Dissertations / Theses on the topic 'Adhesive Contact Mechanics'

The applications of inflatable membrane structures are increasing rapidly in the various fields of engineering and science. The geometric, material, force and contact non-linearities complicate this subject further, which in turn increases the demand for computationally efficient methods and interpretations of counter-intuitive behaviors noted by the  scientific community. To understand the complex behavior of membranes in biological and medical engineering contexts, it is necessary to understand the mechanical behavior of a membrane from a physics point of view.  The first part of the  present work studies the pre-stretched circular membrane in contact with a soft linear substrate. Adhesive and frictionless contact conditions are considered during inflation, while only adhesive contact conditions are considered during deflation. The peeling of membrane during deflation is studied, and a numerical formulation of the energy release rate is proposed. It is observed that the pre-stretch is having a considerable effect on the variation of the energy release rate. In the second part, free and constrained inflation of a cylindrical membrane is investigated. Adhesive and frictionless contact conditions are considered between the membrane and substrate. It is observed that the continuity of principal stretches and stresses depend on contact conditions and the inflation/deflation phase. The adhesive traction developed during inflation and deflation arrests the axial movement of material points, while an adhesive line force created at the contact boundary is responsible for a jump in stretches and stresses at the contact boundary. The pre-stretch produces a softening effect in free and constrained inflation of cylindrical membranes. The third part of the thesis discusses the instabilities observed for fluid containing cylindrical membranes. Both limit points and bifurcation points are observed on equilibrium branches. The secondary branches emerge from bifurcation points, with their directions determined by an eigen-mode injection method. The occurrence of critical points and the stability of equilibrium branches are determined by perturbation techniques. The relationship between eigenvalue analysis and symmetry is highlighted in this part of the thesis.

QC 20150227

The remarkable attachment system of geckos has inspired the development of dry microfiber adhesives through the last two decades. Some of the notable characteristics of gecko-inspired fibrillar adhesives include: strong, directional, and controllable adhesion to smooth and rough surfaces in air, vacuum, and under water; ability to maintain strong adhesion during repeated use; anti-fouling and self-cleaning after contamination. Given these outstanding qualities, fibrillar adhesives promise an extensive range of use in industrial, robotic, manufacturing, medical, and consumer products. Significant advancements have been made in the design of geckoinspired microfiber adhesives with the characteristic properties listed above, with the exception of the anti-fouling and self-cleaning features. The self-cleaning mechanism of the gecko’s adhesion system plays an important role to its ability to remain sticky in various environments. Similarly, enabling self-cleaning capability for synthetic microfiber adhesives will lead to robust performance in various areas of application. Presently, the practical use of fibrillar adhesives is restricted mainly to clean environments, where they are free from contaminants. The goal of this thesis is to conduct a detailed study of the mechanisms and mechanics of contact-based self-cleaning of gecko-inspired microfiber adhesives. This work focuses on contact self-cleaning mechanisms, as a more practical approach to cleaning. Previous studies on the cleaning of microfiber adhesives have mostly focused on mechanisms that involve complete removal of the contaminants from the adhesive. In this thesis, a second cleaning process is proposed whereby particles are removed from the tip of the microfibers and embedded between adjacent microfibers or in grooves patterned onto the adhesive, where they are no longer detrimental to the performance of the adhesive. In this work, a model of adhesion for microfiber adhesives that take the deformation of the backing layer under individual microfiber is developed. The dependence of adhesion of microfiber adhesives on the rate of unloading is also modeled and verified using experiments. The models of adhesion presented are later used to study the mechanics of contact self-cleaning of microfiber adhesives. Three major categories of self-cleaning are identified as wet self-cleaning, dynamic self-cleaning, and contact self-cleaning. A total of seven self-cleaning mechanisms that are associated with these categories are also presented and discussed. Results from the self-cleaning model and experiments show that shear loading plays an important role in self-cleaning. The underlying mechanism of contact self-cleaning due to normal and shear loading for spherical contaminants is found to be the particle rolling between the adhesive and a contacted substrate. Results from the model and experiments also show that small microfiber tips (much less than the size of the contaminants) are favorable for self-cleaning. On the other hand, large microfiber tips (much larger than the size of the contaminants) are favorable for anti-fouling of the microfiber adhesive. Results from this work suggests that the sub-micrometer size of the gecko’s adhesive fibers and the lamellae under the gecko toes contribute to its outstanding self-cleaning performance. The results presented in this thesis can be implemented in the design of microfiber adhesives with robust adhesion, self-cleaning and anti-fouling characteristic, for use in numerous applications and in various environments.

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, May, 2020
Cataloged from the official PDF of thesis.
Includes bibliographical references (pages 65-68).
Projection Micro-Stereolithography (P[mu]SLA) is one of the most high-throughput additive manufacturing methods, yet preserving the high-resolution characteristic of light-based polymerization techniques. However, further improvement of fabrication speed and precision is usually hindered by the undesired adhesive forces at the curing interface, which is an inevitable consequence of in situ liquid-to-solid phase transition. To overcome this limitation, a bio-inspired super low adhesive interface has been proposed based on the observation of a slippery water layer on the peristome surface of pitcher plant. This hydrophobic layer provides an effective shield to solid adhesion due to its low adhesive energy, and attracting force between fabricated part and UV curing interface is significantly reduced. The introduction of this new lubrication layer not only remarkably improves the fabrication speed, but also increases the refilling rate of liquid pre-polymer resin.
This ultra-low adhesive interface shows promises for pushing the boundaries of continuous 3D printing into a realm of high-throughput additive manufacturing methods ready for industrial applications. In this thesis, I sought to provide a more comprehensive understanding of the solid-solid interaction at the curing interface of 3D photo-polymerizing systems. The state-of- the-art review of current literature suggested that a surface-based cohesive contact theory from a continuum mechanics perspective was the most appropriate model to establish a connection between interfacial material properties and macroscopic measurement results from experiment. Based on that I analyzed the entire mechanical separation process using finite-element method, and provided a semi-quantitative explanation of the stability of such lubricant-infused nano-cavities against peeling forces.
This research lays the ground for elucidating the physical mechanism behind the general adhesion-separation problem, and framework has been constructed in a more general form to allow for analyzing a wide range of interdisciplinary problems involving the dynamics of anisotropic moving contact lines and the propagation of surface instabilities induced by adhesive contact.
by Huifeng Du.
S.M.
S.M. Massachusetts Institute of Technology, Department of Mechanical Engineering

Doctor of Philosophy
Department of Mechanical and Nuclear Engineering
Kevin B. Lease
The overall goal of this study was to assess the effects of surface chemistry on adhesion of polymeric systems underwater. The adhesion is quantified by the thermodynamic work of adhesion (W) when two surfaces are approached and the energy release rate (G) when the surfaces are separated. For some polymeric systems there is a difference between W and G, referred to as adhesion hysteresis. For this study an experimental approach based upon Johnson-Kendall-Roberts (JKR) theory of contact mechanics was utilized to evaluate how surface chemistry affects the adhesion behavior (both W and adhesion hysteresis) in the presence of water. The interfacial interactions were also studied in air and contrasted to those obtained underwater. To accomplish the overall goal of this research, this study was divided into two phases where smooth model surfaces with disparate surface chemistries were used. The model surfaces in the first part included poly(dimethysiloxane) (PDMS), glass surfaces chemically functionalized to display hydrophilic to medium to hydrophobic characteristics, and thin films of wood-based biopolymers. The functionalities used to modify glass surfaces included polyethylene oxide (PEO) with hydrophilic nature; amine, carbomethoxy, and mercapto (thiol) with intermediate characteristics; cyclohexyl, fluorocarbon, and methyl with hydrophobic behavior. In addition to these surfaces, flat PDMS and clean glass surfaces were also used for means of comparison. The wood-derived polymers included two different cellulose types (natural cellulose and regenerated cellulose) as well as one lignin surface (from hardwood milled lignin). These surfaces were probed with native PDMS hemispheres, which are hydrophobic. The results showed that in air the value of W for all model surfaces was independent of the surface chemistry, except fluorocarbon which was lower. Underwater W was significantly affected by the surface hydrophilicity/ hydrophobicity. The adhesion hysteresis both in air and underwater was significantly dependent on the structure of the probed surface. For the second phase PDMS hemispheres were chemically modified with amine functionality to probe model surfaces with hydrophilic and intermediate behavior. These surfaces included glass surfaces functionalized with PEO and amine as well as PDMS sheets that were functionalized with amine. Native PDMS flat surfaces were also used for means of comparison. The results showed that for the selected surfaces both W and hysteresis were affected by the surface chemistry in both media.

Friction management and control of adhesion at the wheel/rail interface is vital for an efficient and cost effective railway network. The understanding of how the friction management products (grease and friction modifiers) work and effectively test these products is necessary to improve the performance of a railway network. The papers presented concern the effective benchmarking of wayside curve lubricants (grease) in a twin disc test rig. They compare the effectiveness of several greases in respect to adhesion, wear protection and retentivity (number of cycles of adequate lubrication). A new method for assessment of grease carry down has been trialled in the field. The modified pendulum was able to detect the difference between a dry and lubricated rail gauge face. Top of rail friction modifiers (TOR-FMs) have been tested at two different laboratory test scales. The results showed the difference in operational behaviour of the chosen TOR-FM when used in a laboratory versus the field. The ‘wet-rail’ phenomena, where low adhesion as a result of water on the rail head, has been investigated at two scales of laboratory test and results have been used to generate a model to predict adhesion coefficients for a range of water and iron oxide mixtures. The results presented show how the addition of small amounts of water to a wheel/rail contact can cause reduced adhesion to ‘low/ultra-low’ levels when combined with third body materials (iron oxides, wear debris etc.). A novel treatment method to protect the rail head using hydrophobic solutions was investigated using twin disc and pendulum testing. Tests showed that these products, when sufficiently diluted, do not reduce friction to dangerous levels or isolate the vehicle from the track circuit. However, the benefits of use in the field are questioned.

Dans cette thèse, les mécanismes d’endommagement du verre revêtu sous contact glissant ont été étudiés à l’aide d’essais microscratch in situ. Dans un premier temps, la fissuration du verre revêtu d’une monocouche de Si3N4 a été comparée à la résistance à la rayure du verre nu. Les résultats sont discutés en termes de friction et de différence de module entre le revêtement et le substrat. Ensuite, un système bicouches Si3N4/Ag a été étudié. L'impact de l'épaisseur du revêtement sur l’amorçage d’une rayure a été étudié expérimentalement et numériquement. Enfin, un empilement multicouches modèle, constitué de couches de nitrure/oxyde et d'argent d'environ 100nm d'épaisseur, a été étudié. Les résultats montrent que l’amorçage de la rayure et la propagation stationnaire résultent de deux mécanismes très différents. Les propriétés interfaciales (coefficient de frottement et énergie d’adhésion) ont été mesurées et leur impact sur le mécanisme d’amorçage d’une rayure a été évalué par des analyses en éléments finis.

Friction force microscopy was employed for the tribological investigation of human head hair in: a dry atmosphere and in de-ionized water. The effects of bleaching, conditioning, and immersion in methanolic KOH were quantified using the relative coefficient of friction (u). The coefficient of friction in both environments increased in the order: virgin < virgin-conditioned < damaged-conditioned < bleached < KOH-damaged hair. All categories of hair exhibited higher friction coefficients in the aqueous environment, attributed mainly to the higher elastic compliance of the fibres due to water absorption. Secondary ion mass spectroscopy was used as a complementary technique to examine the presence 0 f fatty acids on the cuticular surface 0 r the different categories of hair as well as the conditioner distribution. Conditioner species were detected along the whole cuticular surface. The combination of SIMS with FFM data suggests that the main reason for the different tribological properties of the categories of hair examined is the altered chemistry of the surface, i.e., partial removal of the covalently bound lubricant layer of 18-MEA and, also, of the unbound fatty acids (stearic, palmitic, myristic, etc.) by the damaging agents. Atomic and friction force microscopy was employed to investigate friction and adhesion between polar and between non-polar self-assembled mono layers in pure solvents as well as in heptane/acetone mixtures of varying polarity. The two polar interfaces examined were l l-rnercapto-l-undecanol vs l l-rnercapto-l-undecanol (MUT), and II-mercapto-l- undecanol vs Oiethoxy-Phosphatoethyl-Triethoxysilane (OPTS), while the non-polar interface examined was l-dodecanethiol vs l-dodecanethiol (000). For the MUT and OPTS interfaces the pull-off forces were found to decrease with increasing static dielectric constant of the medium (e), while for the 000 interface the opposite trend was observed. A simple model based on functional group H-bond parameters was found adequate to describe the adhesive interactions for the MUT and OPTS interfaces in terms of the degree of so lvation of the functional groups of the mono layers by the medium. In the heptane/acetone mixtures the pull-off force was observed to correlate excellently to the free energy of interaction as predicted by the model. For the 000 interface an approximation of the Lifshitz theory predicted satisfactorily the pull-off forces in media of weak hydrogen bond donor ability. The friction-load relationship at the 'wearless' regime (L < 20 nN) was found to be dependent on the strength of adhesion as well as to the molecular properties of the medium. Systems of lowest adhesion obeyed Arnonton's law, while as adhesion increased their offset displaced towards tensile loads and their slope increased, but strongly adhering systems provided a sublinear Friction - Load relation best fitted by the Derjaguin-Muller-Toporov (DMT) model of contact mechanics. Exceptionally, in n-octanol a two-sloped linear relation was observed for all interfaces, attributed to its ability to strongly physisorb on them, decreasing the friction coefficient considerably and eliminating the frictional differences of the three interfaces at loads lower than about 3 nN. These observations were rationalized by friction being considered to be the sum of an interfacial and a plowing term. The Fr-L data for all interfaces were successfully modeled, under the assumption that DMT contact mechanics is obeyed, with an adhesion-independent friction coefficient due to plowing, and an adhesion-dependent shear strength due to interfacial friction. For all interfaces the shear strength was found to increase as the pull-off force increased, thus explaining Amontori's law as the limit of zero shear strength at very weakly adhesive systems. For the MUT and OPTS interfaces in the mixtures, the shear strength was found to correlate closely to the free energy of interaction predicted by the H-bonding model used. The OPTS interface exhibited lower adhesion but higher friction than the MUT one, while the DOD interface exhibited the lowest friction. Analysis with the model used resulted in very close friction coefficients for the DOD and MUT films, but considerably higher one for the OPTS films. The shear strength of the MUT and DPTS was similar, but that of the DOD monolayer was clearly lower. These findings demonstrate the importance of packing effects and hydrogen bonding to friction.

Thesis (Ph. D.)--Mechanical Engineering, Georgia Institute of Technology, 2005.
J. Carson Meredith, Committee Member ; J. Carlos Santamarina, Committee Member ; G. Paul Neitzel, Committee Member ; Richard F. Salant, Committee Member ; Jeffrey L. Streator, Committee Chair. Includes bibliographical references.

Zasadniczym celem pracy było wyznaczenie i analiza wytrzymałości oraz trwałości zmęczeniowej połączeń zakładkowych wykonanych z materiałów o różnych właściowściach mechanicznych. Badaniom poddano połączenia stali konstrukcyjnej C45, stopu aluminium AW-5754 oraz tworzywa sztucznego ABS, wykonane przy użyciu kleju metakrylowego Plexus MA300

La présence d'eau sur la route affecte l'adhérence des pneumatiques. Un film d'eau s'interpose entre le pneumatique et la rugosité du sol routier entraînant d'abord une perte de l'adhérence par adhésion provenant du contact direct gomme-sol. Ensuite, la réduction de la déformation de la gomme induit une diminution de l'adhérence par indentation. C'est ce second aspect qui sera étudié. Le film d'eau génère une portance par un phénomène de lubrification qui diminue la déformation du pneumatique et ainsi son adhérence. Cette étude est basée sur une théorie prédisant la diminution de l'amplitude de la rugosité en fonction des propriétés des surfaces et des conditions de contact. L'équation de Reynolds est résolue par des techniques de calcul efficaces. La viscoélasticité est implémentée dans les calculs de contact sec et lubrifié. L'objectif de cette étude est de proposer un modèle de prédiction de la diminution de l'adhérence d'un pneumatique sur sol mouillé. La meilleure compréhension du contact permettra d'améliorer le choix du matériau composant les bandes de roulements des pneumatiques
Water presence on road affects tyre friction. A water film interposed between tyre and road asperities results first in a loss of adhesion friction coming from the direct contact between rubber and road. Then, the rubber deformation reduction leads to a diminution of the hysteretic friction. This study focuses on this last topic. The water film generates a hydrodynamic force by a lubrication phenomenon which decreases rubber deformation and friction. This study is based on a theory which predicts roughness amplitude reduction as a function of surface properties and contact conditions. Reynolds equation is solved using efficient computing methods. Viscoelasticity is implemented in dry and lubricated calculations. The aim of this research is to suggest a predictive model of the tyre friction decrease on wet roads. A better understanding of the contact will enable to improve the tyre tread material design

Ce travail de thèse met en évidence deux phénomènes microscopiques dissipatifs au voisinage de la ligne de contact dans les phénomènes de capillarité et d'adhésion. L'étude s'appuie sur des expériences dynamiques et des modélisations théoriques. La mesure expérimentale de quantités macroscopiques et la théorie hydrodynamique permettent d'extraire les informations dynamiques localisées au voisinage la ligne de contact. Les différents phénomènes dissipatifs localisés au voisinage de la ligne de contact ont pour origines les propriétés des substrats sur lesquels se déplace la ligne de contact. Pour une surface rigide hétérogène, nous avons développé un modèle rhéologique de la ligne de contact fondé sur l'hydrodynamique, permettant d'établir théoriquement l'évolution temporelle de la ligne de contact et de ses déformations. Une décomposition modale fondée sur la réduction de l'énergie par la théorie du chemin de réaction fournit une prédiction quantitative de la dynamique thermiquement activée de la ligne de contact, en accord avec l'expérience réalisée. Pour un substrat déformable, à partir de l'analyse de deux expériences différentes de mouillage dynamique et de l'estimation de la dissipation dans le substrat, fondée sur sa viscoélasticité, nous avons développé une compréhension générale du comportement dynamique d'une ligne de contact sur un substrat viscoélastique. Pour finir, ce modèle de dissipation viscoélastique est appliqué au cas de l'adhésion réversible, où expérimentalement nous mesurons la dynamique de pelage et de recollement sur un substrat viscoélastique. Cette extension à l'adhésion permet de relier les phénomènes interfaciaux en une compréhension générale
This thesis reveals two dissipative microscopic phenomena close to the contact line in the fields of capillarity and adhesion. The study is based on dynamic experiments and theoretical predictions. Experimental measurement of macroscopic quantities and the hydrodynamic theory give access to dynamic information located close to the contact line. The different dissipative phenomena, located close to the contact line, originate from the properties of the substrates on which the contact line moves. For a heterogeneous rigid surface, we have developed a rheological model of the contact line based on hydrodynamics, in order to theoretically establish the temporal evolution of the contact line and its deformations. A modal decomposition based on the reduction of the energy by the reaction path theory allows a quantitative prediction of the thermally activated dynamics of the contact line, in agreements with the experiment carried out. For a deformable substrate, based on the analysis of two different experiments of wetting dynamics and on the estimation of the dissipation in the substrate founded on its viscoelasticity, a general understanding of the dynamical behavior of contact lines on viscoelastic substrates is achieve. Finally, this viscoelastic model is applied to the case of weak adhesion, where experimentally we measured the peeling dynamics from a viscoelastic substrate. This extension to adhesion bridge the gap between different interfacial phenomena into a general understanding

Les pneus sont un organe déterminant dans la tenue de route des véhicules. Cette thèse porte sur la modélisation du contact entre la bande de roulement d'un pneumatique et une route sèche, afin de comprendre les différents phénomènes physiques mis en jeu ainsi que leurs rôles relatifs dans le frottement. La rugosité multi échelle des sols routiers les rendent difficiles à modéliser avec une simulation par élements finis standard. En utilisant l'hypothèse que la gomme de la bande de roulement est très grande devant la taille des rugosités, elle peut être considérée comme un massif semi-infini. Il est alors possible de résoudre efficacement le problème de contact en ne discrétisant que la surface du massif de gomme. Cette résolution est faite à l'aide d'un algorithme de Gradient Conjugué, au cours duquel les calculs matriciels sont effectués par Transformée de Fourier Rapide (FFT). La viscoélasticité de la gomme est prise en compte en régime transitoire. Les interactions à l'interface entre la gomme et le sol sont modélisés par une loi de frottement ainsi que par une énergie d'adhésion. Les résultats montrent le rôle primordial de la viscoélasticité qui, couplée à la rugosité multi-échelle du sol, modifie la surface du contact au cours des différents étapes de mise en glissement d'un pneumatique, faisant ainsi varier le frottement
Tires are a key component for the handling and safety of personal vehicles. In this thesis a model of the contact between the tire tread and a dry road is described. It aims at understanding the different physical phenomena taking place in such a contact and their relative role in tire friction. Modeling the multiple scales of road roughness is difficult using a standard Finite Element Method. The hypothesis that the rubber of the tire tread is very large compared to the largest scale of surface roughness is made, so that it can be considered as a semi-infinite half-space. This way, the contact problem can be solved by discretizing the rubber surface only. The solver is a specific Conjugate Gradient iterative method, in which the matrix-vector products are performed with Fast Fourier Transforms. Transient viscoelasticity is accounted for with a step-by-step approach. The algorithm is able to model surface interactions such as Coulomb friction and adhesion. Results show the crucial role played by viscoelasticity. Coupled with the road roughness, it changes the contact surface during the different steps of tire sliding, which in turns impacts friction

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

Master of Science
Department of Mechanical and Nuclear Engineering
Kevin B. Lease
This research was intended to measure the interfacial shear strength between fiber/ matrix systems and to investigate the relation between structure-mechanical properties and performance of fiber/matrix systems. This work conducted a systematic study on model fiber/matrix systems to enhance the fundamental understanding on how variation of polymeric compositions (and hence, different structures), different curing conditions, and fiber surface treatments influence the interactions between the fiber and matrix. In order to measure the interfacial shear strength of fiber/matrix systems, the microdroplet technique was used. In this technique a polymer droplet was deposited on a fiber in the liquid state. Once the droplet was cured a shear force was applied to the droplet in order to detach the droplet from the fiber. The amount of the force needed to de-bond the droplet was directly related to the strength of the bonds formed between the fiber and matrix during the curing process. In addition, the micro-droplet technique was used to evaluate effects of different crosslinker ratio of fiber/ matrix system and also to see if different curing conditions affect the interfacial shear strength of fiber/ matrix system. Surface treatment was also conducted to evaluate its effects on the interfacial shear strength of the fiber/ matrix system using microdroplet technique. The interfacial shear strength of fiber/ matrix system increased along with the increase of crosslinker ratio to a limiting value, and it decreased as long as the crosslinker ratio increased. Curing condition also caused the interfacial shear strength of fiber/ matrix system to increase when it was cured at higher temperature. Fiber surface treatment exhibited a significant effect to the interfacial shear strength as well as the fiber/ matrix contact angle measurement.

L’adhérence et le frottement existent dans de nombreux systèmes techniques ainsi que dans les systèmes naturels. Ces deux phénomènes ont une influence importante sur la durabilité et l’efficacité des dispositifs techniques. Une approche reconnue pour ajuster précisément ces caractéristiques - outre le fait de modifier les propriétés physico-chimiques - est la texturation des surfaces en contact. Les surfaces de feuilles de plantes sont souvent décorées avec des morphologies de surface diverses, et présentent ainsi des fonctionnalités de surface remarquables. Cette thèse visait à réaliser une étude systématique de la mécanique de l’adhérence et du frottement sur des surfaces micro-structurées, répliquées à partir de surfaces de feuilles végétales, en contact avec une sonde qui s’inspire de l’organe adhérent d’un insecte. Les morphologies de surface de trois feuilles végétales différentes ont été directement transférées sur un polymère viscoélastique. Pour ce faire, trois approches différentes de reproduction ont fait l’objet d’une étude approfondie. La microscopie électronique à balayage et la microscopie confocale à balayage laser ont été utilisées pour l'évaluation qualitative et quantitative de la qualité de reproduction. Concernant l’étude de la mécanique du contact, un nano-indenteur a été modifié, permettant d’enregistrer les images in situ des contacts réels. Des tests de pull-off ont été menés afin d’évaluer quantitativement l’effet de la pré-charge sur la force d’adhésion et pour comprendre les modes distincts de collage/décollement. Des essais de frottement ont été effectués afin d’examiner l’effet de la charge normale et de la vitesse de glissement sur la force de frottement. Les résultats ont été discutés en fonction de la topographie de chaque surface
Adhesion and friction exist in many technical systems as well as in natural ones. Both phenomena have a profound influence on the durability and efficiency of technical systems. A well-recognised way to tune these characteristics - besides altering the physicochemical properties - is the texturing of the interacting surfaces. Inspiringly, plant leaf surfaces are often decorated with diverse surface morphologies, and so show remarkable functionalities. This thesis aimed to perform a systematic investigation of adhesion and friction mechanics on micro-structured surfaces replicated from plant leaves, in contact with a probe, which was inspired from an insect’s adhesive pad. Surface morphologies of three different plant leaves were directly transferred onto a viscoelastic polymer. For this, three different replication approaches were comprehensively explored. Scanning electron microscopy and confocal laser scanning microscopy were used for the qualitative and quantitative evaluation of replication ability. For the contact mechanics investigation, a high-resolution nanoindenter was modified, with incorporating a unique feature to record the in-situ real-contact images. Pull-off tests were carried out to quantitatively evaluate the effect of pre-load on adhesion force characteristics and to understand distinct attachment-detachment modes. Friction investigations were performed to examine the effect of normal load and sliding speed on the friction force. Results were discussed with regard to each surface’s topography

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

Capillarity is often exploited in self-cleaning, drag reducing and fluid absorption/storage (sanitary products) purposes just to name a few. Formulating the underlying physics of capillarity helps future design and development of optimized structures. This work reports on developing computational models to quantify the capillary pressure and capillary forces on the fibrous surfaces. To this end, the current study utilizes a novel mass-spring-damper approach to incorporate the mechanical properties of the fibers in generating virtual fibrous structures that can best represent fibrous membranes. Such virtual fibrous structures are then subjected to a pressure estimation model, developed for the first time in this work, to estimate the liquid entry pressure (LEP) for a hydrophobic fibrous membrane. As for accurate prediction (and not just estimation) of the capillary pressure, this work also presents an energy minimization method, implemented in the Surface Evolver code, for tracking the air–water interface intrusion in a hydrophobic fibrous membrane comprised of orthogonally oriented fibers. This novel interface tracking algorithm is used to investigate the effects of the membrane’s microstructure and wetting properties on its resistance to water intrusion (i.e., LEP). The simulation method developed in this work is computationally affordable and it is accurate in its predictions of the air–water interface shape and position inside the membrane as a function of pressure. Application of the simulation method in studying effects of fiber diameter or contact angle heterogeneity on water intrusion pressure is reported for demonstration purposes. Capillary forces between fibrous surfaces are also studied experimentally and numerically via the liquid bridge between two parallel plates coated with electrospun fibers. In the experiment, a droplet was placed on one of the polystyrene- or polyurethane-coated plates and then compressed, stretched, or sheared using the other plate and the force was measured using a sensitive scale. In the simulation, the liquid bridge was mathematically defined for the Surface Evolver finite element code to predict its 3-D shape and resistance to normal and shearing forces, respectively, in presence of the contact angle hysteresis effect. Despite the inherent non-uniformity of the fibrous surfaces used in the experiments and the simplifying assumptions considered for the simulations, reasonable agreement was observed between the experiments and simulations. Results reveal that both normal and shear force on the plates increase by increasing the liquid volume, or decreasing the spacing between the plates.

Recently, powders of the size d (0.1 μm < d < 10 μm) have been referred to ultrafine particles. The particle shape considered is assumed to be a sphere of the diameter d. The handling of powders is of great importance for processing of pharmaceuticals, cement, chemicals and other products. Most of these technological processes involve powder compaction, storage, transportation, mixing, etc, therefore, understanding of the fundamentals of particles interaction behaviour is very essential in the design of machines and equipment as well as in powder technology, cleaning of environment and other areas. The dynamic behaviour of particulate systems is very complicated due to the complex interactions between individual particles and their interaction with the surroundings. Understanding the underlying mechanisms can be effectively achieved via particle scale research. The problem of a normal contact may be resolved in a number of ways. In spite of huge progress in experimental techniques, direct lab tests with individual particles are still rather time-consuming and expensive. The interaction of particles as solid bodies is actually a classical problem of contact mechanics. In the case of ultrafine particles, the reduction of the particle size shifts the contact zones into the nanoscale or subnanoscale. Thus, steadily increasing contribution of adhesion has to be considered in the development of the physically correct constitutive models and numerical tools. Consequently, it may... [to full text]
Ultrasmulkios dalelės yra šiuolaikinės chemijos, farmacijos, maisto ir kitų pramonės šakų produktų sudėtinė dalis. Tiriant pramoninius technologinius procesus, neišvengiamai reikalingos teorinės žinios apie ultrasmulkių dalelių elgseną. Išsamus supratimas įmanomas tik atlikus įvairius tyrimus. Pastaruoju metu milteliai, klasifikuojami kaip ultrasmulkios (0,1 < d < 10 μm) dalelės, imti plačiai naudoti pramoniniuose procesuose, todėl suprasti ultrasmulkių dalelių elgsenos fundamentalumą miltelių technologijoje yra labai svarbu. Ultrasmulki dalelė yra itin maža, todėl su ja atlikti fizinį eksperimentą, kuris reikalauja specialios įrangos bei žinių, labai sunku. Tokiu atveju dažniausiai naudojamas skaitinis eksperimentas, kurį galima atlikti virtualiai. Skaitinio eksperimento metu yra tiriamos dinaminės ultrasmulkios dalelės savybės bei sprendžiamas dinaminis uždavinys. Taikant skaitinius modelius bei dalelės judėjimą aprašančias jėgų lygtis, naudojami sąveikos modeliai, apimantys adhezinę, klampią, tamprią bei tampriai plastinę sąveikas. Mikroskopinis adhezinės sąveikos modeliavimas – aktualus mechanikos mokslo uždavinys. Taikant sąveikos modelius, svarbu pritaikyti ir diskrečiųjų elementų metodą, kadangi, norint aprašyti dalelių elgseną, visų pirma reikia su-vokti ir aprašyti dalelės modelį. Dalelės elgsenos skaitiniam modeliavimui siūlomi teoriniai modeliai leidžia tirti dalelės sąveiką su dalele ar tampria puserdve bei sąveikos dinamiką. Šie modeliai galėtų būti pritaikyti... [toliau žr. visą tekstą]

L'aptitude à coller instantanément, dès leur mise en contact avec des substrats variés, est l'une des qualités premières que doivent présenter les adhésifs dans de nombreux domaines d'application (adhésifs thermofusibles ou sensibles à la pression par exemple). Cependant, cette qualité, communément appelée pégosité (tack en anglais) est difficile à définir d'un point de vue physico-chimique. En effet, elle dépend aussi bien des propriétés de volume que des propriétés de surface des matériaux considérés. La présente étude consiste en une analyse fondamentale des phénomènes influençant l'adhésion à différentes échelles de temps de contact, de la milliseconde à quelques dizaines de minutes. Ainsi, la pégosité d'élastomères modèles acrylonitrile-butadiène (NBR), de polarité différente (33 et 41 % d'acrylonitrile) et de masse moléculaire entre noeuds de réticulation variant de 14000 à 45000 g/mol, a été étudiée. Le test de mesure utilisé repose sur le principe du test à la sonde, améliorée par la mesure d'aire de contact. La géométrie du test est celle du test de Johnson, Kendall et Roberts (J. K. R. ), à savoir contact entre un hémisphère d'élastomère et une plaque de verre. Une décroissance linéaire de l'énergie d'adhésion avec la vitesse de retrait, suivant une échelle logarithmique, pour un temps de contact et des conditions expérimentales donnés, a été observée. Ce comportement, contraire à celui prédit par l'approche de Gent et Schultz a été attribué à la différence de géométrie et à une forte variation de volume de la zone déformée à faibles vitesses de séparation. Le résultat majeur est l'augmentation de l'énergie d'adhésion G avec le temps de contact, suivant une fonction puissance. Cet accroissement a été attribué à l'évolution de l'énergie de surface au cours du temps. De même, l'interdiffusion de courts segments de chaînes, aux temps très courts (quelques millisecondes) ainsi que pour des réseaux réticulés a été mise en évidence. En résumé, cette étude a permis d'identifier les différentes contributions à l'énergie d'adhésion, suivant la plage de temps considérée et a clairement mis en évidence le rôle joué par les phénomènes de réorganisation moléculaire aux interfaces.

Adhesion between solid bodies plays a prominent role in a wide variety of situations ranging from tribological applications to dust coagulation initiating the formation of planets. It can be due to various reasons like capillary, electrostatic, van der Waals, and hydrophobic forces. Among these, adhesion due to van der Waals force| which has its origin in permanent or instantaneous electric dipoles present in all atoms and molecules|is of special significance as it is present in all cases. Computational studies on adhesion due to van der Waals force commonly assume it as a surface force due to its short effective range, which is about a few tens of nanometers, in comparison to the length-scales commonly encountered. However, such restrictions are often violated in various important problems. For example, the characteristic dimensions of asperities| which are the smallest roughness elements interacting to cause friction and wear| are usually of nanometer length-scale. In addition, the assumptions inherent in development of surface force model are exact only when the deformations are small. In all such situations, the van der Waals force must be assumed as distributed over the volume. In this work, a computational model is developed by incorporating van der Waals force and short-range repulsion (steric repulsion or Pauli repulsion) as body forces distributed over the volume in a large deformation, static/transient, finite element framework. First the development of the general formulation is discussed, and then it is specialized for various considerations like handling symmetry and interaction between an elastic body and a rigid half-space, which offer significant computational advantages over the general formulation. The applicability of the model is illustrated by using a number of benchmark and practical problems. The comparison of the analysis results and well-established analytical models are provided, which validates our method. As a specific example, the smooth change of interaction force from a thin-rod model to a at-plate model on increasing the cross-sectional areas of two interacting elastic rods is demonstrated. The impact of elastic bodies in presence adhesion, and the associated energy loss is an important concern in studies regarding the origin of friction. Therefore, adhesive impact of elastic rods and spheres is studied using our formulation. Emphasis of the study is on finding the apparent energy loss during impact, which represents the part of energy lost to elastic stress waves remaining in the body after the impact, and hence not available for rebound motion. In case of impact of elastic rods on a rigid half-space, it is shown that the apparent energy loss is a unique function of the tensile strain energy developed in the rod due to van der Waals attraction. A one-dimensional model is developed for this case to determine the energy loss based on the specified problem parameters, which can be used to predict practically relevant phenomena like capture. In case of impact of elastic spheres, which is often correlated with asperity interactions, the energy loss is found to be significant only if adhesion-induced instabilities occur. The behavior shown by rods and spheres are probably at the two extremes with regards to energy loss during impact of elastic bodies in presence of adhesion. Practical use of the formulation is demonstrated by applying it to the study of amplitude variation and phase shifts in tapping-mode atomic force microscopy. Specifically, the advantage of operating the AFM cantilever just below its natural frequency as compared to operating it just above the natural frequency is demonstrated. Bistable behavior, which is the coexistence of two stable vibration modes under exactly same operating conditions, is shown to be severe when the driving frequency is higher than the natural frequency of AFM cantilever even in the absence of adhesion, which can result in spurious contrast-reversal artifacts during imaging. The hysteresis loop associated with the bistable behavior may lead to erroneous conclusions regarding presence of adhesion. Since this model overcomes the limitations of lumped parameter models and the computational models based on surface force approximation, the results can be used for much more realistic interpretation of experimental data. Computational framework developed in this study achieves the capability for analysis of adhesive contact problems directly from van der Waals interaction and steric repulsion. Such a model can be used for revisiting the fundamental problems in contact mechanics, as well as for providing better insights into experimental observations.

Adhesion between solid bodies plays a prominent role in a wide variety of situations ranging from tribological applications to dust coagulation initiating the formation of planets. It can be due to various reasons like capillary, electrostatic, van der Waals, and hydrophobic forces. Among these, adhesion due to van der Waals force| which has its origin in permanent or instantaneous electric dipoles present in all atoms and molecules|is of special significance as it is present in all cases. Computational studies on adhesion due to van der Waals force commonly assume it as a surface force due to its short effective range, which is about a few tens of nanometers, in comparison to the length-scales commonly encountered. However, such restrictions are often violated in various important problems. For example, the characteristic dimensions of asperities| which are the smallest roughness elements interacting to cause friction and wear| are usually of nanometer length-scale. In addition, the assumptions inherent in development of surface force model are exact only when the deformations are small. In all such situations, the van der Waals force must be assumed as distributed over the volume. In this work, a computational model is developed by incorporating van der Waals force and short-range repulsion (steric repulsion or Pauli repulsion) as body forces distributed over the volume in a large deformation, static/transient, finite element framework. First the development of the general formulation is discussed, and then it is specialized for various considerations like handling symmetry and interaction between an elastic body and a rigid half-space, which offer significant computational advantages over the general formulation. The applicability of the model is illustrated by using a number of benchmark and practical problems. The comparison of the analysis results and well-established analytical models are provided, which validates our method. As a specific example, the smooth change of interaction force from a thin-rod model to a at-plate model on increasing the cross-sectional areas of two interacting elastic rods is demonstrated. The impact of elastic bodies in presence adhesion, and the associated energy loss is an important concern in studies regarding the origin of friction. Therefore, adhesive impact of elastic rods and spheres is studied using our formulation. Emphasis of the study is on finding the apparent energy loss during impact, which represents the part of energy lost to elastic stress waves remaining in the body after the impact, and hence not available for rebound motion. In case of impact of elastic rods on a rigid half-space, it is shown that the apparent energy loss is a unique function of the tensile strain energy developed in the rod due to van der Waals attraction. A one-dimensional model is developed for this case to determine the energy loss based on the specified problem parameters, which can be used to predict practically relevant phenomena like capture. In case of impact of elastic spheres, which is often correlated with asperity interactions, the energy loss is found to be significant only if adhesion-induced instabilities occur. The behavior shown by rods and spheres are probably at the two extremes with regards to energy loss during impact of elastic bodies in presence of adhesion. Practical use of the formulation is demonstrated by applying it to the study of amplitude variation and phase shifts in tapping-mode atomic force microscopy. Specifically, the advantage of operating the AFM cantilever just below its natural frequency as compared to operating it just above the natural frequency is demonstrated. Bistable behavior, which is the coexistence of two stable vibration modes under exactly same operating conditions, is shown to be severe when the driving frequency is higher than the natural frequency of AFM cantilever even in the absence of adhesion, which can result in spurious contrast-reversal artifacts during imaging. The hysteresis loop associated with the bistable behavior may lead to erroneous conclusions regarding presence of adhesion. Since this model overcomes the limitations of lumped parameter models and the computational models based on surface force approximation, the results can be used for much more realistic interpretation of experimental data. Computational framework developed in this study achieves the capability for analysis of adhesive contact problems directly from van der Waals interaction and steric repulsion. Such a model can be used for revisiting the fundamental problems in contact mechanics, as well as for providing better insights into experimental observations.

博士
國立中正大學
機械工程學系暨研究所
102
Adhesive mechanism usually offers the ability for species to survival in nature, e.g., hunting, locomotion and escape. The bio-tissue structure and surface energy are the dominant factors for dry adhesion. For example, the geckos’ amazing adhesive ability attracts a lot of attention in our quest to unlock the secret of nature. The adhesion ability depends on the conditions of contact surface. The motivation of our study is to explore the adhesive behavior under different conditions of those species using the contact mechanics and to develop the biomimic adhesive material. In order to understand the characteristic of bio-tissue adhesion such as the gecko, an adhesive contact mechanics model considering the surface energy was established to analyze the dry adhesive property of bio-tissue. The relationship between the bio-tissue adhesion and contact substrate, i.e., smooth, with tiny particle and different material properties, were investigated. In view of the excellent adhesion of multi-walled carbon nanotubes (MWCNTs) arrays, the present research utilizes this kind of material to develop a biomimic array of adhesive material. Various interfacial adhesion characteristics of the MWCNTs arrays under different roughness, wettability, tube size and applied load were characterized using depth sensing technique. The results show that the gecko’s adhesion under the substrate with tiny particle depends strongly on the ratio of particle and spatula size. The generated adhesion is considerably higher when the spatula is in contact with the soft substrate rather than the hard substrate. The adhesive measurement for MWCNTs array exhibits that the adhesion increases with the surface roughness. The adhesion significantly reduces with the increasing cap size due to a reduction in the number of contact sites between the MWCNTs array and the indenter. Finally, the results also show that the contact angle of a water droplet on the MWCNTs surface reduces with the surface roughness. It is inferred that the enhanced adhesion of the MWCNTs arrays with a greater surface roughness is partially due to an increased humidity and wettability of the MWCNTs surface. This study reveals insights of the adhesive properties for bio-tissues through the developed adhesive contact mechanics model considering the elastoplastic effect. This developed model provides a powerful tool for the investigation of adhesive properties when elastoplastic deformation and irregular interface surface are involved. The performance of developed biomimic adhesive array materials is in the order of the adhesive ability of the gecko. Moreover, it demonstrates the ability of depth sensing technique to characterize the pull-off force of an array material.

In the framework of this thesis, a study was conducted dealing with a strategy to change the mechanical properties of microgel particles using inwards-interweaving self-assembly post-synthesis. This technique was invented by my cooperation partners of the Trau group, and they prepared all particles studied. Exemplarily, agarose microparticles were used to interweave a defined shell of complexed poly(allylamine) (PA) and poly(styrenesulfonic acid) (PSS) into such particles. Thereby, the shell thickness can be well-defined. Adjusting the concentration of PA and the incubation time, the filling of the particles can be readily controlled up to complete filling. By adding excessive PSS, the diffusion-controlled shell formation stops by complexation. The shell thickness of individual particles was determined through fluorescence-labeled PA and confocal laser scanning microscopy. The mechanical properties of single particles were inferred by AFM with an attached colloidal probe (CP). Here, a non-linear increase of the elastic modulus (E-modulus) from 10 to 190 kPa was determined while the shell thickness increased from 10 to 24 µm. After adding a second shell, a further gain to 520 kPa on average can be realized. Furthermore, a new concept was developed by Mr. Fery and me to change the surface mechanical properties of mircogel particles by applying a thermal trigger. Meanwhile, a particular focus was laid to maintain constant adhesive properties at every temperature. First, crosslinked poly(N-isopropyl acrylamide) (PNIPAM) particles are prepared by droplet microfluidics. These gelled particles are injected into a second microfluidic device and surrounded by an aqueous solution of uncrosslinked poly(acrylamide) (PAAM). At a second junction, droplets are formed via the cut-off effect of the continuous organic solvent. The droplets now contain the crosslinked PNIPAM core and a thin uncrosslinked PAAM liquid shell. After a short diffusion of PAAM polymers into the core they are crosslinked by UV-light. These experiments were performed by our cooperation partners of the Seiffert group. I applied temperature-controlled CP-AFM to obtain the resulting adhesive and mechanical properties of individual particles. Here, the core-shell particles behaved similarly to plain PNIPAM particles displaying the typical increase in E-modulus at temperatures above 34°C, however lower in magnitude. Further, no temperature effect on the interfacial interaction for these core-shell particles was detected. While one focus of the study above was on constant adhesion, in the following section, a new synthetic approach for mussel inspired underwater adhesives, and their characterization is presented. Based on a peptide sequence of ten amino acids, which is found frequently in natural mussel foot proteins, a new polymerization route was developed by my cooperation partners of the Börner group. Possible reaction pathways were investigated with specifically designed model reaction and analyzed using mass spectroscopy and gel permeation chromatography. The resulting polymers were further characterized using high-performance liquid chromatography and SDS-page. Nanometer thick coatings of these synthetic polymers revealed an excellent persistence even against highly concentrated salt solutions measured by quartz crystal microbalance with dissipation experiments. My contribution was the investigation of the work of adhesion necessary to detach a microparticle from such a coating by CP-AFM in an aqueous environment. Here, the newly developed synthetic polymer provided higher adhesive strength, up to 10.9 mJ m- 2, compared to comparable natural mussel foot proteins.
Im Rahmen dieser Arbeit, wurde eine Studie durchgeführt, welche die Möglichkei¬ten der nachträglichen Elastizitätsveränderung von Mikrogelpartikeln mittels „nach Innen gerichteter, verwebender Selbstassemblierung“ (engl. inwards-interweaving self-as¬sembly) beleuchtet. Diese Technik wurde von meinen Kooperationspartnern aus der Gruppe von Herrn Trau entwickelt. Am Bespiel von Agarose Mikropartikeln, kann mittels dieser Technik eine definierte Schale aus Polyallylamin (PA) und Polystyrolsulfonsäure (PSS) in das Partikel verwoben werden. Die Schalendicke kann dabei kontrolliert variiert werden, bis hin zur vollständigen Ausfüllung des Partikels, in dem die Konzentration von PA und die Inkubationszeit angepasst werden. Durch Zugabe eines Überschusses an PSS wird der diffusionsgesteuerte Schalenaufbau durch Komplexierung beendet. Die Schalendicke der individuellen Partikel wurde mittels Fluoreszenzmarkierung und konfokaler Laser Raster¬mikroskopie (engl. confocal laser scanning microscopy) ermittelt. Die mechanische Cha¬rakterisierung einzelner Partikel durch AFM und kolloidaler Sonde (engl. colloidal probe, CP) ergab eine nicht lineare Erhöhung des Elastizitätsmoduls (E-Modul) von 10 auf 190 kPa bei einem Schalendicken Zuwachs von 10 auf 24 µm. Durch eine zweite Schale, in der Ersten, konnte der E-Modul auf im Mittel 520 kPa gesteigert werden. Weiterführend, wurde von mir und Herrn Fery ein neues Konzept entwickelt, um eine mechanische, oberflächliche Verhärtung von Mikrogelpartikel durch Temperaturver¬änderung zu induzieren mit einem Augenmerk, dass sich die Adhäsionseigenschaften nicht verändern. Zunächst wurden von meinen Kooperationspartnern aus der Gruppe von Herrn Seiffert vernetzte Poly-N-isopropylacrylamid (PNIPAM) Partikel mittels Tropfenmikroflu¬idik hergestellt. In einem zweiten Mikrofluidik Experiment wurde diese Partikel mit einer wässrigen Lösung von Polyacrylamid (PAAM, unvernetzt) umgeben bevor es zu einer Tropfenbildung in der organischen Phase kommt. Nach kurzer Diffusionszeit der PAAM Polymerketten in die Kernpartikel, wurde die PAAM Schale mittels UV-Licht querver-netzt. In temperaturkontrollierten CP-AFM Untersuchungen habe ich die resultierenden Adhäsions- und mechanischen Eigenschaften auf der Einzelpartikel Ebene bestimmt. Hier¬bei konnte bei den Kern-Schale Partikeln der für bloße PNIPAM Partikel typische E-Modul anstieg oberhalb von 34°C nachgewiesen werden, jedoch mit verminderten Absolutwerten. Eine begleitende Veränderung der adhäsiven Eigenschaften der Kern-Schale Partikel konnte dabei nicht beobachtet werden. Lag ein Fokus der vorherigen Arbeit auf konstanten Wechselwirkungen, behandelt der dritte Teil der Ergebnisse, einen neuen Synthese Ansatz zur Herstellung Muschel in¬spirierter Unterwasser Adhäsiva und deren Charakterisierung. Basierend auf einer natürli¬chen Peptidsequenz, wurde eine enzymatische Polymerisationsroute von meinen Koopera¬tionspartner aus der Börner Gruppe entwickelt. Der Reaktionsverlauf wurde durch neu de¬signte Modelreaktion untersucht und mittels Massenspektroskopie und GPC analysiert, das resultierende Polymer zusätzlich mit HPLC und SDS-page. Nanometer dicke Beschichtun¬gen dieser Muschel inspirierten Polymer wiesen eine sehr gute Beständigkeit gegen hoch konzentrierten Salzlösung in QCM-D Experimenten auf. Die Adhäsionsarbeit, welche nö¬tig ist um eine Mikropartikel von diesen in wässeriger Lösung zu entfernen, wurde von mir mittels CP-AFM bestimmt. Nach meiner Erweiterung einer bekannten Adhäsionstheorie, konnten für das synthetische Polymer höhere Werte als für vergleichbare Natürliche von bis zu 10.9 mJ m-2 bestimmt werden.

碩士
國立臺灣海洋大學
機械與機電工程學系
96
This paper focuses on the mechanical interlocking effects on the bump/pad interfacial contact surface of a flip chip on flex with non-conductive adhesive under thermal cycling loads after the bonding/curing process of packaging. The aim of improvement for the bump/pad interfacial contact characteristics was to enhance the reliability of chip set package through a well maintained contact surface. Comparisons of the normal stress on the interfacial contact surface with varied roughness for the true-shaped bump geometric parameters, revealed the mechanical interlocking effects, and induced the increase of equivalent contact stresses. Finite Element Code ANSYS was used to analyze the package, and a 2D model was adopted to proceed with a parametric sensitivity analysis for the material properties of non-conductive adhesive and bump geometric dimensions varied. The results of aging by using thermal cycling showed that the bonding force between bump/pad will be improved by moderate increase of Young’s modulus and thermal expansion coefficient of non-conductive adhesive. A so-called “saw-tooth” fractal surface will raise the normal compressive force, and surpassed the flat surface by the effect of mechanical interlocking. A case-by-case interfacial stress analysis was suggested for the investigation of any realistic microelectronic package due to the complexity among all the interrelated parameters.

An ideal braking system not only ensures safety and ride comfort but also attracts significant cost-benefits through optimum on-time operation and reduction in wheel-rail damage processes. For such a braking system, the adhesion condition information between the wheel and rail at contact interfaces during rail vehicle operation is essential. The adhesion estimation for the braking system is a complex task because it is influenced by operational factors such as non-linear wheel and rail geometry, rail vehicle speed, track irregularities, axle load distributions, etc., plus environmental factors such as third body layer characteristics, presence of friction modifiers, and rail-wheel temperatures. The research objective of this thesis is to estimate the adhesion condition between wheel and rail and implement the obtained adhesion condition information for the improvement of braking performance of a heavy haul wagon. The research methodologies used for the realisation of the defined objective are based on specially designed algorithm development, numerical modelling, and scaled laboratory experiment techniques. A novel real-time multipoint wheel-rail contact model was constructed using a modular approach and validated with the contact point function of Gensys multibody software. Then, a simplified fast-processing adhesion estimation algorithm was developed including the contact model that provides anticipated results in both rolling and braking conditions. The simplified algorithm was further improved by designing a novel observer which analyses the adhesion in real-time. Then a real-time scaled bogie test rig model was developed for the validation of the developed adhesion estimation algorithm with the purpose of its integration in an advanced braking control system. For the transition from the algorithm development and numerical scaled bogie test rig modelling to the physical implementation, the numerical findings need to be compared with a set of experimental results. Therefore, an experimental scaled bogie test rig was re-designed based on an existing heavy haul wagon design and equipped with a newly designed braking system for the conceptual validation of the system. For robust and reliable estimation of the adhesion condition, the acoustic signal emanating from wheel-rail interaction was considered in this project as an additional input parameter. Finally, the noise analysis method and a wheel-rail adhesion observer were integrated into a novel mechatronic brake control system for accurate and reliable estimation of adhesion conditions. The performance analysis of the mechatronic brake system shows that the proposed system can achieve a shorter stopping distance in comparison with both a conventional controller and with no controller. It also indicates that the developed mechatronic brake system can maintain the operational condition even under a degraded adhesion condition. This research work contributes knowledge to the process of adhesion estimation between wheel and rail contact in a real-time mode that is currently unavailable and has therefore not been implemented in existing rollingstock brake control system designs. This study confirms the capability for enhancement of the productivity, efficiencies, and safety of trains that will ultimately be a contribution toward sustainable transportation.