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

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

Mesoscopic modeling of a two-phase flow in the presence of boundaries: the Contact Angle

We present a mesoscopic model, based on the Boltzmann Equation, for the interaction between a solid wall and a non-ideal fluid. We present an analytic derivation of the contact angle in terms of the surface tension between the liquid-gas, the liquid-solid and the gas-solid phases. We study the dependency of the contact angle on the two free parameters of the model, which determine the interaction between the fluid and the boundaries, i.e. the equivalent of the wall density and of the wall-fluid potential in Molecular Dynamics studies. We compare the analytical results obtained in the hydrodynamical limit for the density profile and for the surface tension expression with the numerical simulations. We compare also our two-phase approach with some exact results for a pure hydrodynamical incompressible fluid based on Navier-Stokes equations with boundary conditions made up of alternating slip and no-slip strips. Finally, we show how to overcome some theoretical limitations connected with a discretized Boltzmann scheme and we discuss the equivalence between the surface tension defined in terms of the mechanical equilibrium and in terms of the Maxwell construction.Comment: 29 pages, 12 figure

Computational Studies of Liquid Droplet on Liquid Infused Surfaces

Surfaces that can repel any liquid are highly desired in various aspect of life for their wide range of beneficial applications. They can be utilised to enhance production processes, to simplify maintenance processes, to preserve surfaces from fouling, and many more. Ironically, one effective way to repel a liquid from a surface is by infusing the surface with another liquid which acts as a lubricant. Such surfaces are called liquid infused surfaces (LIS). The presence of the lubricant introduces rich interplay between the interfacial properties of the solid surface and the other fluid phases, which leads to many new interfacial phenomena. We employ two numerical methods for studying the behaviour of a liquid droplet on a LIS. First, we use the lattice Boltzmann method, which is powerful for studying the dynamic evolution of the system. Second, we use the phase field energy minimisation method, which is efficient for finding the equilibrium states of the system. In this thesis, we show how these numerical methods can be exploited to explore a wide range of LIS parameters and to confirm our theoretical predictions. We start by examining the equilibrium properties of the LIS system and demonstrate that the droplet morphology strongly depends on the choice of liquids used for the droplet-lubricant combination. The droplet morphology, in turn, affects the translational and rotational dynamics of the droplet under the influence of an external body force. Interestingly, we found a complex interplay between contact line pinning and viscous dissipation at the lubricant ridge, which become dominant at large and small apparent angles, respectively. Our investigations further demonstrate that the relative importance of viscous dissipation at the lubricant ridge depends on the drop to lubricant viscosity ratio, as well as on the shape of the wetting ridge. Next, we demonstrate spontaneous bidirectional motion of droplets on liquid infused surfaces in the presence of a topographical gradient, in which the droplets can move either toward the denser or the sparser solid fraction area. We show that the key factor determining the direction of motion is the preferential wetting of the droplet on the solid surface and on the lubricant film, which depends on the choice of the droplet-lubricant combination. Finally, we study how the pinning force of droplet on LIS is controlled by the solid surface fraction, the lubricant wetting angles, and the various fluid surface tensions. We derive an analytical prediction for contact angle hysteresis and numerically test the theory. We also discuss why a droplet on a liquid infused surface with partially wetting lubricants typically experiences stronger pinning compared to a droplet on a classical superhydrophobic surface

Capillary Effects on Fluid Transport in Granular Media

Fluid transport phenomena in granular media are of great importance due to various natural and industrial applications, including CO2 sequestration, enhanced oil recovery, remediation of contamination, and water infiltration into soil. Although numerous studies exist in the literature with aims to understand how fluid properties and flow conditions impact the transport process, some key mechanisms at microscale are often not considered due to simplifications of physical phenomenon and geometry, limited computational resources, or limited temporal/spatial resolution of existing imaging techniques. In this Thesis, we investigate fluid transport phenomena in granular media with a focus on the capillary effects. We move from relatively simple scenario on patterned surfaces to more complex granular media, tackling a variety of liquid-transport related problems that all have extensive industrial applications. The bulk of this Thesis is composed of six published papers. Each chapter is prefaced by an introductory section presenting the motivation for the corresponding paper and its context within the greater body of work. This Thesis reveals the impact of some previously neglected physical phenomena at microscale on the fluid transport in granular materials, providing new insights and methodology for describing and modelling fluid transport process in porous media

Exploring Stability Landscapes for Optimal Material Design: Application to Wetting of Structured Surfaces

Nature exhibits a diverse and sophisticated range of complex surface micro- and nanostructures which are highly adapted to manipulating liquids. Many exhibit surfaces which are efficient at shedding even wetting or pressurised liquids, self-cleaning, anti-fouling, antimicrobial, abrasion-resistant or able to produce strongly directional liquid motion; properties which are immensely desirable across a broad range of applications, from water purification to absorbent wipes. Here, optimised surface designs are produced for two wetting applications: superomniphobic surfaces for liquid-repellency, and enclosed fluid diodes for directional flow. In the superomniphobic investigation, we study three key wetting properties: the minimum energy barrier to the breakdown of liquid-repellency, the contact angle hysteresis (liquid mobility), and the critical pressure (maximum sustainable liquid pressure). We then treat all three properties simultaneously to produce optimal superomniphobic designs. In the fluid diode investigation, we study the critical pressures required for liquid to flow into and out of a membrane pore with both chemical and physical gradients. We then maximise the contrast between these two critical pressures, to design pores with optimal liquid directionality. Previously, two major hurdles have existed to such optimal design. The first is that the wetting properties on complex surface structures feature multiple competing mechanisms, which previously have been inefficient or expensive to investigate. In this thesis, we overcome this by employing and developing computationally efficient, high-dimensional energy landscape methods. The second is that when multiple wetting properties are desired, optimisation of one property can diminish another. We show how this can be overcome through simultaneous optimisation

Investigation on thermal and kinetic dynamics of droplets

In recent years, there has been a surge of interest in studying the dynamics of droplets due to their broad range of applications. However, despite their prevalence in nature, accurately predicting and controlling the various behaviours of droplets, such as evaporation and impingement, remains challenging due to the intricate underlying mechanisms involved. This thesis aims to address these challenges by consolidating existing literature and conducting a comprehensive investigation into the thermal and kinetic dynamics of droplets. Both numerical and experimental approaches were employed in this research. The numerical work utilised a multi-component multiphase pseudopotential Lattice Boltzmann model to simulate droplet dynamics. The focus was on studying the Marangoni effect by simulating droplet evaporation under localised heating. By introducing a non-uniform temperature distribution within the droplet, the temperature-induced Marangoni flow was generated, and its impact on the flow field and temperature distribution was analysed. Additionally, under certain conditions, the emergence of an asymmetrical droplet shape was observed, resulting in the droplet sliding on smooth surfaces. Another numerical simulation was conducted to investigate droplet impingement on a conical structure. To achieve an equilibrium wetting state on the inclined surface, a modified boundary condition scheme was proposed. The influence of gravity, surface wettability, and surface temperature on the impingement process was studied, and various outcomes were observed. Furthermore, the individual contributions of each factor were analysed. To complement the numerical investigations, an experimental study was conducted to further analyse droplet impingement on conical obstacles. High-speed cameras were employed to capture the impingement process, allowing for exploring factors not covered in the numerical research, such as cone angles and surface roughness. This study significantly enhanced the understanding of the thermal and kinetic dynamics of droplets and expanded the potential applications of droplets by leveraging their unique characteristics

Not spreading in reverse: the dewetting of a liquid film into a single droplet

Wetting and dewetting are both fundamental modes of motion of liquids on solid surfaces. They are critically important for processes in biology, chemistry and engineering, such as drying, coating and lubrication. However, recent progress in wetting, which has led to new fields such as superhydrophobicity and liquid marbles, has not been matched by dewetting. A significant problem has been the inability to study the model system of a uniform film dewetting from a non-wetting surface to a single macroscopic droplet – a barrier which does not exist for the reverse wetting process of a droplet spreading into a film. Here, we report the dewetting of a dielectrophoresis-induced film into a single equilibrium droplet. The emergent picture of the full dewetting dynamics is of an initial regime, where a liquid rim recedes at constant speed and constant dynamic contact angle, followed by a relatively short exponential relaxation of a spherical-cap shape. This sharply contrasts with the reverse wetting process, where a spreading droplet follows a smooth sequence of spherical-cap shapes. Complementary numerical simulations and a hydrodynamic model reveal a local dewetting mechanism driven by the equilibrium contact angle, where contact-line slip dominates the dewetting dynamics. Our conclusions can be used to understand a wide variety of processes involving liquid dewetting, such as drop rebound, condensation and evaporation. In overcoming the barrier to studying single film-to-droplet dewetting our results provide new ways of fluid manipulation and use of dewetting, such as inducing films of prescribed initial shapes and slip-controlled liquid retraction