MSc Physiotherapy, Faculty of Health Sciences, University of the Witwatersrand, 2009. / INTRODUCTION Rugby is a high impact sport with many injuries reported in the literature. A high rate of ankle injury is reported with resultant recurrence of these injuries. There is however only scarce epidemiological data with minimal detail to highlight clinical findings and prevalence of ankle injuries especially in the club rugby fraternity. AIMS This study investigated the prevalence of clinical signs of ankle injuries in rugby players at club rugby level in the South Gauteng region. The data collected was used to identify the clinical signs related to ankle instability for perceived, mechanical and functional parameters and was applied to determine the difference between players with and those without previous injury. METHODOLOGY The researcher obtained ethical clearance to do the study from the Human Research Ethics Committee of the University of the Witwatersrand. Permission was obtained from the Golden Lions Gauteng Rugby Union to use players in the South Gauteng region. One hundred and eighty players from nine clubs in the region participated in the study. Informed consent was obtained from all parties concerned and players were asked to complete a battery of tests. To determine the prevalence of clinical signs of perceived instability each player was asked to complete a data questionnaire and the Olerud and Molander questionnaire. The data questionnaire also included questions pertaining to the exclusion criteria. iii Objective testing was done to determine the clinical signs of mechanical instability of both ankles of each player through mechanical tests; the talar tilt and anterior drawer tests. Balance and proprioception were assessed through the Star Excursion Balance Test (SEBT) and Balance Error Scoring System (BESS) which is used to indicate clinical signs of functional instability and these tests were used to determine the prevalence of clinical signs of functional instability and to relate the clinical signs of functional instability to the other clinical findings. RESULTS The prevalence of ankle injuries at club rugby level is discussed for the different parameters of instability. The prevalence of clinical signs of perceived instability based on the Olerud and Molander questionnaire is 47%, as reported by the player and is further described in a sub-analysis of perceived problems. The prevalence of clinical signs of mechanical ankle instability, when laterality is ignored is 38.7%. The prevalence of clinical signs of functional ankle instability depends on the surface and the visual input and is greater as the challenge or protuberance increases in difficulty. The clinical signs of perceived, mechanical and functional ankle instability are further described and related to other clinical findings for two groups, namely those with and those without previous injury to the ankle and as expected clinically significant differences were noted with the players with previous injury recording a higher prevalence for perceived and mechanical parameters. The odds ratios for the presence of certain clinical signs revealed significant p-values for the presence of pain, stiffness and swelling and the need for supports e.g. bracing or taping and the affect on activities of daily living. DISCUSSION In this study there is a high prevalence of clinical signs of ankle instability in club rugby players for perceived, mechanical and functional parameters, compared to the prevalence reported in the literature. From the study the clinical findings associated with the presentation of ankle injuries in club rugby players have been established and related to the perceived, mechanical and functional signs of instability. Differentiation between players with reported ankle injury and those without were also done and significant differences were noted between the two groups for perceived and mechanical parameters but where the functional assessment was done it supported the fact that balance and proprioception tests included the whole kinetic chain and does not view the ankle in isolation. It was evident that previously injured players were more likely to sustain future injury to the ankle and odds-ratios to support this showed an increased risk of the presence of swelling, stiffness and pain for players with previous injury and the greater need for the use of supports and influence on activities of daily life. The information gathered can be used in the future to set up a management plan for pre-season screening, assessing and addressing individual predisposing biomechanical factors, managing acute injuries successfully and rehabilitation in the post-season phase.

Subject to a sufficiently large compression, materials may undergo mechanical instabilities of various types. When the material is homogeneous, creases set in. When the material is a bilayer consisting of a stiff thin film on a thick compliant substrate, wrinkles set in. Creases are localized self-contact regions with large strain deviating from the smooth state, while wrinkles are undulations finite in space with infinitesimal strain deviating from the smooth state. After the formation of wrinkles, if the compression further increases, wrinkles double their period and form localized folds. If the substrate is subject to a sufficiently large pre-tension, wrinkles transit to ridges. This thesis explores different types of mechanical instabilities: creases, wrinkles, folds, and ridges. We start with studying creases in different materials. Soft tissues growing under constraint often form creases. We adopt the model of growth that factors the deformation gradient into a growth tensor and an elastic deformation tensor, and show that the critical conditions for the onset of creases take a remarkably simple form. We then perform simulations to explore creases in strain-stiffening materials. For a solid that stiffens steeply at large strains, as the compression increases, the surface is initially smooth, then forms creases, and finally becomes smooth again. For a solid that stiffens steeply at small strains, creases never form for all levels of compression. In order to better control the formation and disappearance of creases, we design a soft elastic bilayer with same moduli of the film and substrate but the substrate pre-compressed, and show that the bilayer can snap between the flat and creased states reproducibly with tunable hysteresis in a large strain range. We also show that an interface between two soft materials can form creases under compression. We then investigate the critical conditions for the onset of wrinkles and creases in bilayers with arbitrary thicknesses and moduli of the two layers, and show several new types of bifurcation behavior when the film and substrate have comparable moduli and thicknesses. We study the effect of substrate pre-stretch on post-wrinkling bifurcations, and show that pre-tension stabilizes wrinkles while pre-compression destabilizes wrinkles. When the pre-compression is sufficiently large, `chaotic' morphologies emerge. When the pre-tension is sufficiently large, we realize ridge localizations and networks under an equi-biaxial compression, and study the mechanics of ridge formation and propagation. / Engineering and Applied Sciences

Lateral ankle sprains are the most common musculoskeletal injury among the general population and U.S. military personnel. Despite the common perception of being a minor injury, at least 1 out of 3 individuals with a previous ankle sprain will develop chronic ankle instability (CAI). This clinical phenomenon creates a significant barrier for patients to return to their prior level of physical function. Specifically, CAI is associated with reductions in physical activity level, leading to decreases in lower health-related quality of life and increase risk of developing of post-traumatic ankle osteoarthritis. Current evidence has largely focused on characterizing the mechanical and sensorimotor insufficiencies associated with CAI in adolescent and young-adult populations, with little attention on middle- and older-aged adults. This restricts our understanding of how these insufficiencies associated with CAI that develop in early adulthood progress over time and contribute to other chronic diseases such as post-traumatic osteoarthritis. Therefore, the overall objective of this study was to compare self-reported and physical function between three age groups: 1) young, 2) middle-aged, and 3) older-aged adults with and without CAI. We hypothesized participants with CAI would have age-related changes in self-reported and physical function compared to non-injured individuals across the lifespan. The objective of this dissertation was to compare regional and global health- related quality of life (HRQoL), static and dynamic balance, spinal reflex excitability of the soleus muscle, open- and closed-kinetic chain dorsiflexion range of motion and spatiotemporal gait parameters between those with and without CAI across the lifespan. Her callIt was hypothesized that all self-reported and physical characteristics would be decrease with age, but significantly more in those with CAI compare to non-injured individuals. Results from the first study demonstrated participants with CAI had worse regional HRQoL compared to healthy-controls as evidenced by the lower Foot and Ankle Disability Index scores. Likewise, participants with CAI reported having worse overall physical function and pain interference during activity compared to healthy-controls. There was no significant interaction for Injury (CAI and healthy-control) and Age group (young, middle, and old) for any dependent variable. In the second, it was determined that static and dynamic balance, spinal reflex excitability, ankle (dorsiflexion and plantarflexion) and hip extension torque were all lower in the older-aged participants compared to the younger-aged adults. In addition, it was determined that participants with CAI had decreased dorsiflexion range of motion, ankle (dorsiflexion and plantar flexion) and hip extension peak isometric torque compared to the healthy-control group. However, no significant interaction was found for Injury (CAI & healthy-control) and Age (young, middle, old) for any dependent variable. In the third study, there were no differences in spatiotemporal gait parameters between groups (CAI vs. healthy-controls) or age categories. It can be concluded from this dissertation that regardless of the age, individuals with CAI have worse region-specific HRQoL, lower overall physical function, greater pain interference, limited dorsiflexion range of motion, and decreased ankle and hip peak isometric torque compared to healthy-controls. Several age-related observations were found including decreased static and dynamic balance, ankle and hip strength, and spinal reflex excitability. Though no relationship was found between CAI and age, several interactions were found to be trending towards significance. Therefore, future work is needed to better understand the consequences of CAI on middle- and older-aged adults.

Monolayer molybdenum disulfide (MoS2) is a three-atom-thick direct band gap semiconductor, which has received considerable attention for use as a channel material in atomically thin transistors, photodetectors, excitonic LED’s, and many other potential applications. It is also a mechanically exceptional material with a large stiffness and flexibility, and can withstand very large strains (11%) before rupture. In this dissertation we investigated the mechanics of the stiffness and adhesion forces in atomically thin MoS2 membranes, and how biaxial strains can be used to induce large modulations in the band structure of the material. First, we used chemical vapor deposition (CVD) to grow MoS2 crystals that are highly impermeable to gas, and used a pressure difference across suspended membranes to induce large biaxial strains. We demonstrated the continuous and reversible tuning of the optical band gap of suspended monolayer membranes by as much as 500 meV, and induced strains of as much as 5.6% before rupture. We observed the effect of strain on the energy and intensity of the peaks in the photoluminescence (PL) and Raman spectra and found their linear strain tuning rates, then report evidence for the strain tuning of higher level optical transitions. Second, we determined the Young’s modulus and works of separation and adhesion of MoS2 membranes, and found that adhesion hysteresis is an important effect in determining the behavior of our systems. Finally, we investigated the use of atomically thin materials as nanofiltration membranes, by perforating the material with nanopores which selectively permit the transport of smaller molecules while rejecting larger ones. We studied ion transport through nanopores in graphene membranes and demonstrate that in-situ atomic force microscope measurements in liquid are a powerful way to reveal occlusions and contaminants around the pores - work which will aid future researchers in further unveiling the properties of these fascinating systems.

Mechanical instabilities such as wrinkling and buckling-driven delamination in thin film-substrate systems have historically been considered as one of structural failure mechanisms, which should be avoided. The past decade has witnesssed rapid growth in harnessing such surface instabilities for a wide range of tunable surface related properties and functionalities, especially in soft materials on small scales. Compared to extensively studied wrinkling on soft substrates and localized buckling driven delamination on stiff substrates, the fundamental mechanics underpinning ordered buckle-delamination on soft substrate over large area and its guidance for potential implications in engineering innovation remain largely to be explored. This thesis aims to partially bridging such a knowledge gap. In this thesis, I exploit how to generate the controllable and globally periodic delaminated buckling patterns in thin films on highly prestrained elastomeric substrates, and then explore the fundamental mechanics of this spontaneous extreme buckling driven periodic delamination, as well as its implications in design of extremely stretchable electronics and interfacial mechanical properties measurement. Compared to wrinkling, one of the benefits of extremely buckling driven delamination is the extraordinarily high aspect ratio of buckles. The large surface roughness and high local curvature could potentially enable extreme surface topographies related properties, such as adhesion, wetting, friction, and optics, as well as augment the extreme stretchability in stretchable optical and electronic devices. In the aim of harnessing this extreme buckling driven delamination, I first explore the formation and evolution of extraordinarily high-aspect-ratio delaminated buckles of thin films on 400% pre-strained elastomers, as well as uncovered the underlying deformation mechanism through combining quantitative theoretical analysis and experimental and numerical approaches. A theoretical framework is developed to describe the formation and evolution process of periodic delaminated buckles, which includes three deformation stages, i.e. onset of localized blisters (Stage I), growth and propagation of delamination (Stage II), and post-buckling after delamination arrest (Stage III). I show that under extreme large compressive strain, the profile of periodic blisters changes from sinusoidal shape to jig-saw-like shape with relative high aspect ratio, which have potential applications for design of extremely stretchable electronics. Equipped with the fundamental mechanics of buckle-delamination in thin films, I then exploit harnessing the spontaneous buckling driven periodic delamination to achieve high stretchability in both metal and silicon films. Experimentally I observe periodic buckle-delaminated patterns over large area, accompanied by highly ordered transversely cracking patterns, which can be theoretically predicted by simple crack fragments model. I hypothesize that when the width of ribbons is set to be equal or smaller than the theoretically predicted crack fragment width, there would be no cracking fragmentation. This criteria for designing crack-free micro-ribbons is further validated by related experiments. Guided by the validated criteria, I successfully design crack-free and spontaneous delaminated ribbons on highly prestrained elastomer substrates, which provides a high stretchability of about 120% and 400% in Si and Au ribbons, respectively. I further extend the buckling instability-based metrology to systematically measure the mechanical properties of 2D organic conjugated polymer nano-films, which have tremendous promising applications in organic integrated circuits, solar cells, and stretchable devices. I develop a new fabrication strategy to generate buckle-delaminated free-standing organic conjugated polymeric (P3BT/C60) nanosheets. Through both experiments and theoretical analysis, I show that the free-standing buckle-delaminated organic P3BT/C60 nanosheets have significant advantages over the traditional spin-coated wrinkled nanosheets, including the enhanced mechanical properties, a higher level of stretchability with lower electrical resistance, and a wider range of controllable wettability modulation. / Mechanical Engineering

La plupart des digues sont constituées de matériaux granulaires compactés. Elles sont ainsi perméables et constamment soumises à des écoulements d’eau dans leur volume. Dans certaines conditions, ces écoulements peuvent altérer leur microstructure par érosion interne et générer des instabilités mécaniques responsables de ruptures inopinées lors de crues. Cette thèse s’intéresse à l’analyse multi-échelle des instabilités mécaniques dans les matériaux granulaires soumis à l'érosion interne. Dans ce travail, le comportement mécanique de ces matériaux est simulé en 3D à l’échelle de volumes élémentaires représentatifs, et ce, pour différents états de contraintes et gradients hydrauliques. Grâce à l’utilisation du critère du travail du second ordre et d’outils micromécaniques, leur stabilité est analysée avant et après l’application d’un écoulement interne. Il est établi que l’origine micro-inertielle des instabilités observées provient du déconfinement et de la flexion des chaînes de force ainsi que des déformations plastiques importantes résultant de leur effondrement. Par leur capacité à enrayer rapidement le développement de telles déformations plastiques, il est montré que les particules libres contribuent à assurer la stabilité mécanique des matériaux granulaires. Ce résultat est fondamental pour analyser les conséquences de l’érosion interne en termes de stabilité mécanique car les particules libres sont facilement transportables sous l’action d’un écoulement interne. Selon si elles sont colmatées ou érodées, un écoulement interne aura un effet stabilisateur ou déstabilisateur vis-à-vis du comportement mécanique des matériaux granulaires soumis à l’érosion interne / Dikes are most of the time built of compacted granular materials that are permeable and continuously subjected to internal fluid flows. In some cases, microstructure modifications resulting from internal erosion generate mechanical instability that will lead to unexpected failures in case of serious flooding. This thesis focuses on multi-scale analysis of mechanical instability in granular materials subjected to internal erosion. In this work, the mechanical behavior of such materials is simulated in three dimensions at the scale of representative elementary volumes subjected to different stress states and hydraulic gradients. Thanks to the use of the second order work criterion and micromechanical tools, the mechanical stability of these materials is tested before and after internal erosion. It is established that the micro-inertial origin of the observed instabilities is linked to force chain deconfinement and bending as well as to the development of large plastic strains resulting from force chain collapse. By preventing the development of such plastic strains, it is shown that rattlers contribute to ensure the mechanical stability of granular materials. This key finding is of a particular significance in relation with internal erosion as rattlers can be easily transported under the action of an internal fluid flow. Depending on whether they get clogged or eroded, an internal fluid flow has thus either a stabilizing or a destabilizing effect on the mechanical behavior of granular materials subjected to internal erosion