Wetting property of smooth and textured hydrophobic surfaces under condensation condition

Static and dynamic wetting behaviors of sessile droplet on smooth, microstructured and micro/nanostructured surface under condensation condition are systematically studied. In contrast to the conventional droplet wetting on such natural materials by dropping, we demonstrate here that when dropwise condensation occurs, the sessile droplet will transit from the Cassie-Baxter wetting state to the Wenzel wetting state or partial Cassie-Baxter wetting state on the microstructured surface or the micro/nanostructured surface, which leads to a strong adhesion between the droplet and the substrate. In contrast, the apparent contact angle and the sliding angle on the smooth surface changes a little before and after the condensation because of small roughness. Theoretical analysis shows that the roughness factor controls the adhesion force of the droplet during condensation, and a theoretical model is constructed which will be helpful for us to understand the relationship between the adhesion force and the geometry of the surface.     

Influence of superalloy substrate roughness on adhesion and oxidation behavior of magnetron-sputtered NiCoCrAlY coatings

The present study has been conducted in order to determine the influence of superalloy substrate roughness on adhesion and oxidation behavior of magnetron-sputtered NiCoCrAlY coatings. Six types of coating samples with different substrate roughness were tested. The surface roughness and real surface area of both the substrates and coatings were studied by atomic force microscopy (AFM) techniques. The scratch tests performed at progressive loads were employed to evaluate the adhesion of the coatings. Cyclic oxidation tests were performed at 1100 °C in air for 50 cycles, each cycle consisting of 1 h heating in the tube furnace followed by 15 min cooling in the open air. The AFM measurements exhibit that the surface roughness of the sputtered NiCoCrAlY coating increases with the increasing of the superalloy substrate roughness. The NiCoCrAlY coatings present slightly lower roughness than the corresponding superalloy substrate. The scratch adhesion tests indicate that the coatings on substrates with a smoother surface possess better adhesion than on those with a rougher surface. Both the real surface area and oxidation weight gain of the coatings decrease with the decreasing of the superalloy substrate roughness. The NiCoCrAlY coating sputtered on the superalloy substrate with lower roughness provides relatively higher antioxidant protection than that provided by the coating with rougher substrate.     

Adhesive properties of contacts between elastic bodies with randomly rough self-affine surfaces: A simulation with the method of reduction of dimensionality

Adhesive properties (adhesion force and adhesion coefficient) of contacts between elastic bodies with rough surfaces have been investigated and adhesion maps constructed showing the dependence of adhesive properties on the roughness, rms slope of the surface, elastic modulus, surface energy and fractal dimension. Simulations have been carried out in the frame of the method of reduction of dimensionality.     

Effects of roughness on interfacial performances of silica glass and non-polar polyarylacetylene resin composites

The influence of roughness on interfacial performances of silica glass/polyarylacetylene resin composites was investigated. In order to obtain different roughness, silica glass surface was abraded by different grits of abrasives and its topography was observed by scanning electron microscopy and atomic force microscopy. At the same time, the failure mechanisms of composites were analyzed by fracture morphologies and the interfacial adhesion was evaluated by shear strength test. The results indicated that shear strength of silica glass/polyarylacetylene resin composites firstly increased and then decreased with the surface roughness of silica glass increased. The best surface roughness range of silica glass was 40-60 nm. The main mechanism for the improvement of the interfacial adhesion was physical interlocking at the interface.     

表面黏附导致的接触行为转变

应用大规模分子动力学方法, 模拟了具有不同原子级粗糙形貌的两种刚性球形探头与弹性平面基体的黏附接触行为. 研究了载荷与真实接触面积、接触界面排斥力与真实接触面积, 以及黏附力与真实接触面积之间的关系. 分子模拟得到的载荷与真实接触面积的关系, 与连续力学接触理论预测很好地定性一致. 无论是原子级光滑探头还是粗糙探头, 黏附接触下的排斥力与真实接触面积的关系, 都与无黏附接触时的规律相一致, 即黏附力对接触行为的影响作用, 可以等效为附加在真实外载荷基础上的虚拟载荷, 将对黏附接触行为的分析转变为无黏附接触分析. 两种探头的黏附力随真实接触面积都呈幂函数形式的增长, 但是, 原子级光滑探头的幂指数大于1, 而原子级粗糙探头的幂指数小于1. 关键词： 接触行为 表面黏附 分子动力学模拟     

Elasticity and nanomechanical response of Aspergillus niger spores using atomic force microscopy

The elasticity and nanomechanical response of Aspergillus niger spores determined using atomic force microscopy (AFM) and nanoindentation are discussed. The force-displacement curve of the spore surfaces shows that the average surface roughness of spores was approximately 33 nm and that the adhesion force ranged from 9 to 28 nN. The Young's modulus of the A. niger spores ranged from 0.1 to 21.4 GPa and the hardness ranged from 0.01 to 0.17 GPa. The critical buckling load of the spore membrane is 290 μN.     

Modeling of the rough spherical nanoparticles manipulation on a substrate based on the AFM nanorobot

In this paper, dynamic behavior of the rough spherical micro/nanoparticles during pulling/pushing on the flat substrate has been investigated and analyzed. For this purpose, at first, two hexagonal roughness models (George and Cooper) were studied and then evaluations for adhesion force were determined for rough particle manipulation on flat substrate. These two models were then changed by using of the Rabinovich theory. Evaluations were determined for contact adhesion force between rough particle and flat substrate; depth of penetration evaluations were determined by the Johnson–Kendall–Roberts contact mechanic theory and the Schwartz method and according to Cooper and George roughness models. Then, the novel contact theory was used to determine a dynamic model for rough micro/nanoparticle manipulation on flat substrate. Finally, simulation of particle dynamic behavior was implemented during pushing of rough spherical gold particles with radii of 50, 150, 400, 600, and 1,000 nm. Results derived from simulations of particles with several rates of roughness on flat substrate indicated that compared to results for flat particles, inherent roughness on particles might reduce the rate of critical force needed for sliding and rolling given particles. Given a fixed radius for roughness value and increased roughness height, evaluations for sliding and rolling critical forces showed greater reduction. Alternately, the rate of critical force was shown to reduce relative to an increased roughness radius. With respect to both models, based on the George roughness model, the predicted rate of adhesion force was greater than that determined in the Cooper roughness model, and as a result, the predicted rate of critical force based on the George roughness model was closer to the critical force value of flat particle.     

Contact mechanics analysis of oscillatory sliding of a rigid fractal surface against an elastic–plastic half-space

Oscillatory sliding contact between a rigid rough surface and an elastic–plastic half-space is examined in the context of numerical simulations. Stick-slip at asperity contacts is included in the analysis in the form of a modified Mindlin theory. Two friction force components are considered – adhesion (depending on the real area of contact, shear strength and interfacial adhesive strength) and plowing (accounting for the deformation resistance of the plastically deformed half-space). Multi-scale surface roughness is described by fractal geometry, whereas the interfacial adhesive strength is represented by a floating parameter that varies between zero (adhesionless surfaces) and one (perfectly adhered surfaces). The effects of surface roughness, apparent contact pressure, oscillation amplitude, elastic–plastic properties of the half-space and interfacial adhesion on contact deformation are interpreted in the light of numerical results of the energy dissipation, maximum tangential (friction) force and slip index. A non-monotonic trend of the energy dissipation and maximum tangential force is observed with increasing surface roughness, which is explained in terms of the evolution of the elastic and plastic fractions of truncated asperity contact areas. The decrease of energy dissipation with increasing apparent contact pressure is attributed to the increase of the elastic contact area fraction and the decrease of the slip index. For a half-space with fixed yield strength, a lower elastic modulus produces a higher tangential force, whereas a higher elastic modulus yields a higher slip index. These two competing effects lead to a non-monotonic dependence of the energy dissipation on the elastic modulus-to-yield strength ratio of the half-space. The effect of interfacial adhesion on the oscillatory contact behaviour is more pronounced for smoother surfaces because the majority of asperity contacts deform elastically and adhesion is the dominant friction mechanism. For rough surfaces, higher interfacial adhesion yields less energy dissipation because more asperity contacts exhibit partial slip.     

Adhesion between an elastic body and a randomly rough hard surface

I have developed a theory of adhesion between an elastic solid and a hard randomly rough substrate. The theory takes into account that partial contact may occur between the solids on all length scales. I present numerical results for the case where the substrate surface is self-affine fractal. When the fractal dimension is close to 2, complete contact typically occurs in the macro-asperity contact areas, while when the fractal dimension is larger than 2.5, the area of (apparent) contact decreases continuously when the magnification is increased. An important result is that even when the surface roughness is so high that no adhesion can be detected in a pull-off experiment, the area of real contact (when adhesion is included) may still be several times larger than when the adhesion is neglected. Since it is the area of real contact which determines the sliding friction force, the adhesion interaction may strongly affect the friction force even when no adhesion can be detected in a pull-off experiment. Received 3 April 2002     

Tuning the adhesion of layer-by-layer films to the physicochemical properties of inner limiting membranes using nanoparticles

Retinal trauma is a serious concern for patients undergoing inner limiting membrane (ILM) peeling to correct for various vitreoretinal interface conditions. This mechanical trauma can be prevented by modifying the surface of surgical instruments to increase adhesion to the ILM. To this effect, we have studied the effects of roughness and surface charge on the adhesive properties of ILMs by utilizing layer-by-layer (LbL) films with embedded gold nanoparticles (LbL-AuNP films). LbL films were assembled on atomic force microscopy (AFM) tipless cantilevers. Topographical analysis of these films, with and without nanoparticles, showed that LbL films with nanoparticles had a higher rms roughness compared to films alone or unmodified cantilevers. Nanoparticle-modified LbL films significantly increased the adhesion forces at the cantilever-ILM interface, compared to LbL films without particles. Surprisingly, adsorption of gold nanoparticles onto the AFM cantilevers caused increases in adhesion forces greater than those measured with LbL-AuNP films. These results have important implications for the design of surface modifications for vitreoretinal surgical instruments.     

纳米接触过程中黏着规律的变化

本文应用大规模分子动力学方法, 模拟了两种具有不同粗糙形貌的、刚性球形探头与弹性平面基体之间的纳米尺度接触, 计算了探头与基体之间的拉离力和黏着功, 研究了接触过程中界面黏着力随载荷的变化规律, 分析了接触界面原子的法向应力分布. 研究发现, 原子级光滑接触的黏着力随着载荷的增大而线性增大, 而原子级粗糙接触的黏着力-载荷曲线分为以不同斜率增长的两个阶段. 相比于原子级光滑探头, 原子级粗糙探头与基体之间具有较小的拉离力和黏着功, 却在接触过程中形成了较大的黏着力. 因此, 拉离力和黏着功不能表征出纳米接触过程中原子吸引作用对界面法向力的贡献大小.     

Surface Modification of Poly (ether ether ketone) with a Medlite C6 (ND-YAG Q-Switched) Skin Treatment Laser

Poly ether ether ketone (PEEK), a synthetic polymer, is expected to be useful as a biomaterial due to its appropriate mechanical, chemical, and biocompatibility properties. However, this polymer is biologically inert, requiring surface modification to improve its adhesion to bone cells for use as a bone substrate. Surface properties, such as roughness and hydrophilicity, are important factors in the adhesion of biomaterials to the surrounding tissue; therefore, in this study, laser treatment was performed for surface modification. The aim of the research described here was to investigate the effect of two laser parameters, fluency and wavelength, on the surface roughness and hydrophilicity to determine the optimum parameters for improving surface adhesion. The surface topography and average roughness (R_a) were investigated by atomic force microscopy (AFM). Surface morphology was also observed with an optical microscope, and the hydrophilicity of the surfaces was investigated with static contact angle tests. The results obtained showed that the samples treated at the wavelength of 532?nm with fluency of 8?J/cm², compared to fluencies of 4 and 12?J/cm², showed improved surface properties. However, in terms of radiation wavelength, the wavelength of 1064?nm at these three fluencies showed the most promising results for enhancing the surface properties of PEEK for bone implant applications.     

In situ atomic force microscopy studies of reversible light-induced switching of surface roughness and adhesion in azobenzene-containing PMMA films

Thin films in the range 40-80 nm of a blend of PMMA with an azobenzene derivative have been studied directly during UV and blue light irradiation by atomic force microscopy (AFM), revealing highly reversible changes in the surface roughness and the film adhesion. UV light induces an ≈80% increase in surface roughness, whereas illumination by blue light completely reverses these changes. Based on the observed surface topography and transition kinetics a reversible mass flow mechanisms is suggested, where the polarity changes upon switching trigger a wetting-dewetting transition in a surface segregation layer of the chromophore. Similar AFM measurements of the pull-off force indicate a decrease upon UV and an increase after blue light illumination with a complex kinetic behavior: a rapid initial change, attributed to the change in the cis isomer fraction of the azobenzene derivative, and a more gradual change, indicative of slow structural reorganization.     

Adhesion studies of latex film surfaces on the meso- and nanoscale

In this paper the effects of surface roughness and annealing temperature (T) of latex coating films on adhesion are discussed for the different stages of the film formation process. The surface free energy of latex films was assessed in terms of practical work of adhesion (W) (or adherence) using a custom-built adhesion-testing device (ATD), atomic force microscopy (AFM), and contact angle measurements. For preannealed latex films surface roughness averages (R_a) were determined from AFM height images and were related to the values of W obtained from ATD measurements at room temperature. The results obtained using these tests exhibiting surface behavior on different length scales indicate a dependence of the measured adhesion on surface roughness and temperature, as well as on the length scale of the measurements.First preannealed samples were studied, which were obtained by heat treatment above the respective glass transition temperatures (T_g). Increasing the temperature of preannealing resulted in a decrease of the adherence observed in ATD experiments at room temperature. However, on the nanoscale, using AFM, no significant variation of the adherence was observed. This observation can be explained by roughness arguments. Preannealing decreases roughness which results in lower adherence values measured by ATD while for essentially single asperity AFM experiments roughness has an insignificant effect. Specimens were also annealed over a constant period of time (90 min) at different temperatures. At the end of the heat treatment, adhesion was measured at the treatment temperature by ATD. The amplified effect of temperature observed in this case on adherence is attributed to the combination of roughness decrease and increasing test temperature. In a third set of experiments completely annealed samples were studied by ATD as well as by AFM as a function of temperature. With increasing T values ATD showed a decrease in adherence, which is attributed to a decreasing surface free energy of the annealed films at elevated T values. AFM, on the other hand, showed an opposite trend which is assigned to increasing penetration of the tip into the tip/wetting polymer samples versus increasing temperature. Finally, annealing isotherms as a function of time were investigated by ATD in situ at different temperatures. This last set of experiments allowed us to optimize annealing time and temperature to achieve complete curing.     

An AFM study of the effect of chemical treatments on the surface microstructure and adhesion properties of flax fibres

The mechanical properties of fibre-reinforced polymer composites are largely dependant on the adhesion between the matrix and the fibre. In order to enhance the interaction between flax fibres and unsaturated polyester resins, raw fibres were chemically modified using sodium hydroxide, sodium hydroxide plus acetic anhydride and formic acid-based treatments. The physical properties of the modified fibres were investigated by means of the atomic force microscopy. At first, the morphological analysis of the surfaces shows that after the chemical treatments, the fibres surface appear to be less heterogeneous in topology and smoother. Nonetheless, no significant roughness difference was found between the different treatments. Secondly, adhesion forces measurements were performed between a standard AFM silicon nitride tip and the fibres. The adhesion forces were found to vary according to the chemical treatment. The sodium hydroxide-based treatment was found to increase the adhesion force between the fibre and the AFM tip whereas the lowest adhesion force was found for the formic acid- based treated fibre. These results were attributed to the different hydrophilic character of the modified fibres. Due to the importance of the water layer adsorbed on the fibres, the adhesion forces between the AFM tip and the different samples are found to be mainly dominated by capillary forces in relation with the fibre's surface hydrophilicity.     

Surface microstructure of the oxide protective layers grown on vanadium-free Ti alloys for use in biomedical applications

In this work, we present a surface study by SFM (scanning force microscopy) of three new Ti alloys of composition (in wt%) Ti-7Nb-6Al, Ti-13Nb-13Zr and Ti-15Zr-4Nb, developed for biomedical applications. V was not included in these alloys since this element has been reported to be cytotoxic. The surface of these materials has been modified by a thermal treatment in air at 750 °C for different times. As a consequence of this treatment an oxide layer develops on the surface, resulting in both an improvement of the corrosion resistance and an increase of the roughness, which enhances the adhesion of the tissue cells to the implant. SFM has been used to characterize the surface structure and topography of the oxide layers grown on the three alloys. The surface roughness analysis obtained by SFM points to a correlation between the mean square roughness, the thickness of the oxide layer, and the α-phase/β-phase ratio in the base material.     

覆冰垂直粘结强度的测试研究

本文利用自行设计的新型表面覆冰垂直粘结强度测试装置,测试了不同材料表面的覆冰垂直粘结强度,并探讨了基体表面粗糙度、冻冰时间、冰层厚度、冻冰温度等因素对同一材料表面覆冰垂直粘结强度的影响。结果发现,覆冰垂直粘结强度随着材料表面粗糙度增加而增大,随着冰冻温度的升高而降低。而冻冰时间与冰层厚度对覆冰垂直粘结强度的影响较为复杂。     

Hydrophobic recovery of UV/ozone treated poly(dimethylsiloxane): adhesion studies by contact mechanics and mechanism of surface modification

Silicone elastomers (Sylgard 184 and 170), based on poly(dimethylsiloxane) (PDMS), were surface treated by a combined exposure to UV and ozone. The effects of the treatments were analyzed as a function of time elapsed after stopping the treatments using different standard surface characterization techniques, such as water contact angle measurements, XPS and atomic force microscopy (AFM). However, the primary focus of this study was to apply the Johnson–Kendall–Roberts (JKR) contact mechanics approach to investigate PDMS samples prior to and following UV/ozone surface treatment. A gradual formation of a hydrophilic, silica-like surface layer with increasing modulus was observed with increasing UV/ozone exposure. A subsequent hydrophobic recovery after UV/ozone exposure was observed, as indicated by increasing contact angles. This supports the hypothesis that the hydrophobic recovery is mainly caused by the gradual coverage of a permanent silica-like structure with free siloxanes and/or reorientation of polar groups. PDMS containing a homogenously dispersed filler (Sylgard 184), exhibited a decreasing surface roughness (by AFM) when the oxidized surface region “collapsed” into a smooth SiO_x layer (final surface roughness <2 nm). PDMS containing heterogeneously distributed, aggregated filler particles (Sylgard 170), exhibited an increasing surface roughness with treatment dose, which was attributed to the “collapse” of the oxidized surface region thus exposing the contours of the underlying filler aggregates (final surface roughness ∼140 nm). A dedicated device was designed and built to study the contact mechanics behavior of PDMS prior to, and following surface treatment. The value of the combined elastic modulus obtained for PDMS lens and semi-infinite flat surface system showed an increase in full agreement with the formation of a silica-like layer exhibiting a high elastic modulus (compared with untreated PDMS). The work of adhesion observed in JKR experiments exhibited an increasing trend as a function of treatment done in agreement with contact angle data. JKR experiments showed hydrophobic recovery behavior as anticipated from contact angle measurements. Single pull-off force measurements by JKR and numerical analysis of full-approach JKR curves were in quantitative agreement regarding practical work of adhesion values.     

Effect of surface roughness and release layer on anti-adhesion performance of the imprint template

UV imprint lithography has been initiated as an enabling, cost-effective technique to achieve 100 nm resolution patterning in recent years. However, the adhesion between resist and imprint template is one of the critical problems for the industrial application of imprint lithography. In this paper, two kinds of measures, including increase of surface roughness of template and application of a fluorinated release agent as self-assembled monolayers (SAMs) to the template surface, were taken to overcome the adhesion between resist and template. The test results of contact angle showed that the appropriate increase of surface roughness could improve hydrophilicity of template surface greatly, and improved the hydrophobicity of template surface when it was combined with self-assembled monolayers. The XPS, DRIR spectra indicated that the fluorinated release layers were successfully prepared on the surface of template using the process in the paper. The surface free energy of the template was 16.6 mN/m, and less than that of PTFE (18 mN/m). The imprint experiment results also showed that the anti-adhesion performance of treated template was improved greatly during detaching procedure, and the demolding force decreased by 56.64% in comparison with that of untreated template.     

Preparation and in vitro characterization of aerosol-deposited hydroxyapatite coatings with different surface roughnesses

Hydroxyapatite (HA) coatings with different surface roughnesses were deposited on a Ti substrate via aerosol deposition (AD). The effect of the surface roughness on the cellular response to the coating was investigated. The surface roughness was controlled by manipulating the particle size distribution of the raw powder used for deposition and by varying the coating thickness. The coatings obtained from the 1100 °C-heated powder exhibited relatively smooth surfaces, whereas those fabricated using the 1050 °C-heated powder had network-structured rough surfaces with large surface areas and were superior in terms of their adhesion strengths and in vitro cell responses. The surface roughness (R_a) values of the coatings fabricated using the 1050 °C-heated powder increased from approximately 0.65 to 1.03 μm as the coating thickness increased to 10 μm. The coatings with a rough surface had good adhesion to the Ti substrate, exhibiting high adhesion strengths ranging from 37.6 to 29.5 MPa, depending on the coating thickness. The optimum biological performance was observed for the 5 μm-thick HA coating with an intermediate surface roughness value of 0.82 μm.