Wetting Characteristics of Insect Wing Surfaces

Biological tiny structures have been observed on many kinds of surfaces such as lotus leaves,which have an effect on thecoloration of Morpho butterflies and enhance the hydrophobicity of natural surfaces.We investigated the micro-scale andnano-scale structures on the wing surfaces of insects and found that the hierarchical multiple roughness structures help in enhancingthe hydrophobicity.After examining 10 orders and 24 species of flying Pterygotan insects,we found that micro-scaleand nano-scale structures typically exist on both the upper and lower wing surfaces of flying insects.The tiny structures such asdenticle or setae on the insect wings enhance the hydrophobicity,thereby enabling the wings to be cleaned more easily.And thehydrophobic insect wings undergo a transition from Cassie to Wenzel states at pitch/size ratio of about 20.In order to examinethe wetting characteristics on a rough surface,a biomimetic surface with micro-scale pillars is fabricated on a silicon wafer,which exhibits the same behavior as the insect wing,with the Cassie-Wenzel transition occurring consistently around apitch/width value of 20.     

Effects of Methanol on Wettability of the Non-Smooth Surface on Butterfly Wing

The contact angles of distilled water and methanol solution on the wings of butterflies were determined by a visual contact angle measuring system. The scale structures of the wings were observed using scanning electron microscopy, The influence of the scale micro- and ultra-structure on the wettability was investigated. Results show that the contact angle of distilled water on the wing surfaces varies from 134.0° to 159.2°. High hydrophobicity is found in six species with contact angles greater than 150°. The wing surfaces of some species are not only hydrophobic but also resist the wetting by methanol solution with 55% concentration. Only two species in Parnassius can not resist the wetting because the micro-structure （spindle-like shape） and ultra-structure （pinnule-like shape） of the wing scales are remarkably different from that of other species. The concentration of methanol solution for the occurrence of spreading/wetting on the wing surfaces of different species varies from 70% to 95%. After wetting by methanol solution for 10 min, the distilled water contact angle on the wing surface increases by 0.8°-2.1°, showing the promotion of capacity against wetting by distilled water.     

Wet adhesion and adhesive locomotion of snails on anti-adhesive non-wetting surfaces

Creating surfaces capable of resisting liquid-mediated adhesion is extremely difficult due to the strong capillary forces that exist between surfaces. Land snails use this to adhere to and traverse across almost any type of solid surface of any orientation (horizontal, vertical or inverted), texture (smooth, rough or granular) or wetting property (hydrophilic or hydrophobic) via a layer of mucus. However, the wetting properties that enable snails to generate strong temporary attachment and the effectiveness of this adhesive locomotion on modern super-slippy superhydrophobic surfaces are unclear. Here we report that snail adhesion overcomes a wide range of these microscale and nanoscale topographically structured non-stick surfaces. For the one surface which we found to be snail resistant, we show that the effect is correlated with the wetting response of the surface to a weak surfactant. Our results elucidate some critical wetting factors for the design of anti-adhesive and bio-adhesion resistant surfaces.     

Compound Microstructures and Wax Layer of Beetle Elytral Surfaces and Their Influence on Wetting Properties

A beetles’ first line of defense against environmental hazards is their mesothoracic elytra – rigid, protective forewings. In order to study the interaction of these wings with water, the surface microstructures of various beetles’ elytra were observed by Environment Scanning Electron Microscopy (ESEM) and Atomic Force Microscopy (AFM). Chemistry components were ascertained using X-ray photoelectron spectroscopy (XPS). All the beetles of various habitats (including desert, plant, dung, land and water) exhibited compound microstructures on their elytra. The wetting properties of these elytra were identified using an optical contact angle meter. In general the native elytra exhibited hydrophilic or weak hydrophobic properties with contact angles (CAs) ranging from 47.5° to 109.1°. After treatment with chloroform, the CAs all increased on the rougher elytral surfaces. The presence of wax is not the only determinant of hydrophobic properties, but rather a combination with microscopic structures found on the surfaces. Irregularities and the presence or absence of tiny cracks, hairs (or setae), pores and protrusions are important factors which influence the wetting properties. Rougher elytral surfaces tended to present a stronger hydrophobicity. Effects on hydrophobicity, such as surface microstructures, chemistry, environment and aging (referring to the time after emergence), are also included and discussed. Our results also provide insights into the motion of water droplets when in contact with beetle elytra.     

Modeling Superhydrophobic Contact Angles and Wetting Transition

It is well known that surface roughness has a very important effect on superhydrophobicity.The Wenzel and Cassie-Baxtermodels,which correspond to the homogeneous and heterogeneous wetting respectively,are currently primary instructions fordesigning superhydrophobic surfaces.However,the particular drop shape that a drop exhibits might depend on how it is formed.A water drop can occupy multiple equilibrium states,which relate to different local minimal energy.In some cases,both equilibriumstates can even co-exist on a same substrate.Thus the apparent contact angles may vary and have different values.Wediscuss how the Wenzel and Cassie-Baxter equations determine the homogeneous and heterogeneous wetting theoretically.Contact angle analysis on hierarchical surface structure and contact angle hysteresis has been put specific attention.In particular,we study the energy barrier of transition from Cassie-Baxter state to Wenzel state,based on existing achievement by previousresearchers,to determine the possibility of the transition and how it can be interpreted.It has been demonstrated that surfaceroughness and geometry will influence the energy required for a drop to get into equilibrium,no matter it is homogeneous orheterogeneous wetting.     

Anisotropism of the Non-Smooth Surface of Butterfly Wing

Twenty-nine species of butterflies were collected for observation and determination of the wing surfaces using a ScanningElectron Microscope(SEM).Butterfly wing surface displays structural anisotropism in micro-,submicro- and nano-scales.Thescales on butterfly wing surface arrange like overlapping roof tiles.There are submicrometric vertical gibbosities,horizontallinks,and nano-protuberances on the scales.First-incline-then-drip method and first-drip-then-incline method were used tomeasure the Sliding Angle(SA)of droplet on butterfly wing surface by an optical Contact Angle(CA)measuring system.Relatively smaller sliding angles indicate that the butterfly wing surface has fine self-cleaning property.Significantly differentSAs in various directions indicate the anisotropic self-cleaning property of butterfly wing surface.The SAs on the butterfly wingsurface without scales are remarkably larger than those with scales,which proves the crucial role of scales in determining theself-cleaning property.Butterfly wing surface is a template for design and fabrication ofbiomimetic materials and self-cleaningsubstrates.This work may offer insights into how to design directional self-cleaning coatings and anisotropic wetting surface.     

Measurements of conformational changes during adhesion of lipid and protein (polylysine and S-layer) surfaces

The adhesion forces between various surfaces were measured using the "surface forces apparatus" technique. This technique allows for the thickness of surface layers and the adhesion force between them to be directly measured in controlled vapor or liquid environments. Three types of biological surfaces were prepared by depositing various lipid-protein monolayers (with thicknesses ranging from 1 to 4 nm) on the inert, molecularly smooth mica surface: (i) hydrophobic lipid monolayers; (ii) amphiphilic polyelectrolyte surfaces of adsorbed polylysine; and (iii) deposited bacterial S-layer proteins. The adhesion, swelling, and wetting properties of these surfaces was measured as a function of relative humidity and time. Initial adhesion is due mainly to the van der Waals forces arising from nonpolar (hydrophobic) contacts. Following adhesive contact, significant molecular rearrangements can occur which alter their hydrophobic-hydrophilic balance and increase their adhesion with time. Increased adhesion is generally enhanced by (i) increased relative humidity (or degree of hydration); (ii) increased contact time; and (iii) increased rates of separation. The results are likely to be applicable to the adhesion of many other biosurfaces, and show that the hydrophobicity of a lipid or protein surface is not an intrinsic property of that surface but depends on its environment (e.g., on whether it is in aqueous solution or exposed to the atmosphere), and on the relative humidity of the atmosphere. It also depends on whether the surface is in adhesive contact with another surface and-when considering dynamic (nonequilibrium) conditions-on the time and previous history of its interaction with that surface. (c) 1993 John Wiley & Sons, Inc.     

Geckos as Springs: Mechanics Explain Across-Species Scaling of Adhesion

One of the central controversies regarding the evolution of adhesion concerns how adhesive force scales as animals change in size, either among or within species. A widely held view is that as animals become larger, the primary mechanism that enables them to climb is increasing pad area. However, prior studies show that much of the variation in maximum adhesive force remains unexplained, even when area is accounted for. We tested the hypothesis that maximum adhesive force among pad-bearing gecko species is not solely dictated by toepad area, but also depends on the ratio of toepad area to gecko adhesive system compliance in the loading direction, where compliance (C) is the change in extension (Δ) relative to a change in force (F) while loading a gecko’s adhesive system (C = dΔ/dF). Geckos are well-known for their ability to climb on a range of vertical and overhanging surfaces, and range in mass from several grams to over 300 grams, yet little is understood of the factors that enable adhesion to scale with body size. We examined the maximum adhesive force of six gecko species that vary in body size (~2–100 g). We also examined changes between juveniles and adults within a single species (Phelsuma grandis). We found that maximum adhesive force and toepad area increased with increasing gecko size, and that as gecko species become larger, their adhesive systems become significantly less compliant. Additionally, our hypothesis was supported, as the best predictor of maximum adhesive force was not toepad area or compliance alone, but the ratio of toepad area to compliance. We verified this result using a synthetic “model gecko” system comprised of synthetic adhesive pads attached to a glass substrate and a synthetic tendon (mechanical spring) of finite stiffness. Our data indicate that increases in toepad area as geckos become larger cannot fully account for increased adhesive abilities, and decreased compliance must be included to explain the scaling of adhesion in animals with dry adhesion systems.     

Wetting transition of nanodroplets of water on textured surfaces: a molecular dynamics study

The crossover behaviour of water droplet's state from the Wenzel state to the Cassie state with varying pillar height and surface fraction is examined critically using molecular dynamics. We report the effect of the system size on the wetting behaviour of water droplets by examining the contact angle for both regimes. We observe that when the droplet size is comparable to the pillar dimension, the contact angle of droplets fluctuates with increasing droplet size because of the contact line pinning, which is more pronounced in the Wenzel regime. We further demonstrate the phantom-wall method to evaluate free energy of intermediate wetting states.     

Self-Drying: A Gecko's Innate Ability to Remove Water from Wet Toe Pads

When the adhesive toe pads of geckos become wet, they become ineffective in enabling geckos to stick to substrates. This result is puzzling given that many species of gecko are endemic to tropical environments where water covered surfaces are ubiquitous. We hypothesized that geckos can recover adhesive capabilities following exposure of their toe pads to water by walking on a dry surface, similar to the active self-cleaning of dirt particles. We measured the time it took to recover maximum shear adhesion after toe pads had become wet in two groups, those that were allowed to actively walk and those that were not. Keeping in mind the importance of substrate wettability to adhesion on wet surfaces, we also tested geckos on hydrophilic glass and an intermediately wetting substrate (polymethylmethacrylate; PMMA). We found that time to maximum shear adhesion recovery did not differ in the walking groups based on substrate wettability (22.7±5.1 min on glass and 15.4±0.3 min on PMMA) but did have a significant effect in the non-walking groups (54.3±3.9 min on glass and 27.8±2.5 min on PMMA). Overall, we found that by actively walking, geckos were able to self-dry their wet toe pads and regain maximum shear adhesion significantly faster than those that did not walk. Our results highlight a unexpected property of the gecko adhesive system, the ability to actively self-dry and recover adhesive performance after being rendered dysfunctional by water.     

Effect of Microscale Contact State of Polyurethane Surface on Adhesion and Friction

The effect of microscale contact of rough surfaces on the adhesion and friction under negative normal forces was experimentally investigated. The adhesive force of single point contact - sapphire ball to flat polyurethane did not vary with the normal force. With rough surface contact, which was assumed to be a great number of point contacts, the adhesive force increased logarithmically with the normal force. Under negative normal force adhesive state, the tangential force （more than hundred mN） were much larger than the negative normal force （several mN） and increased with the linear decrease of negative normal force. The results reveal why the gecko＇s toe must slide slightly on the target surface when it makes contact on a surface and suggest how a biomimetic gecko foot might be designed.     

Numerical Study of Wetting Transitions on Biomimetic Surfaces Using a Lattice Boltzmann Approach with Large Density Ratio

The hydrophobicity of natural surfaces has drawn much attention of scientific communities in recent years.By mimicking natural surfaces,the manufactured biomimetic hydrophobic surfaces have been widely applied to green technologies such as self-cleaning surfaces.Although the theories for wetting and hydrophobicity have been developed,the mechanism of wetting transitions between heterogeneous wetting state and homogeneous wetting state is still not fully clarified.As understanding of wetting transitions is crucial for manufacturing a biomimetic superhydrophobic surface,more fundamental discussions in this area should be carried out.In the present work,the wetting transitions are numerically studied using a phase field lattice Boltzmann approach with large density ratio,which should be helpful in understanding the mechanism of wetting transitions.The dynamic wetting transition processes between Cassie-Baxter state and Wenzel state are presented,and the energy barrier and the gravity effect on transition are discussed.It is found that the two wetting transition processes are irreversible for specific inherent contact angles and have different transition routes,the energy barrier exists on an ideally patterned surface and the gravity can be crucial to overcome the energy barrier and trigger the transition.     

The roles of carbonic anhydrases IX and XII in cancer cell adhesion,migration, invasion and metastasis

The main function of carbonic anhydrases (CAs) in cancer cells is the pH regulation through a conversion of H₂O and CO₂ to H⁺ and HCO₃⁻. However, the data of in vitro and in vivo studies have demonstrated that transmembrane isoforms of CA IX and CA XII are involved in various steps of cancer cell migration, invasion and metastasis. According to literature, inhibition of these CAs can affect the expression of multiple proteins. Some scientific groups have reported the possible interactions between CA IX and E-cadherin–catenin system, CA IX and integrins, CA IX, CA XII and ion transporters, which all are highly involved in cell-to-cell adhesion, the formation of membrane protrusions and focal adhesions. Nevertheless, CA IX and CA XII have a high impact on tumour growth and metastases formation. The data discussed in this review are quite recent. It highly support the role of CA IX and CA XII in various cancer metastasis processes through their interactions to other invasion proteins. Nevertheless, all findings show the great potential of these CAs in the context of research and application in clinical use.     

Extracellular factors affecting the adhesion of Pseudomonas fluorescens cells to glass surface

Two factors affecting the adhesion of Pseudomonas fluorescens to glass surfaces were revealed in the culture liquid (CL) of this bacterium. One of these factors, adhesin, which is responsible for cell adhesion, was found to be a protein substance located both at the cell surface and in the CL. Bacterial cells grown in rich LB medium were less adhesive than cells grown in minimal M9 medium. The adhesive capacity of cells was independent of the growth phase. The other factor, anti-adhesion (AA), which reduces cell adhesion, was found only in the CL. AA concentration in the CL increased with the culture age.     

Catch bond-mediated adhesion without a shear threshold: trimannose versus monomannose interactions with the FimH adhesin of Escherichia coli

The FimH protein is the adhesive subunit of Escherichia coli type 1 fimbriae. It mediates shear-dependent bacterial binding to monomannose (1M)-coated surfaces manifested by the existence of a shear threshold for binding, below which bacteria do not adhere. The 1M-specific shear-dependent binding of FimH is consistent with so-called catch bond interactions, whose lifetime is increased by tensile force. We show here that the oligosaccharide-specific interaction of FimH with another of its ligands, trimannose (3M), lacks a shear threshold for binding, since the number of bacteria binding under static conditions is higher than under any flow. However, similar to 1M, the binding strength of surface-interacting bacteria is enhanced by shear. Bacteria transition from rolling into firm stationary surface adhesion as the shear increases. The shear-enhanced bacterial binding on 3M is mediated by catch bond properties of the 1M-binding subsite within the extended oligosaccharide-binding pocket of FimH, since structural mutations in the putative force-responsive region and in the binding site affect 1M- and 3M-specific binding in an identical manner. A shear-dependent conversion of the adhesion mode is also exhibited by P-fimbriated E. coli adhering to digalactose surfaces.     

HeLa cell adhesion on various collagen-grafted surfaces

Cell adhesion is important to develop cell microarrays and biocompatible materials. Collagen has been reported to be able to improve cell adhesion. In this paper, two collagen coating methods (collagen grafted directly on the substrate and chitosan-modified substrate) were carried out, on which the adhesive behaviors of HeLa cells were studied. An atomic force microscope and a surface potential meter were used to characterize morphologies and electric polarization of these surfaces. It was found that surface electric polarization and the its durability and surface topography were key factors to cell adhesion. Collagen (1 mg/mL) grafted on 1% chitosan-modified surface showed the best adhesion of HeLa cell. This work might be helpful to the practical application of cell microarray chips.     

Motion of cells sedimenting on a solid surface in a laminar shear flow.

Cell adhesion often occurs under dynamic conditions, as in flowing blood. A quantitative understanding of this process requires accurate knowledge of the topographical relationships between the cell membrane and potentially adhesive surfaces. This report describes an experimental study made on both the translational and rotational velocities of leukocytes sedimenting of a flat surface under laminar shear flow. The main conclusions are as follows: (a) Cells move close to the wall with constant velocity for several tens of seconds. (b) The numerical values of translational and rotational velocities are inconsistent with Goldman's model of a neutrally buoyant sphere in a laminar shear flow, unless a drag force corresponding to contact friction between cells and the chamber floor is added. The phenomenological friction coefficient was 7.4 millinewton.s/m. (c) Using a modified Goldman's theory, the width of the gap separating cells (6 microns radius) from the chamber floor was estimated at 1.4 micron. (d) It is shown that a high value of the cell-to-substrate gap may be accounted for by the presence of cell surface protrusions of a few micrometer length, in accordance with electron microscope observations performed on the same cell population. (e) In association with previously reported data (Tissot, O., C. Foa, C. Capo, H. Brailly, M. Delaage, and P. Bongrand. 1991. Biocolloids and Biosurfaces. In press), these results are consistent with the possibility that cell-substrate attachment be initiated by the formation of a single molecular bond, which might be considered as the rate limiting step.     

Putative functions and functional efficiency of ordered cuticular nanoarrays on insect wings

The putative functions and functional efficiencies of periodic nanostructures on the surface of cicada wings have been investigated by atomic force microscopy (AFM) used as a tool for imaging, manipulation, and probing of adhesion. The structures consist of hexagonal close-packed protrusions with a lateral spacing of ∼200 nm and may have multiple functionalities. Not only do the structures confer survival value by virtue of camouflage, but they may also serve as antiwetting and self-cleaning surfaces and thus be resistant to contamination. These effects have been demonstrated by exposure to white light, liquid droplets, and AFM adhesion measurements. The dependence of optical reflectivity and surface adhesion on surface topography has been demonstrated using AFM as a nanomachining tool as well as an imaging and force-sensing probe. The intact arrays display exceptionally low adhesion for particles in the size range 20 nm-40 μm. The particles can be removed from the array by forces in the range 2-20 nN; conversely, forces in the range 25-230 nN are required to remove identical particles from a flat hydrophilic surface (i.e., polished Si). Measurements of contact angles for several liquids and particle adhesion studies show that the wing represents a low-surface-energy membrane with antiwetting properties. The inference is that a combination of chemistry and structure constitutes a natural technology for conferring resistance to contamination.     

Distinct combinations of amino acid substitutions in N-terminal domain of Gag-capsid afford HIV-1 resistance to rhesus TRIM5α

TRIM5α is a potent anti-retroviral factor that interacts with viral capsid (CA) in a species-specific manner. Recently, we and others reported generation of two distinct HIV-1 CAs that effectively overcome rhesus TRIM5α-imposed species barrier. In this study, to directly compare the effect of different mutations in the two HIV-1 CAs on evasion from macaque TRIM5-restriction, we newly generated macaque-tropic HIV-1 (HIV-1mt) proviral clones carrying the distinct CAs in the same genomic backbone, and examined their replication abilities in macaque TRIM5-overexpressing human cells and in rhesus cells. Comparative analysis of amino acid sequences and homology modeling-based structures revealed that, while both CAs gained some mutated amino acids with similar physicochemical properties, their overall appearances of N-terminal domains were different. Experimentally, the two CAs exhibited incomplete TRIM5α-resistance relative to SIVmac239 CA and different degrees of susceptibility to various TRIM5 proteins. Finally, two HIV-1mt clones carrying a different combination of the CA mutations were found to grow to a comparable extent in established and primary rhesus cells. Our data show that there could be some distinct CA patterns to confer significant TRIM5-resistance on HIV-1.     

Thermodynamics of short-term cell adhesion in vitro.

A thermodynamic theory of short-term (less than 2 hr) in vitro cell adhesion has been developed which allows calculation of reversible work of adhesion and estimation of a term proportional to cell-substrate contact area. The theory provides a means of determining a parameter related to membrane wetting tension for microscopic cells that does not require special manipulations which might desiccate or denature delicate cell membranes. Semiquantitative agreement between predicted and experimentally-measured cell adhesion obtained for three different cell types (MDCK, RBL-1, and HCT-15) in two different liquid phase compositions of surfactants (Tween-80 and fetal bovine serum) supports concepts and approximations utilized in development of theory. Cell-substrate contact areas were largest for wettable surfaces treated with ionizing corona or plasma discharges and smallest for hydrophobic materials for each cell type studied. Contact area for the continuous dog-kidney cell line MDCK was larger than that of either the leukemic blood cell RBL-1 or the anaplastic human colon cell HCT-15.