Influence of periodic wall roughness on the slip behaviour at liquid/solid interfaces: molecular-scale simulations versus continuum predictions

The influence of surface roughness on the slip behaviour of a Newtonian liquid in steady planar shear is investigated using three different approaches, namely Stokes flow calculations, molecular dynamics (MD) simulations and a statistical mechanical model for the friction coefficient between a corrugated wall and the first liquid layer. These approaches are used to probe the behaviour of the slip length as a function of the slope parameter ka = 2πa/λ, where a and λ represent the amplitude and wavelength characterizing the periodic corrugation of the bounding surface. The molecular and continuum approaches both confirm a monotonic decay in the slip length with increasing ka but the rate of decay as well as the magnitude of the slip length obtained from the Stokes flow solutions exceed the MD predictions as the wall feature sizes approach the liquid molecular dimensions. In the limit of molecular-scale wall corrugation, a Green–Kubo analysis based on the fluctuation–dissipation theorem accurately reproduces the MD results for the behaviour of the slip length as a function of a. In combination, these three approaches provide a detailed picture of the influence of periodic roughness on the slip length which spans multiple length scales ranging from molecular to macroscopic dimensions

Animating physical phenomena with embedded surface meshes

Accurate computational representations of highly deformable surfaces are indispensable in the fields of computer animation, medical simulation, computer vision, digital modeling, and computational physics. The focus of this dissertation is on the animation of physics-based phenomena with highly detailed deformable surfaces represented by triangle meshes. We first present results from an algorithm that generates continuum mechanics animations with intricate surface features. This method combines a finite element method with a tetrahedral mesh generator and a high resolution surface mesh, and it is orders of magnitude more efficient than previous approaches. Next, we present an efficient solution for the challenging problem of computing topological changes in detailed dynamic surface meshes. We then introduce a new physics-inspired surface tracking algorithm that is capable of preserving arbitrarily thin features and reproducing realistic fine-scale topological changes like Rayleigh-Plateau instabilities. This physics-inspired surface tracking technique also opens the door for a unique coupling between surficial finite element methods and volumetric finite difference methods, in order to simulate liquid surface tension phenomena more efficiently than any previous method. Due to its dramatic increase in computational resolution and efficiency, this method yielded the first computer simulations of a fully developed crown splash with droplet pinch off.Ph.D.Committee Chair: Turk, Greg; Committee Member: Essa, Irfan; Committee Member: Liu, Karen; Committee Member: Mucha, Peter J.; Committee Member: Rossignac, Jare

Specialized Inter-Particle Interaction Lbm For Patterned Superhydrophobic Surfaces

SPECIALIZED INTER-PARTICLE INTERACTION LBM FOR PATTERNED SUPERHYDROPHOBIC SURFACES by AMAL S. YAGUB ABSTRACT: Superhydrophobic surface characteristics are important in many industrial applications, ranging from the textile to the military. It was observed that surfaces fabricated with nano/micro roughness can manipulate the droplet contact angle, thus providing an opportunity to control the droplet wetting characteristics. The Shan and Chen (SC) lattice Boltzmann model (LBM) is a good numerical tool, which holds strong potentials to qualify for simulating droplets wettability. This is due to its realistic nature of droplet contact angle (CA) prediction on flat smooth surfaces. But SC-LBM was not able to replicate the CA on rough surfaces because it lacks a real representation of the physics at work under these conditions. By using a correction factor to influence the interfacial tension within the asperities, the physical forces acting on the droplet at its contact lines were mimicked. This approach allowed the model to replicate some experimentally confirmed Wenzel and Cassie wetting cases. Regular roughness structures with different spacing were used to validate the study using the classical Wenzel and Cassie equations. This work highlights the strength and weakness of the SC model and attempts to qualitatively conform it to the fundamental physics, which causes a change in the droplet apparent contact angle, when placed on nano/micro structured surfaces. In the second part of this work, the model is used also to analyze the sliding of droplets in contact with flat horizontal surfaces. This part identifies the main factors, which influence the multiphase fluids transport in squared channels. Effects of dimensionless radius, Weber number, Reynolds number and static contact angles are evaluated by calculating the power required for moving single droplets in comparison to the power needed for moving the undisturbed flow in the channel. Guidelines for optimizing the design of such flow are presented. In last part of work, the sliding of droplets on sloped surfaces with and without roughness is numerically investigated. The Shan and Chen (SC) Lattice Boltzmann model (LBM) is used to analyze the effect of pinning on the movement of droplets placed on sloped surfaces. The model is checked for conformance with the Furmidge equation which applies to tilted unstructured surfaces. It is shown that a droplet sliding on a perfectly smooth surface requires very minimal slope angle and that pinning due to the inhomogeneous nature of manufactured smooth surfaces is the key factor in determining the minimal slope angle. The model is also used on sloped rough surfaces to check the effects of roughness on the movement of single droplets. The numerical outcomes are compared with published experimental results for validation and a dimensionless number is suggested for quantifying the degree of pinning needed to control the behavior of sliding droplets on sloped surfaces

Adhesion at Solid/Liquid Interfaces

The adhesion at solid/liquid interface plays a fundamental role in diverse fields and helps explain the structure and physical properties of interfaces, at the atomic scale, for example in catalysis, crystal growth, lubrication, electrochemistry, colloidal system, and in many biological reactions. Unraveling the atomic structure at the solid/liquid interface is, therefore, one of the major challenges facing the surface science today to understand the physical processes in the phenomena such as surface coating, self-cleaning, and oil recovery applications. In this thesis, a variety of theory/computational methods in statistical physics and statistical mechanics are used to improve understanding of water adhesion at solid/liquid interfaces. In here, we addressed two separated, but interconnected problems: First, we consider water adhesion on fiber/surface, responsible for the emergence of droplet residue upon droplet detachment. In this project, we study the mechanism of water droplet detachment and retention of residual water on smooth hydrophilic fibers and surfaces using nonequilibrium molecular dynamics simulations. We investigate how the applied force affects the breakup of a droplet and how the minimal detaching force per unit mass decreases with droplet size. We extract scaling relations that allow extrapolation of our findings to larger length scales that are not directly accessible by molecular models. We find that the volume of the residue on a fiber varies nonmonotonically with the detaching force, reaching the maximal size at an intermediate force and associated detachment time. The strength of this force decreases with the size of the drop, while the maximal residue increases with the droplet volume, V, sub-linearly, in proportion to the V2/3. Second, we address the adhesion on conducting graphene. We improved the graphene model by incorporating the conductivity of graphene sheet using the fluctuating charge technique of Constant Potential Molecular Dynamics (CPMD). We evaluated the wettability by measuring the contact angle of cylindrical water drops on a conducting graphene sheet. We found that the CA of a water droplet on a graphene sheet supported by water is lower than in the absence of water under graphene. Our calculations reveal effective attractions between partial charges of equal sign across the conducting graphene sheet. Attractive correlations are attributed to the formation of the highly localized image charges on carbon atoms between the partially charged sites of water molecules on both sides of graphene. By performing additional computations with nonpolar diiodomethane, we confirm that graphene transmits both polar and dispersive interactions. These findings are important in applications including sensors, fuel cell membranes, water filtration, and graphene-based electrode material to enhance the supercapacitor performance. A challenge for future work concerns dynamic polarization response of wetted graphene at alternating (AC) field condition

Modelling Multiphase Flow Using a Dynamic Pore Network Model for Imbibition

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On the use of a friction model in a Volume of Fluid solver for the simulation of dynamic contact lines

We consider the implementation of a friction contact angle model in a Navier-Stokes VoF-CSF solver for the simulation of moving contact lines at the nano-scale. A liquid-liquid interface confined in a Couette flow generated by two solid walls moving at the same velocity in opposite directions is considered to discuss the relevance of the friction model. The simulations are compared with a reference case obtained using MD simulations by Qian et al. [46]. We show that the Navier Stokes simulations are able to reproduce the MD simulations for both the interface shape and the velocity field. The appropriate contact line friction is found to be grid convergent and of the same order as the friction measured in MD simulations. A detailed investigation of the interface shape has revealed an auto-similar linear profile in the center of the channel. Close to the wall the interface shape follows the classical Log evolution given by the Cox relation despite the wall confinement

Interface-Resolving Simulations of Gas-Liquid Two-Phase Flows in Solid Structures of Different Wettability

This PhD study is devoted to numerical investigations of two-phase flows on and through elementary and complex solid structures of varying wettability. The phase-field method is developed and implemented in OpenFOAM®. The numerical method/code is verified by a series of test cases of two-phase flows, and then applied to investigate: (1) droplet wetting on solid surfaces; (2) air bubble rising and interacting with cellular structures and (3) gas-liquid interfacial flows in foam structures

Pore network modelling of wettability effects on waterflood oil recovery from Agbada sandstone formation in the Niger Delta, Nigeria

A thesis Submitted to the School of Chemical and Metallurgical Engineering, Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Doctor of Philosophy Johannesburg, 2016Wettability of a porous reservoir rock is an important factor that affects oil recovery during waterflooding. It is recognized as being important for multiphase properties. Understanding the variation of these properties in the field, due to wettability trends and different pore structures, is very critical for designing efficient and reliable processes and projects for enhanced hydrocarbon recovery. After primary drainage the reservoir wettability changes: if it was oil-wet initially, it gradually changes to water-wet during waterflooding. This change in reservoir wettability towards water-wet will reduce the residual oil saturation and improve the oil displacement efficiency. However, knowledge of the constitutive relationship between the pore scale descriptors of transport in the porous system is required to adequately describe wettability trend and its impact on oil recovery, particularly during waterflooding. In this work, the petrophysical properties that define fluid flow in the Agbada, Nigeria sandstone reservoir were determined using conventional experimental and x-ray CT scanning methods. Experimentally measured average porosity is 0.28, average permeability is 1699 mD, while the initial and irreducible water saturation is 0.22. Permeability in the x, y and z directions, ranging from 50 to 200 mD, were calculated from the pore network extracted from the Agbada sandstone rock. Results obtained from the Amott-Harvey wettability measurement method indicate that the reservoir is strongly water-wet, with Amott-Harvey index of about 0.9. The cross-over between the water and oil relative permeabilities occurred at saturations of the samples above 0.5, giving an indication of strong water-wetness. The work summarizes the mechanism of wettability alteration and characterizes the performance of the reservoir during waterflooding from injecting water, and relates the residual oil saturation, relative permeability and volumes of water injected to wettability and its effects on oil recovery. Waterflood oil recovery is computed using the Buckley-Leverett method based on the reservoir rock and fluid properties. Computed waterflood oil recovery using this method was about 60% of the oil initially in place. Plots of spontaneous imbibition rate show that the injection rate for optimal oil recovery is 40 bbls of injected water per day. At this rate, both the mobility and shock front mobility ratios are less than 1, leading to a stable flood front and absence of viscous fingering. Waterflooding is by far the most widely applied method of improved oil recovery over the years with good results in conventional and unconventional (tight oil) reservoirs It is relatively simple and cost effective: abundance and availability of water. Waterflood oil recovery factor is affected by internal and external factors. The placement of the injection and production wells, for example, impacts on the effectiveness of the waterflooding process. I considered the placement of the wells in a five-spot pattern as elements of an unbounded double periodic array of wells and assumed the reservoir to be homogeneous, infinite and isotropic, with constant porosity and permeability. Both fluids are treated as having slight but constant compressibility and their flow governed by Darcy’s law. The average pressure in the reservoir satisfies quasi-static flow or diffusion equation. I then assumed piston-like displacement of oil by injected water that takes account of viscosity diffence between both fluids and proposed a model based on the theory of elliptic functions, in particular Weierstrass p-functions functions. Oil-water contact movement, dimensionless time for water breakthrough at the production well, areal sweep and average reservoir pressures were modeled. The model was tested using Wolfram Mathematica 10 software and the results are promising. The thesis has therefore established that the Agbada sandstone reservoir is strongly water-wet and that waterflooding is a viable option for enhanced oil recovery from the reservoir.MT201