Wetting of anisotropic sinusoidal surfaces - experimental and numerical study of directional spreading

Directional wettability, i.e. variation of wetting properties depending on the surface orientation, can be achieved by anisotropic surface texturing. A new high precision process can produce homogeneous sinusoidal surfaces (in particular parallel grooves) at the micro-scale, with a nano-scale residual roughness five orders of magnitude smaller than the texture features. Static wetting experiments have shown that this pattern, even with a very small aspect ratio, can induce a strong variation of contact angle depending on the direction of observation. A comparison with numerical simulations (using Surface Evolver software) shows good agreement and could be used to predict the fluid-solid interaction and droplet behaviour on textured surfaces. Two primary mechanisms of directional spreading of water droplets on textured stainless steel surface have been identified. The first one is the mechanical barrier created by the textured surface peaks, this limits spreading in perpendicular direction to the surface anisotropy. The second one is the capillary action inside the sinusoidal grooves accelerating spreading along the grooves. Spreading has been shown to depend strongly on the history of wetting and internal drop dynamics

Roughness Signature of Tribological Contact Calculated by a New Method of Peaks Curvature Radius Estimation on Fractal Surfaces

This paper proposes a new method of roughness peaks curvature radii calculation and its application to tribological contact analysis as characteristic signature of tribological contact. This method is introduced via the classical approach of the calculation of radius of asperity. In fact, the proposed approach provides a generalization to fractal profiles of the Nowicki's method [Nowicki B. Wear Vol.102, p.161-176, 1985] by introducing a fractal concept of curvature radii of surfaces, depending on the observation scale and also numerically depending on horizontal lines intercepted by the studied profile. It is then established the increasing of the dispersion of the measures of that lines with that of the corresponding radii and the dependence of calculated radii on the fractal dimension of the studied curve. Consequently, the notion of peak is mathematically reformulated. The efficiency of the proposed method was tested via simulations of fractal curves such as those described by Brownian motions. A new fractal function allowing the modelling of a large number of physical phenomena was also introduced, and one of the great applications developed in this paper consists in detecting the scale on which the measurement system introduces a smoothing artifact on the data measurement. New methodology is applied to analysis of tribological contact in metal forming process

Influence of roughness on ZDDP tribofilm formation in boundary lubricated fretting

Influence of initial surface topography on tribofilm formation in ZDDP lubricated contact was analysed. A small displacement fretting tests with sinusoidal motion were carried out in classical sphere/plane configuration. A range of surfaces with different initial roughness were prepared by milling and grinding processes. Tests were carried out using variable displacement method where amplitude of imposed displacement was gradually increased after every 1000 cycles from 2 to 30 µm. The surfaces after tribological tests were measured by interferometric profiler. Main findings confirm that initial roughness has a significant influence on antiwear tribofilm formation in boundary lubricated contact. Tribofilm form faster and require less energy to activate in case of rough surface obtained by milling process than in case of smooth grinded surface. However, in contact lubricated by ZDDP additive a significant transfer of material occurred from plane to sphere specimen

Surface Texturization of Breast Implants Impacts Extracellular Matrix and Inflammatory Gene Expression in Asymptomatic Capsules:

Background: Texturing processes have been designed to improve biocompatibility and mechanical anchoring of breast implants. However, a high degree of texturing has been associated with severe abnormalities. In this study, the authors aimed to determine whether implant surface topography could also affect physiology of asymptomatic capsules. Methods: The authors collected topographic measurements from 17 different breast implant devices by interferometry and radiographic microtomography. Morphologic structures were analyzed statistically to obtain a robust breast implant surface classification. The authors obtained three topographic categories of textured implants (i.e., “peak and valleys,” “open cavities,” and “semiopened cavities”) based on the cross-sectional aspects. The authors simultaneously collected 31 Baker grade I capsules, sorted them according to the new classification, established their molecular profile, and examined the tissue organization. Results: Each of the categories showed distinct expression patterns of genes associated with the extracellular matrix (Timp and Mmp members) and inflammatory response (Saa1, Tnsf11, and Il8), despite originating from healthy capsules. In addition, slight variations were observed in the organization of capsular tissues at the histologic level. Conclusions: The authors combined a novel surface implant classification system and gene profiling analysis to show that implant surface topography is a bioactive cue that can trigger gene expression changes in surrounding tissue, even in Baker grade I capsules. The authors’ new classification system avoids confusion regarding the word “texture,” and could be transposed to implant ranges of every manufacturer. This new classification could prove useful in studies on potential links between specific texturizations and the incidence of certain breast-implant associated complications

The Relative Importance of Topography and RGD Ligand Density for Endothelial Cell Adhesion

The morphology and function of endothelial cells depends on the physical and chemical characteristics of the extracellular environment. Here, we designed silicon surfaces on which topographical features and surface densities of the integrin binding peptide arginine-glycine-aspartic acid (RGD) could be independently controlled. We used these surfaces to investigate the relative importance of the surface chemistry of ligand presentation versus surface topography in endothelial cell adhesion. We compared cell adhesion, spreading and migration on surfaces with nano- to micro-scaled pyramids and average densities of 6×102–6×1011 RGD/mm2. We found that fewer cells adhered onto rough than flat surfaces and that the optimal average RGD density for cell adhesion was 6×105 RGD/mm2 on flat surfaces and substrata with nano-scaled roughness. Only on surfaces with micro-scaled pyramids did the topography hinder cell migration and a lower average RGD density was optimal for adhesion. In contrast, cell spreading was greatest on surfaces with 6×108 RGD/mm2 irrespectively of presence of feature and their size. In summary, our data suggest that the size of pyramids predominately control the number of endothelial cells that adhere to the substratum but the average RGD density governs the degree of cell spreading and length of focal adhesion within adherent cells. The data points towards a two-step model of cell adhesion: the initial contact of cells with a substratum may be guided by the topography while the engagement of cell surface receptors is predominately controlled by the surface chemistry

Dureté des revêtements : quel modèle choisir ?

La caractérisation des propriétés mécaniques des revêtements, et plus particulièrement de leur dureté est d’une grande importance dans le milieu industriel et fait l’objet de nombreuses études. Le plus souvent, la mesure directe de cette grandeur n’est pas possible, puisque sa valeur est influencée par les caractéristiques du substrat sur lequel le revêtement est déposé. Dans cet article, nous nous proposons d’étudier la robustesse de différents modèles utilisés dans la littérature, avec prise en compte de l’effet de la force sur la dureté du film et du substrat, pour déduire la dureté du revêtement de celle du « composite » [revêtement – substrat]. Pour cela, nous avons effectué des essais sur une pièce industrielle en acier, revêtue par un dépôt de nickel chimique de 55 µm d’épaisseur. Cette épaisseur est suffisante pour que la dureté du revêtement puisse être mesurée et comparée à celle obtenue par les modèles. En effectuant des essais sous différentes forces appliquées, nous avons mesuré expérimentalement les duretés « composite » et celles du substrat. À partir des valeurs de dureté du revêtement et du substrat (valeurs considérées comme exactes, sans incertitude), nous avons optimisé les coefficients variables de chaque modèle pour tracer une dureté composite, en fonction de l’inverse de la diagonale de l’empreinte, la plus proche possible des résultats expérimentaux. Ces courbes sont considérées ensuite comme les « courbes expérimentales » (sans incertitude), sur lesquelles nous étudions la robustesse des modèles. Cette robustesse est d’abord testée en perturbant les valeurs de dureté « expérimentales » par un bruit (incertitude due à l’appareil de mesure). Une deuxième perturbation est également introduite en supprimant les données relatives aux plus faibles profondeurs de pénétration, afin de déterminer le domaine expérimental (profondeur d’indentation sur épaisseur de revêtement) où la prévision de la dureté du revêtement est possible (il est évident que si les conditions expérimentales sont réunies pour que la dureté du film seul puisse être mesurée, l’utilisation d’un modèle ne s’impose pas). Nous montrons que les modèles deviennent moins stables lorsque le nombre de paramètres variables augmente, et que le modèle de Jönsson et Hogmark donne les meilleures prévisions. Cette méthode peut être utilisée pour définir les conditions expérimentales (choix de l’appareil et gamme de forces) qui permettent de mesurer la dureté d’un revêtement en fonction de sa dureté et de son épaisseur présumées

Analyse des courbes de décharge en nanoindentation dans le cas des polymères massifs

L'interprétation des courbes d'indentation est particulièrement difficile dans le cas des polymères en raison de la complexité de leur comportement mécanique et notamment de leur viscosité. L'analyse des courbes d'indentation montre clairement l'influence de la viscosité par l'enfoncement de la pointe lors du plateau de maintien et par la valeur anormalement élevée de l'exposant de la loi puissance représentant l'équation de la courbe de décharge. En conséquence, la mesure des propriétés élastiques peut être fortement influencée par le comportement visqueux du matériau. Pour s'en affranchir et mesurer de manière fiable le module d'élasticité et la dureté du matériau, il est important d'effectuer des essais avec un protocole d'indentation et des conditions expérimentales adaptées. Des essais effectués avec un chargement à vitesse de charge/charge constante ($\dot{F}/F$ = Cst) suivis d'un plateau de maintien suffisamment long (600 s) et d'une courbe de décharge à vitesse de décharge/charge constante ont été menés sur deux polymères amorphes (polycarbonate, polyméthacrylate de méthyle). En utilisant la méthode d'Oliver et Pharr et la pente initiale d'un ajustement par une loi puissance de la courbe de décharge, nous obtenons des valeurs calculées du module d'élasticité apparent, proches de celle déterminées par des essais de traction

L'apport de la science dans l'entraînement sportif : l'exemple de la course de fond

International audienceThe purpose of this paper is to analyze the turning machinability of a martensitic steel, according to the cutting speed, and through signal analyses of the morphology of the machined surface. We initially carried out the classification of a large number of parameters of roughness, on the basis of their relevance with regard to cutting speed. The originality of the proposed method lies in the combination of the classical technique of analysis of variance with the statistical technique of resampling of data, called Bootstrap. Another characteristic of the study consists in the addition to the traditional categories of roughness parameters (Amplitude, Frequency, Morphological and Hybrid parameters) to analyze multiscale aspect of surface topography through fractal analysis. According to the analysis carried out, the fractal dimension and the slope of the signal (dz/dx) of the topographical signal of the studied surface appear much more relevant than all the other Euclidian parameters. The fractal dimension and the slope of profile allow us to estimate a critical transition speed between the cutting states by generalized strain hardening and those by localized strain hardening. This parameter is also more relevant than the others, because it allows a good analysis of the influence of cutting speed, within each of the two machining modes. The obtained result is relevant because it provides a practical and inexpensive method for the quality control of the machined surface, to manufacturers and engineering companies, without removing some mechanical part, but only through a direct analysis of the slopes of the profile, with, in particular, the help of a portable instrument. We establish later that the transition between disorder and order of the aspect of the observed profiles is essentially due to an instability, which we analyze by the chaos theory. For that purpose, we propose an original construction of an attractor that presents a fixed point for low cutting speeds. This attractor characterizes, beyond the critical cutting speed, an instability described by a phenomenon of successions of states on the attractor between work hardening by localized shear plastic deformation and softening due to the rise in temperature