Dans cette thèse, nous nous intéressons à l’adhésion d’élastomères sur des substrats rigides (interactions de van der Waals). Nous revisitons ainsi, en nous appuyant sur une approche expérimentale, deux situations classiques qui permettent la mesure de l’énergie d’adhésion. Dans une première partie dédiée à l’étude du pelage, nous montrons que le frottement peut modifier le processus de détachement d’une bande d’élastomère. Ceci est tout d’abord mis en évidence lors du pelage parallèlement à l’interface, où nous montrons qu’il conduit à la progression d’une zone de frottement jusqu’au détachement de la bande pour une force proportionnelle à l’aire de contact. Nous généralisons par la suite nos résultats au pelage selon un angle quelconque. En particulier, nous montrons comment la dissipation par frottement peut augmenter de façon significative la force de pelage pour des angles faibles. Dans une deuxième partie, nous nous tournons vers la mesure d’adhésion dans une géométrie JKR, c’est à dire lors de l’indentation d’une sphère élastique adhésive avec un substrat rigide. Nous utilisons des coques élastiques dont la réponse mécanique est beaucoup plus souple bien que plus complexe. Nous montrons qu’il est néanmoins possible d’obtenir une mesure de l’adhésion par une méthode très simple et très robuste, puisqu’elle ne suppose pas de connaissance a priori de la mécanique du système. Nous espérons que cette technique permettra dans le futur d’accéder à des énergies d’adhésion très faibles ou mettant en jeu des tissus biologiques particulièrement mous. / In this thesis, we are devoted to study the adhesion of elastomers to rigid substrates through van der Waals interactions. We review, from an experimental point of view, two classical methods to measure the adhesion energy. A first part is dedicated to the study of the peeling system, we show that friction can modify the detachment process of an elastomer strip. This is firstly observed in the lap-test configuration, where a sliding front propagates on the interface up until the end of the strip, for a pulling force proportional to the initial contact area. We generalize this results for the finite peeling angle case. In particular, we show how the friction dissipation significantly increases for small peeling angles. In the second part, we study the adhesion measure in the JKR geometry, i.e. for the indentation of an adhesive elastic sphere into a rigid plate. We use elastic thin shells, which elastic response is much softer, as well it is much more complex. However, it is possible to measure the adhesion energy through a very simple and robust method with no need of much details of the mechanical response of the system. We hope this technique will allow to measure the effect of very weak and sensitive adhesive systems such us biological tissues which are particularly soft.

The ability to accurately measure surface and interfacial energies affects our understanding of friction, wear, bonding and adhesion. Although there are accurate ways to measure the surface energies of liquids, the surface energies of solids have been harder to characterize. In order to broaden the knowledge of adhesion of solids, a modification to the constrained blister test is proposed. Most of the previous work on constrained blisters has examined the debonding of the blister from the surface underneath as pressure is applied from below. In this thesis, the contact of the constrained blister with the flat surface above it is considered. In addition, the blister is given specified boundary conditions at its outer radius, which has a fixed value. Three models of the blister behavior are considered: linear plate, nonlinear plate, and membrane. The contact of the blister with the substrate above it is modeled with no adhesion, the JKR-type of adhesion, and the DMT-type of adhesion. Several substrate heights are considered, along with several values for the work of adhesion in the JKR analysis, and several combinations of force magnitude and gap size in the DMT analysis. The effect of adhesion on the contact radius is investigated. Sometimes the contact radius changes discontinuously as the pressure is increased or decreased. Results from the three models of blister behavior and the different models of adhesion are compared. / Master of Science

The understanding of topics such as friction, wear, lubrication, and adhesive bonds is dependent on the ability to measure surface and interfacial energies. The surface energies of liquids may be measured accurately using a variety of techniques; however, surface energies of solids are much more difficult to accurately measure. In an attempt to develop a method that can be used to measure surface and interfacial energies of solids, this thesis proposes the use of a elastica. The elastica acts as an extremely flexible beam and provides a structure that will permit measurable deformation of the solid by relatively small surface attractions. The ends of the elastica are lifted, bent, and clamped vertically at an equal height and specified distance apart. They are then moved downward, allowing the strip to make contact with a flat, rigid, horizontal surface. Two adhesion models are investigated. First, a JKR-type analysis, which examines the effect of adhesion forces that exist within the area of contact between the elastica and the rigid surface, is considered. Various values for the work of adhesion are examined. A DMT-type analysis, which assumes that the adhesion forces act in the region just outside of the contact area, is also considered. Results are obtained for linear and constant forces. Various values for the maximum DMT force and the vertical separation between the elastica and the rigid substrate at which the adhesion forces terminate are examined. Results from the two types of analyses are compared. / Master of Science

Lorsque deux objets sont mis en contact, on réalise une expérience d’adhésion si on les sépare et une expérience de friction si on les fait glisser l’un sur l’autre. Lors de ces expériences, on mesure les forces d’adhésion et de friction contrôlées par un paramètre fondamental qui est l’aire réelle de contact. Cette aire réelle dépend fortement de la rugosité des surfaces. Afin de mieux comprendre le rôle de la rugosité, des expériences d’adhésion et de friction ont été réalisées entre des sphères d’élastomère en PDMS et des surfaces texturées (dures ou molles) constituées d’une rugosité modélisée par un réseau hexagonal de plots cylindriques de hauteurs, diamètres et espacements micrométriques.Dans les expériences d’adhésion, un dispositif de type JKR (pour Johnson, Kendall et Roberts) a été utilisé permettant d’observer le contact entre une sphère élastique et un plan texturé tout en contrôlant la force entre les surfaces. À faible force d’appui, la sphère reste au sommet des plots et le contact est dit « posé ». Lorsque la force entre les surfaces augmente, un contact total, où les plots sont écrasés (« contact intime »), apparaît au centre du contact, entourée d’une couronne de contact « posé ». Un modèle d’évolution du contact intime a été réalisé en prenant en compte l’adhésion entre les plots et les caractéristiques mécaniques des surfaces. De plus, en utilisant une analyse similaire à l’analyse classique de type JKR, il a été possible de mesurer les énergies d’adhésion effectives entre les surfaces. L’étude de l’évolution de ces énergies d’adhésion en fonction de la densité surfacique de plots sous le contact sphère-plan s’est révélée complexe. Finalement, des mesures de la force d’arrachement ont été réalisées, confirmant le rôle très important de la nature du contact sur l’évolution des énergies d’adhésion effective.Pour les expériences de friction, un tribomètre développé au laboratoire a été utilisé pour mesurer la force de friction dynamique. Durant ces expériences, les deux types de contact précédemment cités ont également été observés. Dans le cas où le contact reste « posé », il est naturel d’introduire une contrainte de friction égale à la force de friction divisée par l’aire réelle de contact. Il a été montré que cette contrainte de friction augmente sur des surfaces texturées (par rapport au cas lisse) et que cette augmentation dépend de façon complexe de la géométrie des plots utilisés. De plus, il a été montré que pour des petits rayons de courbure des sphères frottantes, la contrainte de friction n’est plus indépendante de l’aire réelle de contact. Finalement, nous avons montré que la contrainte de friction dans la zone de contact intime est la même que pour des surfaces lisses.Ce travail ouvre la voie à des développements théoriques et numériques nouveaux sur l’analyse du champ de contraintes et de déformations pour des contacts texturés modèles. / When two objects are in contact, an adhesion experiment is carried out if they are separated and a friction experience if one object slides on the other. A fundamental parameter which controls the adhesion and friction forces is the real area of contact between the surfaces which is largely determined by the surface roughness. To better understand the role of roughness, adhesion and friction experiments were performed with spheres of PDMS elastomer and textured surfaces (hard or soft). The latter’s roughness is modeled by an hexagonal network of cylindrical pillars with micrometrical dimensions and spacing.In adhesion experiments, a JKR set up (for Johnson, Kendall and Roberts) was used to observe the contact between an elastic sphere and a textured surface while controlling the force between the surfaces. At low normal force, the sphere remains at the top of the pillars and the contact is called "top". When the force between the surfaces increases, a full area of contact, where pillars are collapsed ("intimate contact"), appears in the center contact, surrounded by a crown of "top" contact. A model of evolution of this intimate contact which takes into account the adhesion between the pillars and the mechanical properties of surfaces has been achieved. Furthermore, it was possible to measure effective energies of adhesion between the surfaces using a similar analysis to the classical JKR analysis. Studying the evolution of these adhesion energies as a function of the pillars’ surface density below the sphere-plan contact proved to be a challenging task. At last, measurements of the pull off force were realised, corroborating the important role of the nature of contact on the evolution of effectif energies of adhesion.For friction experiments, a tribometer developed in the laboratory was used to measure the dynamic frictional force. During these experiments, the two kinds of contact previously reported were observed. When the contact remains "top", it is natural to introduce a friction stress equal to the friction force divided by the real area of contact. It has been shown that friction stress increases on textured surfaces (relative to the smooth case) and that this increase depends in a complex manner on the geometry of the pillars. Moreover, it has been shown that for small curvature radii of the friction spheres, the friction stress is no longer independent of the real area of contact. Finally, we have shown that the friction stress in the zone of intimate contact is the same as on smooth surfaces.The experimental results obtained in this thesis will serve to validate future numerical models.

Development of new scientific disciplines such as bioengineering and micro-nano engineering adopting nonconventional materials requests innovative methodologies that can accurately measure the mechanical properties of soft biological materials and characterize surface energy and adhesion properties of them, independent of measurement conditions. One of emerging methods to measure the deformation of materials under stress is digital image correlation (DIC) technique. As a noncontact strain measurement method, DIC has the advantages of prevention of experimental errors caused by the use of contact type sensors and of flexibility in its application to soft materials that are hard to be tested by conventional method. In the first part of the thesis, 2 dimensional and 3 dimensional DIC codes were developed and optimized, and then applied to two critical applications: 1) determining the stress-strain behaviour of polydimethylsiloxane (PDMS) sample, as a model soft material, using the optical images across large deformation region, and 2) detecting the stiffness variation within the gel mimicking the breast tumour using ultrasound images. The results of this study showed the capability of DIC as a strain sensor and suggested its potential as a diagnosing tool for the malignant lesion causing local stiffness variation. In the characterization of surface energy and adhesion properties of materials, two most common methods are contact angle measurement and JKR-type indentation test. In the second part of the thesis, the experimental set-up for these methods were developed and verified by using the PDMS in static (quasi equilibrium) state. From the dynamic tests, it showed its possible usage in studying adhesion hysteresis with respect to speed. The adhesion hysteresis was observed at high speed condition in both contact angle measurement and JKR-type indentation tests.

Development of new scientific disciplines such as bioengineering and micro-nano engineering adopting nonconventional materials requests innovative methodologies that can accurately measure the mechanical properties of soft biological materials and characterize surface energy and adhesion properties of them, independent of measurement conditions. One of emerging methods to measure the deformation of materials under stress is digital image correlation (DIC) technique. As a noncontact strain measurement method, DIC has the advantages of prevention of experimental errors caused by the use of contact type sensors and of flexibility in its application to soft materials that are hard to be tested by conventional method. In the first part of the thesis, 2 dimensional and 3 dimensional DIC codes were developed and optimized, and then applied to two critical applications: 1) determining the stress-strain behaviour of polydimethylsiloxane (PDMS) sample, as a model soft material, using the optical images across large deformation region, and 2) detecting the stiffness variation within the gel mimicking the breast tumour using ultrasound images. The results of this study showed the capability of DIC as a strain sensor and suggested its potential as a diagnosing tool for the malignant lesion causing local stiffness variation. In the characterization of surface energy and adhesion properties of materials, two most common methods are contact angle measurement and JKR-type indentation test. In the second part of the thesis, the experimental set-up for these methods were developed and verified by using the PDMS in static (quasi equilibrium) state. From the dynamic tests, it showed its possible usage in studying adhesion hysteresis with respect to speed. The adhesion hysteresis was observed at high speed condition in both contact angle measurement and JKR-type indentation tests.

The measurement of the work of adhesion is of significant technical interest in a variety of applications, ranging from a basic understanding of material behavior to the practical aspects associated with making strong, durable adhesive bonds. The objective of this thesis is to investigate a novel technique using an elastica loop to measure the work of adhesion between solid materials. Considering the range and resolution of the measured parameters, a specially designed apparatus with a precise displacement control system, an analytical balance, an optical system, and a computer control and data acquisition interface is constructed. An elastica loop made of poly(dimethylsiloxane) [PDMS] is attached directly to a stepper motor in the apparatus. To perform the measurement, the loop is brought into contact with various substrates as controlled by the computer interface, and information including the contact patterns, contact lengths, and contact forces is obtained. Experimental results indicate that due to anticlastic bending, the contact first occurs at the edges of the loop, and then spreads across the width as the displacement continues to increase. The patterns observed show that the loop is eventually flattened in the contact region and the effect of anticlastic bending of the loop is reduced. Compared to the contact diameters observed in the classical JKR tests, the contact length obtained using this elastica loop technique is, in general, larger, which provides potential for applications of this technique in measuring interfacial energies between solid materials with high moduli. The contact procedure is also simulated to investigate the anticlastic bending effect using finite element analysis with ABAQUS. The numerical simulation is conducted using a special geometrically nonlinear, elastic, contact mechanics algorithm with appropriate displacement increments. Comparisons of the numerical simulation results, experimental data, and the analytical solution are made. / Master of Science

Lorsque deux solides séparés par un liquide se rapprochent, le drainage s’accompagne de forces visqueuses normales aux parois. Si elles sont très rapprochées, de l’ordre de quelques nanomètres, les parois sont indentées par ces forces : c’est le "confinement élastique". Indenter un solide par un liquide permet de supprimer l’adhésion, qui limite la résolution en termes de module d’Young des tests classiques d’indentation par un solide, surtout pour les films supportés minces et mous, par exemple des élastomères d’épaisseur micrométrique. Or leurs propriétés, qui peuvent sensiblement différer des propriétés en volume, sont essentielles dans des domaines aussi variés que la microfluidique, l’électronique ou l’usure mécanique. Nous présentons les calculs qui relient les forces normales aux propriétés mécaniques du liquide et des parois lors d’un confinement élastique. Les résultats sont comparés à des expériences de nanorhéologie réalisées sur une machine à forces de surface très sensible. Cette sensibilité nous permet de montrer que l’effet du confinement élastique se manifeste même sans film mou déposé : cela implique que la rhéologie apparente d’un liquide confiné est toujours affectée par les déformations des parois, même très rigides.Nous montrons enfin que nous pouvons effectivement mesurer avec précision des modules d’Young autour du MPa dans des films d’élastomères de quelques centaines de nanomètres à quelques micromètres d’épaisseur. Si le module de stockage ne varie presque pas avec l’épaisseur, un module de pertes apparaît, augmentant sensiblement lorsque l’épaisseur diminue, témoignant d’une visco-élasticité que nous attribuons à la présence d’une couche interfaciale. / When two solids separated by a liquid layer are moving towards each other, the resulting drainage is associated with viscous forces normal to the walls. If these are very close to each other (several nanometers), they are indented by these forces : this is the notion of “elastic confinement”. Indenting a solid by a liquid solves the problem of adhesion, which limits the ability of classical indentationtests to provide accurate measurements on Young’s modulii. Adhesion is especially problematic for soft thin films, for example micrometric layers of elastomers, which mechanical properties can strongly differ from the bulk, but are of the highest importance in various fields such as microfluidics, electronics or mechanical wear. We present here the calculations which link the solid and liquid mechanical properties to the resulting forces in a liquid indentation test. The corresponding results are compared to nanorheology experiments using a very sensitive Surface Force Apparatus. Its sensitivity enables us to show that the elastic confinement is also measurable without any soft films, which implies that a confined liquid apparent rheology is always affected bythe deformations of even very rigid confining walls. Finally, we demonstrate that we are indeed able to measure precisely Young’s modulii in the MPa range for films as thin as several hundreds of nanometers. While the storage modulii are found to be almost independent ofthe film thicknesses, we identify the presence of loss modulii increasing with decreasing thicknesses. We attribute this unexpected viscoelastic behaviour to the presence of an interfacial layer.

The development of the layer-by-layer (LbL) technique has turned out to be an efficient way to physically modify the surface properties of different materials, for example to improve the adhesive interactions between fibers in paper. The main objective of the work described in this thesis was to obtain fundamental data concerning the adhesive properties of wood biopolymers and LbL films, including the mechanical properties of the thin films, in order to shed light on the molecular mechanisms responsible for the adhesion between these materials. LbLs constructed from poly(allylamine hydrochloride) (PAH)/poly(acrylic acid) (PAA), starch containing LbL films, and LbL films containing nanofibrillated cellulose (NFC) were studied with respect to their adhesive and mechanical properties. The LbL formation was studied using a combination of stagnation point adsorption reflectometry (SPAR) and quartz crystal microbalance with dissipation (QCM-D) and the adhesive properties of the different LbL films were studied in water using atomic force microscopy (AFM) colloidal probe measurements and under ambient conditions using the Johnson-Kendall-Roberts (JKR) approach. Finally the mechanical properties were investigated by mechanical buckling and the recently developed SIEBIMM technique (strain-induced elastic buckling instability for mechanical measurements). From colloidal probe AFM measurements of the wet adhesive properties of surfaces treated with PAH/PAA it was concluded that the development of strong adhesive joints is very dependent on the mobility of the polyelectrolytes and interdiffusion across the interface between the LbL treated surfaces to allow for polymer entanglements. Starch is a renewable, cost-efficient biopolymer that is already widely used in papermaking which makes it an interesting candidate for the formation of LbL films in practical systems. It was shown, using SPAR and QCM-D, that LbL films can be successfully constructed from cationic and anionic starches on silicon dioxide and on polydimethylsiloxane (PDMS) substrates. Colloidal probe AFM measurements showed that starch LbL treatment have potential for increasing the adhesive interaction between solid substrates to levels beyond those that can be reached by a single layer of cationic starch. Furthermore, it was shown by SIEBIMM measurements that the elastic properties of starch-containing LbL films can be tailored using different nanoparticles in combination with starch. LbL films containing cellulose I nanofibrils were constructed using anionic NFC in combination with cationic NFC and poly(ethylene imine) (PEI) respectively. These NFC films were used as cellulose model surfaces and colloidal probe AFM was used to measure the adhesive interactions in water. Furthermore, PDMS caps were successfully coated by LbL films containing NFC which enabled the first known JKR adhesion measurements between cellulose/cellulose, cellulose/lignin and cellulose/glucomannan. The measured adhesion and adhesion hysteresis were similar for all three systems indicating that there are no profound differences in the interaction between different wood biopolymers. Finally, the elastic properties of PEI/NFC LbL films were investigated using SIEBIMM and it was shown that the stiffness of the films was highly dependent on the relative humidity. / <p>QC 20110923</p>

La compréhension des mécanismes qui pilotent la transmission des contraintes aux interfaces déformables est au centre de nombreuses problématiques touchant des applications actuelles utilisant un film mince de polymère souple comme couche interfaciale. Arriver à caractériser de tels films fins est encore un défi aujourd’hui car l’analyse des mesures expérimentales destinées à extraire les contributions dues aux films est complexe et délicate et les techniques usuelles de caractérisation sont peu adaptées aux systèmes. Ce travail étudie la réponse mécanique de deux types de systèmes modèles au moyen de deux techniques de caractérisation différentes. Le premier système que nous avons élaboré et caractérisé mécaniquement par le test JKR, est constitué de films d’élastomère réticulé d’épaisseurs micrométriques (de 5 à 100µm) et déposés sur des wafers de silicium. Les mesures expérimentales ont été analysées par comparaison à un modèle semi-analytique récent proposé par E. Barthel dans le but d’extraire le module élastique de chaque film et de répondre à la question de savoir si l’épaisseur du film influe sur la valeur de ce module. Nous avons montré que ce modèle permet de rendre compte quantitativement du raidissement lié à la présence d’un solide supportant le film mais que la précision sur les mesures de modules de Young reste limitée (de l’ordre de 35 %).Le deuxième système modèle est constitué de brosses de polymères greffées (PDMS) par une extrémité à la surface de wafers de silicium et gonflées dans un bon solvant (47V20). Nous avons analysé la réponse mécanique dans plusieurs régimes de distance et de fréquence en utilisant un appareil à forces de surface (SFA) dans lequel on contrôle l’approche d’une sphère millimétrique d’un plan sur lequel sont greffées les polymères. En statique, nous avons vérifié que la réponse en compression était celle d’une brosse de type Alexander-de Gennes. En mode dynamique, quand la sphère est loin de la couche gonflée, nous avons vérifié que la réponse dissipative était celle d’un écoulement de Reynolds qui décrit normalement l’écoulement d’un fluide simple newtonien entre une sphère et un plan solide. Ceci nous a permis de montrer que l’écoulement du solvant pénètre partiellement à l’intérieur de la couche greffée sur une profondeur de l’ordre du tiers de l’épaisseur gonflée de la couche. Dans le régime ou les brosses sont comprimées, il n’y a pas d’accord entre les mesures réalisées et le modèle classique de Fredrickson et Pincus. Ceci s’explique par les expériences que nous avons réalisées sur un substrat nu (sans polymère) montrant pour la première fois la déformation des substrats solides qui sont indentés par l’écoulement de liquide et qu’il faut prendre en compte cette déformation dans les analyses de nanorhéologie. Finalement, une annexe est consacrée à la fabrication de surfaces hydrophobes silanisées optimisées en vue d’étudier le glissement d’un liquide simple et d’électrolytes à la paroi. / Understanding how stresses are transmitted to deformable interfaces is a key-point in numerous issues having everyday life applications which use a thin polymer film as an interfacial layer. Still, characterizing the mechanical properties of such elastic films remains a challenge because the usual employed techniques are destructive of the surface and because of the complexity of the associated analysis. In this work, we study the mechanical response of two types of home-made model systems using two different characterization techniques. The first system – studied with a JKR test- is composed of reticulated elastomeric films of micrometric thickness (5 to 100 µm) and stuck to a silicon wafer. We analyse the experimental data with E.Barthel’s recently published semi-analytical model in order to determine the elastic modulus of each indented film and see if the thickness of the film had any influence on its value. We show that this model is in a quantitatively good agreement with our data but that we only have a 35% accuracy on the elastic modulus values thanks to the set-up. The second system we studied consists in polymer brushes end-grafted onto the surface of silicon wafers and of nanometric thickness. To characterize the mechanical response of those brushes and the effect of both their molecular organization and ingredients on their ability to transmit stresses at the interface, we use a surface force apparatus in the dynamic mode as a soft fluid indenter. We use a millimetric sphere to create a liquid flow of the solvent in which the brushes are immerged and swollen. This flow induces hydrodynamic forces whose range we can control by varying the excitation frequency and the distance of approach. We obtain the following results : first with the static response we checked that the response of the polymer layers are well-described by the Alexander-de Gennes approximation. In the dynamical mode, when the sphere is far from the solid surface, we showed that the dissipative response was well-described by the Reynolds force. Thanks to those results, we succeeded in localizing the limit of penetration of the liquid flow inside the brushes at one third of the thickness of the swollen brush; second, when the brushes are compressed, we showed that the existing models (Fredrickson & Pincus) are insufficient to explain the dynamic responses of the brushes. This disagreement is explained by experiments we performed on the bare solid substrate, which show for the first time, the deformation of the substrate due to the liquid. Thus, the mechanical response of the underlying substrate has to be taken into account in the analysis of the nanorheological results on the brushes even though the substrate is much stiffer than the polymer layers. Finally, we present how we fabricated hydrophobic (silanized) surfaces in order to study the sliding of simple liquids at the wall with the same surface force apparatus.