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Virtually, every investigation and application of ionic liquids (ILs) involves gas-liquid, liquid-liquid and liquid-solid interactions. Therefore, understanding the behavior of ILs at those interfaces is critical. In this work, such things are investigated in both macroscopic and microscopic aspects by looking into the impact of chemical structures of ILs on their interfacial properties and nanostructures. On a macroscopic level, we studied the interfacial properties of protic and aprotic ILs with N-alkylimidazolium and 1-alkyl-3-methylimidazolium as cations and bis(trifluoromethylsulfonyl)imide, methanesulfonate, and trifluoromethanesulfonate as anions. The surface tension of these ILs is measured. The contact angle measurements are performed at 293.15 K on three solid substrates. Dispersive and non-dispersive components of the IL surface energy are determined from the experimental data using Fowkes theory. The most interesting result is that the protic ILs have lower surface tension and smaller contact angles than the equivalent aprotic ILs, despite the presence of high charge density on the proton associated with one of the nitrogens of the cation. On the other hand, from a microscopic view, we report the layered structures of four ILs on both unbiased and biased highly ordered pyrolytic graphite (HOPG) and Pt(111) surfaces, as determined by atomic force microscopy (AFM). The ILs investigated are 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf₂N]), 1-ethyl-3-methylimidazolium perfluorobutylsulfonate ([emim][C₄F₉SO₃]), 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene bis-(trifluoromethylsulfonyl)imide ([MTBD][Tf₂N]), and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene perfluorobutylsulfonate ([MTBD][C₄F₉SO₃]). On an unbiased surface, these ILs form a cation layer at the IL-solid interface, followed by a layer of anions. [Emim]+ and [MTBD]+ have similar orientation at the surface but [MTBD]+ forms a thinner layer compared to [emim]+ on both HOPG and Pt(111). When the potential is applied to the solid surface, the very first ion layer near the solid surface has the opposite charge of the potential applied to the electrode; i.e., the first layer is a cation when a negative potential is applied and it is an anion layer when a positive potential is applied. We observe oscillating ion layer profiles up to 1 nm away from the solid surface. [emim]+ and [MTBD]+ cations are also found to have a preferably flat and parallel structure to the solid surface.

Over the past few decades, various types of filler materials have been employed to develop the advanced resin-based dental composites, enhancing the lifetime of the restorations. However, further effort in the research on the multi-functional composite that is comparable to dental tissue in mechanical strength, as well as offering the improved antibacterial function and the better aesthetics, is continuously required. In this thesis, micro-sized short S-glass fibres and halloysite nanotubes (HNTs) are employed to serve as excellent load-carrying filler members and antibacterial agent in the dental composites. The mechanical reinforcement mechanism and the interfacial behaviours between filler and resin matrix have been precisely investigated through the multiscale analysis from atomistic to macro by utilising the combined experimental, theoretical, and computational methods. The surface modification process on the short S-glass fibres, named selective atomic-level metal etching, has been developed, which enables to strengthen the interfacial bond between resin matrix and glass fibre by increasing the surface roughness and reactive sites on the fibre. The influence of the surface treatment on the interfacial strength and mechanical properties of the resulted composites were examined through the single-fibre pull-out tests. Also, the modified Lewis-Nielsen model has been developed, where the effective fibre length factor is applied to accurately predict the modulus of the short fibre reinforced composites. For better understanding of the atomistic interfacial bonding and fracture behaviours between glass fibre and resin matrix, molecular dynamics simulations were conducted. The numerical results of the single fibre pull-out and the uniaxial composite tension simulations were validated with the experimental findings. The optimised computational design and analysis methods were established for developing new dental and bio-composites with the accurate prediction on the mechanical performances. The surface modification process on the HNTs was developed to promote the mechanical reinforcement effect and to add an antimicrobial functionality in the composites. The composite reinforced with 2.0 wt.% of chitosan grafted HNTs showed an increased efficacy in flexural strength and modulus up to 8.1% and 14.1%, respectively, and exhibited an improved antibacterial functionality against S. mutans with 39% reduction, making it a desirable dental material.

Abstract: Among all the competing methods to enhance the phase separation of water-in-oil emulsions in chemical industries, electric field is considered as one of the best treatment options. To better understand the mechanism by which electric field assists water droplets coalescence and the corresponding physics of skin formation, the interfacial properties of the water/oil interface in a water-in-model oil emulsion system, including interfacial tension, shape factor, crumpling ratio and dilatational rheology, are studied with a tensiometer. The pendant water drop deforms immediately at the onset of electric field application, into either a prolate shape or an oblate shape in the direction of electric field, depending on the properties of the continuous phase. The degree of drop deformation is of positive correlation with the experienced electric field strength and is also affected by the electric field direction and the drop position in between the electrodes. An external electric field is found to enhance the formation of interfacial asphaltene layer elucidated by a higher crumpling ratio, lower interfacial tension, higher shape factor and higher elasticity with increasing electric field strength. This could be due to the induced electrohydrodynamic flow, which triggers the interfacial convection and accelerates asphaltene movements and adsorption. The same trend holds true for measurements with various asphaltene concentration, aging time, water phase pH, and water salinity under electric field application. Although the electric-assisted formation of a stronger interfacial barrier is against drop-drop coalescence, a hypothesis is proposed that the electric-induced drop deformation may dominant, enlarging the interfacial area and generating a patchy film with weak spots for easier drop-drop coalescence.

With its superior mechanical properties and durability, ultra-high-performance concrete (UHPC) has been used as an alternative grout material to reduce longitudinal cracking and other degradation of shear keys in adjacent box beam bridges. This study evaluates UHPC shear key performance in field and laboratory investigations supplemented with high fidelity 3-dimensional finite element models (FEMs) of the complex behavior at the interface between HSC-UHPC joints. The results from the validated and calibrated FEM were used to optimize a new shear key shape, which was then experimentally tested.The parameters of primary importance to shear key performance were identified from the response data collected during the initial field study. The maximum relative displacement for all load cases was 5.9 mil, insufficient to damage the UHPC shear key interface. In the laboratory, the direct shear test showed the average maximum shear capacity for the UHPC shear key without shear reinforcement bars exceeds that previously observed for any other grout material and any shear key configuration. The direct tension test showed that the average maximum tensile stress for the UHPC-HSC specimens with a smooth interface exceeded previously reported values from the literature, even those with a rough interface. The adhesion, cohesion, and friction angle results from the direct tension tests and slant-shear tests reported in a previous study, were used to back-calculate the friction coefficients using Mohr-Coulomb theory. The FEMs simulating the direct shear, flexural, and direct tension tests were calibrated and validated with experimental results. The models, along with different shear key configurations, were used to investigate the load transfer through previously proposed connections. After comparing the simulated shear keys in terms of the maximum load capacity, an optimized shear key shape (Type OPT) design was selected to balance economy and strength. This Type OPT UHPC shear key was experimentallytested. The direct shear test measured an average maximum shear capacity which exceeded any previously determined value for shear keys regardless of grout material and configuration. Also, the contribution of the shear key configuration to the joint strength was determined.

Nanoscale materials have demonstrated unique properties in condensed matter studies and played a crucial rule in modern devices. Fundamental research on nanoscale systems provides deep understanding into science and enables advancing and reshaping of nanotechnology. Scanning tunneling microscopy is a comprehensive tool which is capable of characterizing topographic and electronic properties of nanoscale materials in real space down to atomic scale. To date many emerging techniques have been combined with scanning tunneling microscopy to discover and explore new phenomena. This dissertation explicitly demonstrates novel application of synchrotron X-ray scanning tunneling microscopy which combines synchrotron X-ray and scanning tunneling microscopy to study magnetic, electronic, and structural properties of materials interfaces. A scanning tunneling microscope tip is used to capture X-ray magnetic circular dichroism and near edge X-ray absorption fine structure signals, which explain charge transfer and magnetic properties of oxide materials interfaces with chemical and elemental sensitivities. Using X-ray absorption spectroscopy and spectroscopic imaging with a scanning tunneling microscope tip, the effect of charge transfer at the interfaces formed by transition metals of cobalt and nickel in nanoscale clusters and islands on a Cu (111) surface has been explored. Finally, X-ray standing wave formed by the interference of the incident and diffracted X-ray beams is used to characterize the structural properties of a cobalt thin film grown on a Au (111) surface. These results open novel research directions where material characterizations will be able to perform simultaneous chemical, structural and magnetic contrast potentially down to atomic scale.

Abstract: Asphaltenes are the heaviest components in crude oil. It is generally believed that asphaltenes adsorbed at oil/water interface can form a protective layer to stabilize the water-in-oil emulsions. Therefore, it is of both fundamental and practical importance to understand the adsorption kinetics of asphaltenes to the oil/water interface. In this work, the effects of asphaltene concentration and temperature on the dynamic interfacial tension (IFT) of oil/water interface were investigated using pendent drop shape method. The adsorption process showed three stages as a function of adsorption time. It was found that the reduction kinetics of interfacial tension in the initial state (Regime I) was diffusion-controlled, during which asphaltenes were adsorbed to the oil/water interface spontaneously. Diffusion coefficient was found to increase with increasing temperature and decreasing asphaltene concentration. While asphaltene concentration showed a dominant influence on the diffusion coefficient as compared with temperature. With increasing adsorption time, in the Regime II, the steric hindrance arisen from the adsorbed asphaltenes at oil/water interface tended to inhibit the further adsorption. Continuous adsorption of asphaltenes to the sublayer of the interface and reconfiguration of adsorbed asphaltenes might contribute to the continuous reduction of dynamic interfacial tension in Regime III. The experimental data was fitted with Gibbs adsorption model, which showed that elevated temperature had a negative impact on the maximum surface excess concentration of asphaltene. Calcium chloride is the main salinity present in low-salinity water flooding which is an efficient process for enhanced oil recovery (EOR). In this thesis, the effect of calcium chloride concentration on the interfacial tension and dilatational interfacial rheology was investigated and further validated by emulsion stability experiments. It was found that with increasing calcium chloride concentration both the dynamic interfacial tension and surface pressure increased. Diffusion coefficient and maximum interfacial excess concentration of asphaltenes were enhanced with the existence of calcium chloride in the aqueous phase. The results of dilatational interfacial rheology and compressibility studies have demonstrated that calcium chloride could induce the rearrangement of asphaltene molecules and form a more rigid film at the oil/water interface. The results of interfacial tension, dilatational interfacial rheology and emulsion stability tests further demonstrate that calcium chloride could significantly influence the stability of water-in-oil emulsions. Our results provide useful information regarding the adsorption kinetics and adsorption mechanism of asphaltenes at oil/water interface in oil production, as well as the correlation between interfacial properties and emulsion stability in the presence of asphaltenes, particularly under high temperature conditions and with calcium chloride.

Modeling the defect structure and mechanical properties of metallic multilayer thin films requires estimates of dislocation parameters such as interfacial line energy, interfacial barrier strength, and resistance to confined layer slip (CLS). A method is presented to estimate these parameters using experimental measurements of hardness and internal stress vs. individual layer thickness, h. Cu/Ni multilayers of varying bilayer thickness (20 nm < BT < 60 nm) and volume fraction (25 < %Ni < 60%) were fabricated via sputtering and characterization performed using x-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Internal stresses for the samples were calculated via peak positions from in-plane XRD and second order elastic constants. The experimental techniques of nanoindentation and micropillar compression data were used to look at flow stresses, hardness, and strain rate sensitivities for the Cu/Ni multilayers. Internal stress was seen to increase with decreasing bilayer thickness and decreasing layer thickness for both layer types. In addition, hardness was seen to increase with decreasing bilayer thickness and decreasing Cu layer thickness.The data acquired via characterization and experimentation was used as inputs within a CLS model in order to extract quantities for interfacial properties. It was seen that that separate values of line energy operate in the Cu and Ni layers and that a single effective line energy for the multilayer is inappropriate. This indicates that dislocation loops will encounter a different resistance at the shared interface depending on whether the dislocation loop originates in Ni or Cu. Analytical models for line energy overestimated the line energy in Cu and underestimated the line energy in Ni. It was seen that the interfacial barrier to dislocation motion increased with increasing bilayer thickness and misfit strain. A maximum value of 444 MPa was extrapolated for the interfacial barrier to dislocation transmission. Pinning stresses increased initially with increasing layer thickness due to the increasing misfit strain. The pinning stresses reached a maximum of 240 MPa in Ni before decreasing for layer thicknesses > 20 nm. Finally, it was seen that coherency stresses derived from lattice parameter mismatch did not completely explain the hardness found in the multilayers. Atomistics had predicted that strength was dominated by the contribution of coherency stresses while these experiments indicated that some contribution from pinning and interfacial stresses was required to fully explain the multilayer strength. The results of the modeling in this dissertation suggest that multilayer strength can be increased by methods other than reducing bilayer thickness. These methods include selecting a seed or buffer layer to place the multilayer in a state of net in-plane compression and increasing the barrier to dislocation transmission by fabricating the compressive phase in thinner quantities compared with the tensile phase.