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An analysis of contact stiffness and frictional receding contactsParel, Kurien Stephen January 2017 (has links)
The tangential contact stiffness for ground Ti-6Al-4V surfaces is measured to linearly decrease with the application of tangential load. At the beginning of the application of tangential load, for ground surfaces, the ratio of the tangential contact stiffness to the normal contact stiffness is seen to be approximately half the Mindlin ratio. This is consistent with many other published experimental studies. Measurements of normal contact stiffness for ground surfaces conform to a model that posits a linear relationship between normal contact stiffness and normal load. An equivalent surface roughness parameter is defined for two surfaces in contact; and the normal contact stiffness for ground surfaces is observed to be inversely proportional to this parameter. Single asperity models were constructed to simulate the effect of different frictional laws and plasticity on the tangential displacement of an asperity contact. Further, multi-asperity modelling showed the effect of different normal load distributions on the tangential behaviour of interfaces. In addition, normal contact stiffness was modelled for a grid of asperities taking into account asperity interactions. A receding contact problem for which the required form of the distributed dislocations is bounded-bounded was solved. Then, a fundamental 2D frictional receding contact problem involving a homogeneous linear elastic infinite layer pressed by a line load onto a half-plane of the same material was analysed. This was done by the insertion of preformed distributed dislocations (or eigenstrains), which take into account the correct form of the separation of the interface at points away from the area of loading, along with corrective bounded-bounded distributions. The general method of solution was further refined and adapted to solve three other receding contact problems. The solutions demonstrated the robustness and applicability of this new procedure.
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Contact Angle Hysteresis: Implications for Fluid FlowAndrade, Cristhian F. 06 1900 (has links)
Contact angle behavior controls the spreading, sticking, or movement of fluid droplets on top of solid substrates, and the immiscible displacement of mixed fluids in porous media. Therefore, it influences applications such as oil recovery, CO2 geological storage, water transport in unsaturated soils, and DNAPL soil remediation techniques. The attraction forces and geometrical-molecular arrangement at the atomic scale define the strength of the interfacial tension that changes in response to changes in temperature, pressure, or the fluid composition within the system. Contact line behavior such as contact line pinning or depinning, microscale roughness, and changes in interfacial tensions influence advancing and receding contact angles.
This study consists of a comprehensive database of published advancing and receding contact angles to understand the underlying mechanisms of contact line pinning and depinning and the implications of these phenomena on advancing and receding contact angles. Calcite experiments that investigate advancing and receding contact angle measurements as a function of ionic concentration complement the published literature. Critical results include: an advancing contact angle trend with calcite as a function of ionic concentration, a point of minimum contact angle hysteresis when brine concentrations are close to 0.1 M, and that contact angle behavior depends on cation type and the calcite surface anisotropy.
Contact line pinning prevents flow and increases contact angle hysteresis. An analysis of the database suggests that the wide range of contact angle hysteresis of calcite and quartz with water results both from hydrogen bonds and microscale roughness at the surface which leads to pinned contact lines. The Jamin effect reduces significantly in calcite when the resultant injection brines have an ionic concentration close to 0.1 M. Thus, the pressure difference required to displace a non-wetting fluid for a wetting fluid reduces, and leads to enhanced recovery of trapped oil, gas or DNAPL.
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Modeling Complex Contact Phenomena with Nonlinear BeamshellsBrink, Adam Ray 19 May 2015 (has links)
No description available.
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High-Speed Flow Visualization and IR Imaging of Pool Boiling on Surfaces Having Differing Dynamic WettabilitiesNicholas Toan-Nang Vu (9760715) 14 December 2020 (has links)
Boiling is used in a wide variety of industries, including electronics cooling, distillation, and power generation. Fundamental studies on the boiling process are needed for effective implementation. Key performance characteristics of boiling are the heat transfer coefficient, which determines the amount of heat flux that can be dissipated for a given superheat, and critical heat flux(CHF), the failure point that occurs when vapor blankets the surface. The wettability of a surface is one of the key parameters that affects boiling behavior. Wetting surfaces(e.g., hydrophilic surfaces), typically characterized by a static contact angle below 90°,have better critical heat flux due to effective rewetting, but compromised heat transfer coefficients due to increased waiting times between nucleation of each bubble. Meanwhile, nonwetting surfaces (e.g., hydrophobic surfaces), characterized by static contact angles greater than 90°, have better heat transfer coefficients due to improved nucleation characteristic, but reach critical heat flux early due to surface dry out. However, recent studies have shown that the static contact angle alone offers and incomplete, and sometimes inaccurate, description of this behavior, which is instead governed entirely by the dynamic wettability. Specifically, the receding contact angle impacts the size and contact area of bubbles forming on a surface during boiling, while the advancing contact angle determines how the bubble departs. With this more complete set of wettability descriptors, three characteristic wetting regimes define the boiling behavior: hygrophilic surfaces having advancing and receding contact angles both under 90°; hygrophobic surfaces having both these dynamic contact angles over 90°;and ambiphilic surfaces having a receding contact angle less than 90°, but an advancing contact angle greater than 90°.The goal of this thesis is to experimentally characterize and compare the behavior of boiling surfaces in each of these regimes, observe the contact line behavior, and explain the mechanisms for their differences in performance.
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