État d'équilibre et cinétique de décollement d'un cylindre rigide en contact avec un élastomère souple

The equilibrium contact of a rigid cylinder applied against the flat and smooth surface of a soft elastomer sample (natural rubber) is examined and a method is proposed for evaluating both the Young's modulus E of the rubber-like material tested and the Dupré energy of adhesion w using a presentation of results similar to that recently described by Chaudhury et al. from the previous work of Barquins. New results on the kinetics of adherence at imposed normal load are presented. They consist of a study of variations of the strain energy release rate G and of the associate dissipation function Φ = ( G - w)/w as a function of the crack propagation speed V at the interface between the rigid cylinder and the elastic solid. As expected, a master curve Φ(V) is found, confirming the variation of Φ as the 0.55 power function of V, as recently established in rolling and rebound experiments by Barquins and Charmer with the same rubber-like material.     

Electrostatic component of the rolling friction force moment

The paper deals with the theory of a newly-suggested mechanism of rolling friction: i.e. that it has an electrostatic component. An electrostatic component develops when a cylinder rolls along the flat surface of a body which has a different physical nature because the the boundary double electric layer is asymmetrical about the mid-point of the contact. It is shown that if the cylinder is made of a dielectric or of semi-conducting material of low conductivity, with a metal substrate or vice versa, the electrostatic component can be a significant factor in rolling friction.

The dependence of the electrostatic component of the moment of rolling friction upon surface conductivity, contact area and gas pressure in the environment is studied. The electrostatic moment of friction on a 1 cm length of cylinder can reach values of some kilogrammes.     

Robust hydrophobic surfaces displaying different surface roughness scales while maintaining the same wettability

A range of surfaces coated with spherical silica particles, covering the size range from nanometer to micrometer, have been produced using Langmuir-Blodgett (LB) deposition. The particles were characterized both in suspension and in the Langmuir trough to optimize the surface preparation procedure. By limiting the particle aggregation and surface layer failures during the preparation steps, well-defined monolayers with a close-packed structure have been obtained for all particle sizes. Thus, this procedure led to structured surfaces with a characteristic variation in the amplitude and spatial roughness parameters. In order to obtain robust surfaces, a sintering protocol and an AFM-based wear test to determine the stability of the deposited surface layer were employed. Hydrophobization of the LB films followed by water contact angle measurements showed, for all tested particle sizes, the same increase in contact angle compared to the contact angle of a flat hydrophobic surface. This indicates nearly hexagonal packing and gives evidence for nearly complete surface wetting of the surface features.     

Surface characterization of recycled tire rubber to be used in cement paste matrix

The surface modification of tire rubber after treatment with saturated NaOH aqueous solution was investigated by HATR infrared analysis, potentiometric titration, and contact angle measurements. Infrared analysis of the powdered treated rubber showed a decrease in absorption at 1540, 1450, and 1395 cm(-1). This decrease is attributed to the removal of zinc stearate, an additive present in tire formulations that often migrates and diffuses to the surface, resulting in poor adhesion between the rubber and other materials. The potentiometric titration of the suspension of powdered rubber in 0.1 M NaCl showed that more hydrochloric acid was consumed by the untreated rubber, most likely a result of the hyrdrolysis of the zinc stearate to the organic acid. Contact angles of flat tire pieces showed an homogeneity enhancement of the treated rubber surface. The decrease of the zinc stearate on the treated rubber surface explains the improvement in the adhesion of this material to the cement matrix, observed in a previous research. The promising results of this study are a starting point for future research on incorporating rubber particles into cementitious materials as a means of successfully utilizing the vast amounts of tire waste currently in landfills.     

Surface charge properties of oxides and hydroxides formed on metal substrates determined by contact angle titration

Interactions of naturally oxidized or chemically treated flat metal surfaces with the environment, such as the atmosphere or a polymer coating are extremely important for the practical applications. These interactions are governed by the surface chemistry defined by the number and ionization constants of the surface hydroxyl groups along with the surface charge properties. In the present study, a robust methodology was developed to characterize the chemistry of flat surfaces using the contact angle titration procedure. The point of zero charge on the surface of a series of metal oxides and hydroxides, determined using the proposed methodology, showed consistent correlation with the corresponding values for oxides and hydroxides studied in the powder form. A new model based on the concept of polar and disperse components of the work of adhesion and a simplified version of the triple layer was proposed to explain the evolution of the contact angle with pH. This model covers the wide range of pH and ionic strengths of the test solutions and directly accounts for the adsorption process and speciation on the surface. The adhesion of polymer coatings to the chemically treated metal substrates was also explored. It was shown that there is a linear correlation between the point of zero charge on the surface and the strength of adhesion determined by means of the feathering test. This finding emphasizes the significance of the surface charge properties for predicting the adhesion of coatings.     

Anisotropic wetting behavior arising from superhydrophobic surfaces: parallel grooved structure

It has been found experimentally that superhydrophobic surfaces exhibit strong anisotropic wetting behavior. This study reports a simple but robust thermodynamic methodology to investigate the anisotropic superhydrophobic behavior for parallel grooved surfaces. Free energy and its barrier and the corresponding contact angle and its hysteresis for various orientations of the groove structure are calculated based on the proposed thermodynamic model. It is revealed that the strong anisotropy of equilibrium contact angle (ECA) and contact angle hysteresis (CAH) is shown in the noncomposite state but almost isotropic wetting properties are exhibited in the composite state. Furthermore, for the noncomposite state, decreasing groove width and spacing or increasing groove depth can amplify the anisotropy for ECA. Meanwhile, decreasing groove width and increasing depth can amplify the anisotropy for CAH, while varying groove spacing can barely influence CAH. For the composite state, however, the surface geometry hardly leads to the anisotropic behavior. In addition, using a fitting approximation, a simple quantitative correlation between wettability and orientation can be established well, which is consistent with the numerical calculations.     

Numerical and experimental study of critical roof collapse conditions in soft lithography

One of the major issues during soft lithographic processes is that, if the pressing force on the stamp becomes too high, the stamp may erroneously come into contact with the substrate in zones where contact is not intended. This decreases the patterning accuracy and may lead to badly or nonperforming electronic devices and is therefore undesired. Design rules, available at an early stage in the design phase, are desired to speed-up the development of this technique. Ultimately, these rules should give an indication of the critical pressure that can safely be applied on the stamp thereby avoiding unwanted contact between the stamp and the substrate. To obtain these critical pressures, numerical analyses of the deformation behavior of two characteristic configurations in the microstructured surface pattern of the rubber stamp are performed. The deformation behavior of the rubber is modeled according to a Gaussian and a non-Gaussian approach, leading to a neo-Hookean and Arruda-Boyce constitutive model, respectively. Besides these material nonlinearities, geometrical nonlinearities are taken into account as well. The calculated pressure at which undesired contact takes place (the roof collapse pressure) is compared to experimentally obtained values for two particular types of structures, and the results are in agreement within the error margins of the experiments and those ensuing from the assumptions of the numerical simulations.     

On the experimental measurement of fracture toughness in SENT rubber specimens

This work provides a direct comparison of several experimental approaches used in the literature to measure fracture toughness of rubber of rubber using single edge notched in tension (SENT) specimens, with the final aim to provide guidelines for an optimal testing procedure. Digital image correlation measurements were used to get new insights into the fracture process. SENT is experimentally advantageous because of the simple preparation from laboratory plates and the small amount of material required. The most common experimental approaches to measure fracture toughness of rubber rely on the energy release rate, measured by the tearing energy or the J-integral parameters. This work points out the importance of experimental conditions and test procedures: long specimens and short notches are preferred, identification of fracture initiation from the front view is necessary, strain energy density should not be evaluated from un-notched specimens at the critical stretch level, rather alternative strategies are shown in this work.     

An investigation of the stable orientations of orthorhombic particles in a thin film and their effect on its critical failure pressure

The effects of shape and contact angle on the behaviour of orthorhombic particles at an interface and in thin films were investigated using Surface Evolver. It is shown that the energetically stable orientations of the particle change with its aspect ratio. Long, wide, flat particles with low contact angles are more stable in flat orientations, i.e. with two faces parallel to the flat film surface. More cubic particles with higher contact angles are more stable in twisted orientations, where the opposite sides of the film can be drawn together at the sharp edges of the particle. The combination of contact angle and orientation has been found to have a large effect on the capillary pressure required to rupture the film. A film containing a particle in a flat orientation will rupture at a capillary pressure up to three times greater than one containing an identical particle in a twisted orientation. Wider, flatter particles with low contact angles stabilise thin liquid films to a greater extent than cubic particles with high contact angles.     

A study of spontaneous rubber/metal adhesion. I. The rolling cylinder test

The spontaneous adhesion of vulcanized filled elastomers to steel has been studied using a rolling cylinder test. Owing to the relatively high degree of crosslinking, heavy cylinders are required to ensure good contact with the hard rubber track. This in turn invokes compression within the bulk of the rubber. A consequence is that hysteresis losses are associated not only with the adhesion phenomenon but also with the compression deformation of the system. Theoretical and experimental analyses have led to the conclusion that the two effects may be treated as additive. Both hysteresis losses can be related to the WLF superposition principle.     

Localized adsorption at solid-liquid interfaces: the sticky site-hard wall model

The interface between a structured solid and a liquid is modelled by a flat surface with sticky sites placed on a regular lattice. It is shown that this three-dimensional adsorption model can be mapped into a two-dimensional lattice-gas model with the interaction energies of the particles on the sites related to the potential of mean force in the inhomogeneous fluid. In the simple case of a liquid of hard spheres, the first layer of liquid at contact with the solid exhibits an order-disorder phase transition and the total surface excess shows a maximum as the bulk density increases.     

The streaming potential method for the characterization of ultrafiltration organic membranes and the control of cleaning treatments

Streaming potential measurement of ultrafiltration (UF) membranes have been realised a new design. This new design is more convenient to determine the streaming potential on a function of the pressure for all kinds of modules (planar, hollow fiber.h.). The effects of pH, ionic strength and size of pores have been studied. Isoelectric points of different materials (polyethersulfone, celloulse acetate, cellulose triacetate and polysulfone membranes) have been experimentally determined from ν variations with pH at a given ionic are, respectively, 3.1, 4.2, 3.4 and 0.5. The study of the charge organic membranes studied has been shown that adsorbing ions are those of water itself. Then the surface charge of the membrane is a dependent on the pH and at the isoelectric point, the charge density and the streaming potential vanished. The polyethersulfone membrane surface has been modified with TX100 adsorption and the modification observed with our design compared to contact angle and permeabilities measurements. The orientation angle of the surfactant at the membrane surface is obtained: θ=5°, and shows that a flat adsorption occurs. The impact of membranes cleaning procedures have been studied in term of permeability completed by streaming potential measurements. It appeared clearly that streaming potential is a useful tool for the control of cleaning procedures.     

Cinétique de décollement d'une sphère rigide coupée en contact avec un élastomère souple

The equilibrium contact and the kinetics of adherence of flat-ended spheres, under an imposed applied load, in contact with the flat and smooth surface of a soft elastomer sample (natural rubber) are examined with the help of concepts from fracture mechanics. The variation of the dissipation function Φ = (G − w)/w, where G is the strain energy release rate and w is the Dupré energy of adhesion, is studied as a function of the crack propagation speed V at the interface between the flat-ended sphere and the elastic solid. As expected, a master curve Φ(V) is found, confirming the variation of Φ as the 0.55 power function of V, which Barquins and Charmet established recently in adherence experiments with the same rubber-like material.     

Adhesive contact based on the Lennard-Jones potential: a correction to the value of the equilibrium distance as used in the potential

A Lennard-Jones type surface law is commonly used in adhesive contact modeling; however, one of its parameters, namely the equilibrium distance z0, is not well defined. In this paper, a self-consistent method is used to derive the Lennard-Jones surface law from the interatomic Lennard-Jones potential. The parameters of the surface law are directly related to the material lattice parameter and surface energy, and the equilibrium distance z0 values are obtained for various materials. The effect of using the z0 proposed in the present work is demonstrated via the study of adhesive contact behavior for a single sphere and a flat surface, as well as the contact between planar rough surfaces. For pull-off force prediction of the contact between a single sphere and a flat surface, the error of using the z0 suggested in previous studies could be as large as 10% at intermediate ranges of a dimensionless adhesion parameter. For the contact between planar rough surfaces, the error of using the previously proposed z0 is larger for smoother cases, and the prediction of pull-off force could be different by as much as a factor of 5.     

Probing athletics tracks degradation using a microscratch technique

Continuous outdoor exposure of athletics tracks can lead to an important degradation of their mechanical and aesthetical properties. In this work, flat laboratory samples prepared from rubber blends of different colours were subjected to natural and artificial ageing, to investigate their effect on the surface properties. Compositional variations demonstrated a generalized oxidization of the outer (top) material layer, together with surfacing of inorganic additives; a small increase of the degradation temperature of the natural rubber component was reported, similar to the one previously observed on bulk track samples. The smooth surface of the present samples allowed their testing using a microscratching technique, able to mechanically probe the material within a few hundred microns below the top surface. The formation of a significantly harder outer crust layer was reported, potentially impacting the track performance since it is exactly the locus of interaction between the athlete and the sport surface. In particular, the increase in scratch hardness is accompanied by a significant reduction in the apparent friction coefficient. These surface modifications, previously unreported in the literature, are independent phenomena with respect to generalized bulk ageing. Microscratch data supported by microscopy evidenced a significantly varying sensitivity to ageing for the different colours (red, blue, green, neutral). Moreover, this sensitivity appeared strongly dependent on the applied ageing protocol (natural vs. artificial). In view of these results, care must be taken when accelerated artificial weathering is used to simulate long-term natural ageing of these materials.     

Design of a superhydrophobic surface using woven structures

The relationship between surface tension and roughness is reviewed. The Cassie-Baxter model is restated in its original form, which better describes the most general cases of surface roughness. Using mechanical and chemical surface modification of nylon 6,6 woven fabric, an artificial superhydrophobic surface was prepared. A plain woven fabric mimicking the Lotus leaf was created by further grafting 1H,1H-perfluorooctylamine or octadecylamine to poly(acrylic acid) chains which had previously been grafted onto a nylon 6,6 woven fabric surface. Water contact angles as high as 168 degrees were achieved. Good agreement between the predictions based on the original Cassie-Baxter model and experiments was obtained. The version of the Cassie-Baxter model in current use could not be applied to this problem since the surface area fractions in this form is valid only when the liquid is in contact with a flat, porous surface. The angle at which a water droplet rolls off the surface has also been used to define a superhydrophobic surface. It is shown that the roll-off angle is highly dependent on droplet size. The roll-off angles of these superhydrophobic surfaces were less than 5 degrees when a 0.5 mL water droplet was applied.     

Anisotropic wetting characteristics on submicrometer-scale periodic grooved surface

Submicrometer-scale periodic structures consisting of parallel grooves were prepared on azobenzene-containing multiarm star polymer films by laser interference. The wetting characteristics on the patterned surfaces were studied by contact angle measurements. Macroscopic distortion of water drops was found on such small-scale surface structures, and the contact angles measured from the direction parallel to the grooves were larger than those measured from the perpendicular direction. A thermodynamic model was developed to calculate the change in the surface free energy as a function of the instantaneous contact angle when the three-phase contact line (TPCL) moves along the two orthogonal directions. It was found that the fluctuations, i.e., energy barriers, on the energy versus contact angle curves are crucial to the analysis of wetting anisotropy and contact angle hysteresis. The calculated advancing and receding contact angles from the energy versus contact angle curves were in good agreement with those measured experimentally. Furthermore, with the groove depth increasing, both the degree of wetting anisotropy and the contact angle hysteresis perpendicular to the grooves increased as a result of the increase in the energy barrier. The theoretical critical value of the groove depth, above which the anisotropic wetting appears, was determined to be 16 nm for the grooved surface with a wavelength of 396 nm. On the other hand, the effect of the groove wavelength on the contact angle hysteresis perpendicular to the grooves was also interpreted on the basis of the thermodynamic model. That is, with the wavelength decreasing, the contact angle hysteresis increased due to the increase in the number of energy barriers. These results may provide theoretical evidence for the design and application of anisotropic wetting surface.     

Large area, molecularly smooth (0.2 nm rms) gold films for surface forces and other studies

Using large-area (cm2) single-crystal mica sheets as the templating substrate, we have created correspondingly large template-stripped (TS) gold films (thickness 82 +/- 2 nm) that appear smooth to within 0.2 nm rms roughness over their entire area. These gold films, created without the use of any releasing solvent, are characterized using AFM, X-ray diffraction, multiple beam interferometric fringes of equal chromatic order (FECO), and contact angle measurements. Being molecularly smooth over large areas and (adjustably) semitransparent, these films are especially suitable for use in the surface force balance (SFB), as shown by measurements of the normal force (F) versus distance (D) profiles between such a flat gold surface and a bare mica surface in water. The F(D) profiles are in good agreement with DLVO theory down to molecular contact and indicate that the gold surface is negatively charged under water.     

A Novel Method for Surface Free-Energy Determination of Powdered Solids

Interfacial solid/liquid interactions play a crucial role in wetting, spreading, and adhesion processes. In the case of a flat solid surface, contact angle measurements are commonly utilized for the determination of the solid surface free energy and its components. However, if such a surface cannot be obtained, then the contact angle can not be measured directly. Usually methods based on imbibition of probe liquids into a thin porous layer or column are applied. In this paper a novel method, also based on the capillary rise, is proposed for the solid surface free-energy components determination. Actually, it is a modification of the thin column wicking method; similar theoretical background can be applied together with that appropriate for the capillary rise method of liquid surface tension determination. The proposed theoretical approach and procedure are verified by using single glass capillaries, and then alumina and ground glass powders were used for the method testing. Thus obtained surface free-energy components for these solids, for both glass and alumina, agree well with the literature values.     

Spreading of liquid drops over dry surfaces

Spreading of thin, axisymmetric, non-volatile, Newtonian liquid drops over a dry, smooth, flat solid surface is considered both theoretically and experimentally in the case of complete wetting. The drop profile is solved analytically by matching the “outer” solution for large film thicknesses, where only the capillary effects are important, with the “inner” solution for small film thicknesses, where the viscous and disjoining pressure effects are comparable to capillary effects. It is shown that the apparent radius of the wetted spot, the apex height of the drop, and the apparent advancing dynamic contact angle follow different power laws in time and the advancing dynamic contact angle follows a power law in capillary number. Both the prefactor and the exponent of each power law are derived theoretically. Good agreement between the theory predictions and experimental measurements is shown for both the prefactor and exponent of each power law. It is necessary to emphasize that the theory suggested does not include any fitting parameters.