面形貌图像和三维立体形貌图上显示涂膜覆盖了金膜表面原有的小孔,表面粗糙度比纯金膜增大.     

纸张表面粗糙度和照射条件对纸张白度的影响

用基于测量仪器和样品散射特性的数学模型研究了纸张白度和纸张表面粗糙度的关系,比较了有和没有散射光中的镜面成分条件下的纸张白度和纸张表面粗糙度的关系,纸张白度是根据ISO/DIS 11475计算获得的.结果发现,所研究的纸张白度取决于纸张的表面粗糙度和所选择的照射条件,而纸张的色度坐标与两者无关.     

The origin of wrinkles on transferred graphene

When two-dimensional graphene is exfoliated from three-dimensional highly oriented pyrolytic graphite (HOPG), ripples or corrugations always exist due to the intrinsic thermal fluctuations. Surface-grown graphenes also exhibit wrinkles, which are larger in dimension and are thought to be caused by the difference in thermal expansion coefficients between graphene and the underlying substrate in the cooling process after high temperature growth. For further characterization and applications, it is necessary to transfer the surface-grown graphenes onto dielectric substrates, and other wrinkles are generated during this process. Here, we focus on the wrinkles of transferred graphene and demonstrate that the surface morphology of the growth substrate is the origin of the new wrinkles which arise in the surface-to-surface transfer process; we call these morphology-induced wrinkles. Based on a careful statistical analysis of thousands of atomic force microscopy (AFM) topographic data, we have concluded that these wrinkles on transferred few-layer graphene (typically 1–3 layers) are determined by both the growth substrate morphology and the transfer process. Depending on the transfer medium and conditions, most of the wrinkles can be either erased or preserved. Our work suggests a new route for graphene engineering involving structuring the growth substrate and tailoring the transfer process.      

The effect of aspect ratio on adhesion and stiffness for soft elastic fibres

The effect of aspect ratio on the pull-off stress and stiffness of soft elastic fibres is studied using elasticity and numerical analysis. The adhesive interface between a soft fibre and a smooth rigid surface is modelled using the Dugdale–Barenblatt model. Numerical simulations show that, while pull-off stress increases with decreasing aspect ratio, fibres get stiffer. Also, for sufficiently low aspect ratio fibres, failure occurs via the growth of internal cracks and pull-off stress approaches the intrinsic adhesive strength. Experiments carried out with various aspect ratio polyurethane elastomer fibres are consistent with the numerical simulations.     

Effect of surface roughness on measurement of light distribution in intralipid suspension

Light distribution in the biological tissue phantom intralipid suspension with different interface roughness was measured for quantitative understanding of surface roughness effect. The results show that the surface roughness strongly affects light distribution inside the approximately semi-infinite biological tissue phantom. The phantom surface possesses a certain degree roughness and the effect of the surface roughness on measurement results of light distribution in tissue is substantial, so the measurement of light distribution in biological tissue needs to take surface roughness into account.     

Strength statistics of adhesive contact between a fibrillar structure and a rough substrate

Equal distribution of load among fibrils in contact with a substrate is an important characteristic of fibrillar structures used by many small animals and insects for contact and adhesion. This is in contrast with continuum systems where stress concentration dominates interfacial failure. In this work, we study how adhesion strength of a fibrillar system depends on substrate roughness and variability of the fibril structure, which are modelled using probability distributions for fibril length and fibril attachment strength. Monte Carlo simulations are carried out to determine the adhesion strength statistics where fibril length follows normal or uniform distribution and attachment strength has a power-law form. Our results indicate that the strength distribution is Gaussian (normal) for both the uniform and the normal distributions for length. However, the fibrillar structure having normally distributed lengths has higher strength and lower toughness than one having uniformly distributed lengths. Our simulations also show that an increase in the compliance of the fibrils can compensate for both the substrate roughness and the attachment strength variation. We also show that, as the number of fibrils n increases, the load-carrying efficiency of each fibril goes down. For large n, this effect is found to be small. Furthermore, this effect is compensated by the fact that the standard deviation of the adhesive strength decreases as 1/ square root n.     

表面微结构金属干式电极制造及细菌粘附性能研究

为开发出高性能生物医用干式电极,提出了利用激光微铣-重铸加工方法,实现了表面具有微结构阵列特征的新型金属干式电极的制造成形。在分析电极表面微观形貌的基础上,研究了电极表面的润湿性能,并重点研究了扫描间距、扫描速度和扫描次数等加工参数对大肠杆菌粘附性能的影响规律。研究结果表明：在一定工艺参数条件下所加工出具有微结构阵列特征的电极的接触角可达150°以上,表现出超疏水的特性。在不同扫描间隙和扫描次数条件下加工出的电极对大肠杆菌的粘附性能具有较大影响,在选择0.1 mm扫描间隙时,电极表面粘附的大肠杆菌数量最少,适当增加扫描次数,也能够有效地减少电极表面大肠杆菌的粘结,从而发挥较好的抗菌效果。通过改变扫描速度加工出的电极则对大肠杆菌的粘附性能影响不大。     

Roles of microstructure,inclusion, and surface roughness on rolling contact fatigue of a wind turbine gear

Contact fatigue is a key feature limiting the service lives and reliabilities of gears. The gear contact fatigue failure mechanism has not been understood fundamentally due to the complexities of structural factors, material properties, and operating conditions. In this work, an integrated finite element model of a megawatt level wind turbine gear is established considering the real gear geometry, material microstructure heterogeneity, existence of nonmetallic inclusion, and the tooth surface roughness. The gear steel material properties are defined based on the crystal elasticity anisotropy framework. The modified Dang Van multiaxial criterion is utilized to estimate the material fatigue failure probability during gear engagement. With the developed model, the roles of microstructure, inclusion, and surface roughness on the gear contact fatigue behaviour are comparatively investigated. Additionally, the influences of different inclusion size and surface roughness profile on gear failure risk are investigated and discussed in detail.