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

Wetting Effect on Patterned Substrates

A droplet deposited on a solid substrate leads to the wetting phenomenon. A natural observation is the lotus effect, known for its superhydrophobicity. This special feature is engendered by the structured microstructure of the lotus leaf, namely, surface heterogeneity, as explained by the quintessential Cassie–Wenzel theory (CWT). In this work, recent designs of functional substrates are overviewed based on the CWT via manipulating the contact area between the liquid and the solid substrate as well as the intrinsic Young\u27s contact angle. Moreover, the limitation of the CWT is discussed. When the droplet size is comparable to the surface heterogeneity, anisotropic wetting morphology often appears, which is beyond the scope of the Cassie–Wenzel work. In this case, several recent studies addressing the anisotropic wetting effect on chemically and mechanically patterned substrates are elucidated. Surface designs for anisotropic wetting morphologies are summarized with respect to the shape and the arrangement of the surface heterogeneity, the droplet volume, the deposition position of the droplet, as well as the mean curvature of the surface heterogeneity. A thermodynamic interpretation for the wetting effect and the corresponding open questions are presented at the end

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

How Heterogeneous Pore Scale Distributions of Wettability Affect Infiltration into Porous Media

Wettability is an important parameter that significantly determines hydrology in porous media, and it especially controls the flow of water across the rhizosphere—the soil-plant interface. However, the influence of spatially heterogeneous distributions on the soil particles surfaces is scarcely known. Therefore, this study investigates the influence of spatially heterogeneous wettability distributions on infiltration into porous media. For this purpose, we utilize a two-phase flow model based on Lattice-Boltzmann to numerically simulate the infiltration in porous media with a simplified geometry and for various selected heterogeneous wettability coatings. Additionally, we simulated the rewetting of the dry rhizosphere of a sandy soil where dry hydrophobic mucilage depositions on the particle surface are represented via a locally increased contact angle. In particular, we can show that hydraulic dynamics and water repellency are determined by the specific location of wettability patterns within the pore space. When present at certain locations, tiny hydrophobic depositions can cause water repellency in an otherwise well-wettable soil. In this case, averaged, effective contact angle parameterizations such as the Cassie equation are unsuitable. At critical conditions, when the rhizosphere limits root water uptake, consideration of the specific microscale locations of exudate depositions may improve models of root water uptake

Lattice Boltzmann Study of Near-Wall Multi-Phase and Multi-Component Flows

Ph.DDOCTOR OF PHILOSOPH

Controlling drop morphology : theory, experiments and applications in printing, self-cleaning coatings and micro-fluidic systems

The accurate control of drop morphology as a drop is placed on a solid surface is an important prerequisite in many applications, such as (inkjet) printing of functional materials, micro-fluidic devices and smart coatings. Carefully patterning the surface on micrometer length scales and combining this with controlled drop placement is shown to allow the creation a variety of drop morphologies in both simple and complex (liquid crystalline) fluids, which is an important parameter in the above-described fields of interest. Starting from the governing thermodynamic equations determining the morphology of drops, the dominant energetic terms are identified for the different length scales of both the drop sizes and micro-structures in the substrate surface. For non-liquid crystalline fluids, these are gravitational potential energy, surface energy and contact line energy. To study the influence on the wetting behaviour on various patterned surfaces, the object of analysis was chosen to be a drop (or droplet) smaller than 1 mm in linear dimension. A combination of experiments, numerical modelling and theoretical analysis is used to explain the oftensurprising drop shape morphologies and their dependence on the deposition method. On surfaces patterned with parallel grooves (i.e. a corrugated surface), drops were found to elongate parallel to the grooves if the drops were deposited using a non-contact method, such as via inkjet printing or careful placement with a needle. However, if the drops were positioned in an overspread position (such as when pressed onto a surface with a contact printing technique such as micro-transfer printing), the drops elongate perpendicular to the corrugations. The key difference is that hysteresis due to contact line pinning is almost completely absent parallel to the corrugations and is present and significant perpendicular to them. Microtransfer printing with nematic thermotropic liquid crystal monomers leads to similar perpendicular elongations under similar experimental conditions, even when the energetic contributions due to the alignment of the liquid crystal director favour elongation parallel to the corrugations in the direction of alignment. Drops of water are shown to be able to exhibit a transition between two important wetting states by employing corrugated surfaces, combined with electrowetting and a high intrinsic contact angle of the surface. The transition from the collapsed (Wenzel) state to the suspended (also known as Cassie-Baxter) state was observed experimentally for the first time without having to heat the drop above the boiling point in order to lift it out of the corrugations. The mechanism of this lifting transition is also investigated in detail with numerical simulations. The analysis shows that only under carefully chosen conditions, which require the elimination of contact line pinning, it is possible to have such a transition spontaneously without other forces such as vibration are employed. The number of achievable morphologies of drops is extended to non-intuitive shapes such as octagons, hexagons, squares and quasi-triangular by employing surfaces patterned with micrometer sized posts. The modulation of the lattice according to which these posts are placed, as well as the shape of the posts itself, creates various drop shapes as the interface de-pins from the posts differently in different directions, also dependent on whether the drop is spreading or retracting. Experimental inkjet printing is combined with microscopy and numerical simulations to elucidate the local pinning of the interface. An important application of smart coatings is self-cleaning materials in for instance windshields, textiles or ship hulls. Liquid repellent surfaces are a particular example with great industrial relevance. An analysis of the stability of the suspended drop states is presented by employing a recently created experimental surface containing raspberry-shaped silica particles covered with lyophobic polymers. By carefully studying the complex wetting states possible and the transitions between them, design rules for stable liquid repellent surfaces are derived. The method of analysis is generalised so that in the future further surfaces can be analyzed in similar fashion. Finally, a number of new potential applications are discussed in a technology review, where also a view to future developments in the field is briefly discussed

How do chemical patterns affect equilibrium droplet shapes?

By utilizing a proposed analytical model in combination with the phase-field method, we present a comprehensive study on the effect of chemical patterns on equilibrium droplet morphologies. Here, three influencing factors, the droplet sizes, contact angles, and the ratios of the hydrophilic area to the hydrophobic area, are contemplated. In the analytical model, chemical heterogeneities are described by different non-linear functions. By tuning these functions and the related parameters, the analytical model is capable of calculating the energy landscapes of the system. The chemically patterned surfaces display complex energy landscapes with chemical-heterogeneity-induced local minima, which correspond to the equilibrium morphologies of the droplets. Phase-field (PF) simulations are accordingly conducted and compared with the predicted equilibrium morphologies. In addition, we propose a modified Cassie–Baxter (CB) model to delineate the equilibrium droplet shapes. In contrast to the classic CB model, our extension is not only restricted to the shape with a spherical cap. Both the energy landscape method and the modified CB model are demonstrated to have a good agreement with the PF simulations

Droplet dynamics on chemically heterogeneous substrates

Slow droplet motion on chemically heterogeneous substrates is considered analytically and numerically. We adopt the long-wave approximation which yields a single partial differential equation for the droplet height in time and space. A matched asymptotic analysis in the limit of nearly circular contact lines and vanishingly small slip lengths yields a reduced model consisting of a set of ordinary differential equations for the evolution of the Fourier harmonics of the contact line. The analytical predictions are found, within the domain of their validity, to be in good agreement with the solutions to the governing partial differential equation. The limitations of the reduced model when the contact line undergoes stronger deformations are partially lifted by proposing a hybrid scheme which couples the results of the asymptotic analysis with the boundary integral method. This approach improves the agreement with the governing partial differential equation, but at a computational cost which is significantly lower compared to that required for the full problem

Droplet dynamics on chemically heterogeneous substrates

Slow droplet motion on chemically heterogeneous substrates is considered analytically and numerically. We adopt the long-wave approximation which yields a single partial differential equation for the droplet height in time and space. A matched asymptotic analysis in the limit of nearly circular contact lines and vanishingly small slip lengths yields a reduced model consisting of a set of ordinary differential equations for the evolution of the Fourier harmonics of the contact line. The analytical predictions are found, within the domain of their validity, to be in good agreement with the solutions to the governing partial differential equation. The limitations of the reduced model when the contact line undergoes stronger deformations are partially lifted by proposing a hybrid scheme which couples the results of the asymptotic analysis with the boundary integral method. This approach improves the agreement with the governing partial differential equation, but at a computational cost which is significantly lower compared to that required for the full problem