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

Lattice Boltzmann parallel simulation of microflow dynamics over structured surfaces

In the present work, a parallel lattice Boltzmann multiphase model was developed to investigate the effects of surface structures on wettabilities and flow dynamics in a microchannel. The theory of wetting transition was firstly discussed. Then three types including triangular, rectangle and hierarchical shaped microstructures were constructed on the surface to examine the effects on wettabilities and drag reduction. It was found that flow behaviour is strongly affected by the surface morphology of the channel. The results indicated that hierarchical structures on the surface could improve the hydrophobicity significantly. For rectangular structures, they can improve the hydrophobicity with the increase of height and distance ratio h/d of the structures, and the improvement will reach its optimal hydrophobicity when the value h/d is over a certain value of 0.6. Moreover, to accelerate computational speed, the Open Multi-Processing (OpenMP) was employed for the parallelization of the model. A maximum speedup of 2.95 times was obtained for 4 threads on a multi-core CPU platform

Correlation between surface topography and slippage: a Molecular Dynamics study

Using Molecular Dynamics simulations of a polymer liquid flowing past flat and patterned surfaces, we investigate the influence of corrugation, wettability and pressure on slippage and friction at the solid-liquid interface. For one-dimensional, rectangular grooves, we observe a gradual crossover between the Wenzel state, where the liquid fills the grooves, and the Cassie state, where the corrugation supports the liquid and the grooves are filled with vapor. Using two independent flow set-ups, we characterize the near-surface flow by the slip length, $\delta$, and the position, $z_\textrm{h}$, at which viscous and frictional stresses are balanced according to Navier's partial slip boundary condition. This hydrodynamic boundary position depends on the pressure inside the channel and may be located above the corrugated surface. In the Cassie state, we observe that the edges of the corrugation contribute to the friction.Comment: 13 pages, 13 figure

Diffuse Interface Modelling of Wetting on Complex Structured Surfaces

Wetting on solid surfaces textured with geometries from simple to highly complex structures is of interest from fundamental physics perspective and for potential applications. From the physics point of view, interesting phenomena can be observed, such as hemiwicking, when perfectly wetting liquids propagate through the corrugation of these structured surfaces, and wetting transition, when liquids initially in a suspended state (Cassie-Baxter state) transition to a collapsed state (Wenzel state). In addition, on a microscopic scale where the wettability is dictated by the intermolecular interactions, distinct wetting phenomena can be observed, such as liquid filling and emptying. From the application perspective, wetting on structured surfaces is key to a broad range of technological and industrial applications, from coating and microfluidic to liquid hydrocarbon recovery. In this thesis, we employ dynamical and quasi-static numerical methods based on diffuse interface model for studying wetting phenomena on structured surfaces. First, we use the Lattice Boltzmann method, which is powerful for studying liquid dynamics. Second, we employ the phase-field energy minimisation method by incorporating distance-dependent solid-liquid interactions to obtain the equilibrium state of the system. Third, we develop a new method based on the phase-field model in the energy minimisation framework, the frozen fluid method, for constructing highly complex geometry structures. We develop a fully analytical model to predict the propagation coefficients for liquids hemiwicking through square and face-centre/hexagonal arrays of micropillars. This is done by balancing the capillary driving force and a viscous resistive force and solving the Navier-Stokes equation for representative channels. The theoretical predictions for the square array case exhibit excellent agreement with the simulation results for a wide range of geometries and improved accuracy compared to previously proposed models. Furthermore, we demonstrate the applicability of the hydraulic-electric circuit analogy approach in approximating the equivalent channel for face-centred/hexagonal arrays of micropillars. In the study of liquid filling and emptying on grooved surfaces, we consider short-range and long-range liquid-solid interactions, with the latter including purely attractive and repulsive interactions and those with short-range attraction and long-range repulsion. Comparing the filling and emptying transitions for complete, partial, and pseudo-partial wetting states, we find that the filling and emptying transitions are reversible for the complete wetting case, while significant hysteresis is observed for the partial and pseudo-partial cases. In agreement with previous studies, we show that the critical pressure for the filling transition follows the Kelvin equation for the complete and partial wetting cases. For the pseudo-partial wetting case, we find that the filling transition can display a number of distinct morphological pathways. Finally, we validate the frozen fluid method through several benchmarking tests, demonstrating its applicability across various solid geometries, including those with flat, curved, and corner features. Subsequently, we utilize the method to investigate the critical pressure of a liquid on superhydrophobic surfaces textured with cylindrical and truncated cone pillars, and mesh geometry. By analyzing the impact of texture parameters, we can optimize superhydrophobic surfaces to enhance their wetting stability

Sintering properties of platinum nanoparticles on different oxide-based substrates

Metal nanoparticles play a significant role in exhaust combustion. They oxidize harmful products like carbon monoxide and hydrocarbons in order to prevent major environmental and health issues. In a converter, platinum nanoparticles (Pt NPs) are impregnated in a thin coating of a porous ceramic oxide. Due to their high surface-to-volume ratio, Pt NPs can provide high catalytic activities; however, elevated temperatures in the exhaust gas flow lead to thermal deactivation of the catalyst via sintering, thereby resulting in large losses in efficiency over the catalyst’s lifetime. In this thesis, the sintering behavior of 5-6 nm sized Pt NPs synthesized via block copolymer micellar nanolithography on various planar oxide-based substrates is investigated. First, their coarsening on both crystalline and amorphous silica (SiO2) and alumina (Al2O3) is evaluated in regard to the mechanisms of Ostwald ripening and particle migration and coalescence. Sinter studies at 750°C in air reveal an enhanced thermal stability on the amorphous alumina-support Al2O3(a). Second, key influencing parameters on the sinter resistivity of the Pt NPs are identified. An increased NP adhesion on the amorphous substrates, a higher roughness and surface potential, as well as a larger contact angle of water on Al2O3(a) are all found to significantly contribute to enhanced sinter stability. Furthermore, the thermal behavior of Pt NPs on dual-structured surfaces is examined at the interface between Al2O3(a) and SiO2 to study the impact of compositional surface heterogeneities. The particles favor the high metal interaction Al2O3(a)-side over the low metal interaction SiO2- side as shown by their diffusion away from the silica. Additionally, structural heterogeneities on sapphire wafers with varying tilt angles, and thus step edges of different height and size, contribute to a smaller increase in Pt NP diameter over time on the more tilted substrates when exposed to 1200°C under vacuum compared to NPs on less tilted substrates. Hereby, larger sintered particles are observed to preferably align along the step edges. This is due to a locally increased surface potential at the edges and because these edges function as Ehrlich-Schwoebel barriers. Thereby they hinder the diffusion of particles on the substrate. Lastly, the sinter stability of Pt NPs is successfully enhanced via the deposition of an isolating silica or alumina layer by solgel techniques. These films are shown not to cover the Pt NPs and also prevent the migration of platinum clusters toward each other during sinter studies at 750°C under atmospheric conditions. Taken together, this data contributes to a better understanding of the thermal stability of Pt NPs catalysts with respect to the underlying support. The information gained from these sinter studies can be harnessed in the design of more thermally stable Pt NP catalysts, which can ultimately contribute to more environmentally sustainable technologies

Droplet wetting on chemically and mechanically structured surfaces

In dieser Arbeit wird das Benetzungsverhalten von Tröpfchen auf chemisch strukturierten und mechanisch strukturierten Oberflächen untersucht. Hier werden die Gleichgewichtsformen und die Quasi-Gleichgewichtsbewegungen von Tröpfchen auf chemisch strukturierten Oberflächen, das Benetzungsverhalten von Mehrphasentröpfchen auf chemisch heterogenen Oberflächen und das Tröpfchenpermeationsverhalten in einer einzelnen Porenstruktur angesprochen. Zu guter letzt wird das Phasenfeldmodell validiert, um die Tröpfchendynamik auf festen heterogenen Oberflächen zu untersuchen, und das validierte Modell wird verwendet, um die steuerbare Bildung von Satellitentröpfchen während des Entwässerungsprozesses für dünne Flüssigkeitsfilme auf chemisch strukturierten Oberflächen zu untersuchen. Für Tröpfchen auf chemisch strukturierten Oberflächen werden zunächst die Gleichgewichtsform von Tröpfchen und die Kontaktlinienbewegung auf chemisch streifenmusterierten Oberflächen untersucht. Es wurde gezeigt, dass das Phasenfeldmodell sehr robust ist, um die Gleichgewichtsform, die Ausbreitungsdynamik und die Phasenänderung von Tröpfchen auf chemisch strukturierten Oberflächen zu simulieren. Man erhält ein morphologisches Diagramm, das zeigt, dass das Tröpfchenaspektverhältnis und die Anzahl der Gleichgewichtsformen eng mit der skalierten Streifenbreite zusammenhängen. Durch die Vergleiche zwischen kondensierenden und verdampfenden Tröpfchen wird ein Hysteresephänomen beobachtet, das beweist, dass die unterschiedlichen Tröpfchenformen über unterschiedliche Bewegungspfade erreicht werden können. Darüber hinaus wird ein präzises mathematisch-physikalisches Modell vorgeschlagen, um die Tröpfchenkonfigurationen auf drei typischen programmierbaren chemisch strukturierten Oberflächen zu beschreiben. Dieses analytische Modell basiert auf der Berechnung der Oberflächenenergielandschaft und wurde erfolgreich gegen Phasenfeldsimulationen und Experimente validiert. Es kann als Anleitung für Experimente und Simulationen dienen, um verschiedene Gleichgewichtsformen ohne blinde Versuche zu finden. Dieses analytische Modell gilt insbesondere für die Situation, in der die Größe der chemischen Heterogenität mit der Tröpfchengröße vergleichbar ist. Basierend auf diesem Konzept wird ein modifiziertes Cassie-Baxter-Modell vorgeschlagen, um die anisotropen Benetzungskonfigurationen zu adressieren. Zusätzlich wird das Mehrphasen-Phasenfeldmodell verwendet, um das Benetzungsverhalten von Mehrphasentröpfchen auf chemisch strukturierten Oberflächen zu untersuchen, und die Wechselwirkung der Flüssig-Flüssig-Grenzfläche, die durch unterschiedliche Werte der Grenzflächenspannungen beeinflusst wird, wird diskutiert. Das Phasenfeldmodell wird weiter validiert, um die Tröpfchenbenetzungszustände in einer Keilstruktur zu untersuchen. Danach wird das Tröpfchenbenetzungsverhalten in einer Porenstruktur theoretisch und numerisch untersucht, um das Kriterium für die Tröpfchenpermeation zu finden. Es ist erwiesen, dass der Öffnungswinkel und die Hydrophobizität des Substrats einen großen Einfluss auf das Tröpfchenpermeationsverhalten haben. Schließlich wird das Cahn-Hilliard-Modell mit Navier-Stokes-Gleichungen gekoppelt, um die Tröpfchendynamik auf chemisch strukturierten Oberflächen zu untersuchen. Wir finden eine neue Strategie zur Kontrolle der Bildung von Satellitentröpfchen durch gezielte Gestaltung der chemischen Muster

Interfacial phenomena on sculpted substrates

Imperial Users onl

Lattice Boltzmann Modelling of Droplet Dynamics on Fibres and Meshed Surfaces

Fibres and fibrous materials are ubiquitous in nature and industry, and their interactions with liquid droplets are often key for their use and functions. These structures can be employed as-is or combined to construct more complex mesh structures. Therefore, to optimise the effectiveness of these structures, the study of the wetting interactions between droplets and solids is essential. In this work, I use the numerical solver lattice Boltzmann method (LBM) to systematically study three different cases of droplet wetting, spreading, and moving across fibres, and droplets impacting mesh structures. First, I focus on partially wetting droplets moving along a fibre. For the so-called clamshell morphology, I find three possible dynamic regimes upon varying the droplet Bond number and the fibre radius: compact, breakup, and oscillation. For small Bond numbers, in the compact regime, the droplet reaches a steady state, and its velocity scales linearly with the driving body force. For higher Bond numbers, in the breakup regime, satellite droplets are formed trailing the initial moving droplet, which is easier with smaller fibre radii. Finally, in the oscillation regime (favoured in the midrange of fibre radius), the droplet shape periodically extends and contracts along the fibre. Outside of the commonly known fully wetting and partial wetting states, there exists the pseudo-partial wetting state (where both the spherical cap and the thin film can coexist together), which few numerical methods are able to simulate. I implement long-range interactions between the fluid and solid in LBM to realise this wetting state. The robustness of this approach is shown by simulating a number of scenarios. I start by simulating droplets in fully, partial, and pseudo-partial wetting states on flat surfaces, followed by pseudo-partially wetting droplets spreading on grooved surfaces and fibre structures. I also explore the effects of key parameters in long-range interactions. For the dynamics demonstration, I simulate droplets in the pseudo-partial wetting state moving along a fibre in both the barrel and clamshell morphologies at different droplet volumes and fibre radii. Finally, I focus on the dynamics of droplets impacting square mesh structures. I systematically vary the impact point, trajectory, and velocity. To rationalise the results, I find it useful to consider whether the droplet trajectory is dominated by orthogonal or diagonal movement. The former leads to a lower incident rate and a more uniform interaction time distribution, while the latter is typically characterised by more complex droplet trajectories with less predictability. Then, focussing on an impact point, I compare the droplet dynamics impacting a single-layer structure and equivalent double-layer structures. From a water-capturing capability perspective (given the same effective pore size), a double-layer structure performs slightly worse. A double-layer structure also generally leads to shorter interaction time compared to a single-layer structure