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

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

Droplet deposition and evaporation dynamics on chemically and topographically patterned surfaces

Inkjet printing is a promising alternative technique for the fabrication of functional devices such as organic light emitting diode displays. However, with ever increasing requirements for finer display resolutions, it becomes increasingly challenging to precisely position inkjet printed droplets. Even once the droplet is in the required location, there are challenges in achieving a uniform particle deposit of the functional material, once the solvent evaporates. In this thesis, a multiphase lattice Boltzmann method is used to investigate the deposition processes of droplets deposited into idealised pixel geometries (square cavities). Specific attention is given to droplets deposited with positioning errors, to see which factors have the greatest influence on the droplets ability to self-align. Additionally, the model is coupled with an energy equation to investigate cavity properties on evaporation rate, internal flows, and particle deposition. A review of different multiphase models leads to the choice of the pseudopotential method, as recent developments allow for the simulation of moderate density ratios, thermodynamic consistency, and the ability to couple with an energy equation to simulate thermal flows with phase change. Implementation is then discussed, with attention given to parallelising the multiphase algorithm to run on high-performance computers. Different wetting models are evaluated, and a new model is suggested, which allows for additional control of adhesive forces over the liquid-vapour interface. Furthermore, the importance of boundary treatment in computing the pseudopotential forces is highlighted. The new wetting model is used to explore the limits of positioning error for the deposition of droplets into square cavities. A regime map is suggested which highlights the conditions required for print success, relating droplet size, cavity size, and printer positioning errors. Finally, investigations of evaporation in heated square cavities show the influence of receding contact angle on evaporation rate, internal flows, and particle deposition

Strukture polja v aktivnih in pasivnih tekočih kristalih

Field structures are developed in passive and active nematic fluids. These are field profiles that are determined by confinement, particles, flow and external fields. Our central methodological approach is numerical modeling based on free energy minimization with finite difference method and flow modeling with hybrid lattice Boltzmann method. We develop structures by combining concepts of topological defects, external confinement and colloidal particles. Ordering properties of horseshoe nematic colloidal particles with planar degenerate anchoring are investigated with numerical modeling, where we optimize their geometrical parameters such that the particle exhibit attractive interactions and can self assemble into 2D and even 3D colloidal crystals. The metamaterial response of horseshoe colloids that perform as split ring resonators is studied. Optical cloaking is achieved by generating polymer microstructures embedded directly within a electric field switchable liquid crystal device. Using numerical modelling we explore the director field structures forming in the vicinity of composite colloidal particles with specially designed conic anchoring, which are assumed to induce high multipoles. Simple rule that allow predictions of multipolar moment from defect configuration is extracted. Starting with a gyroid structure, which is a photonic crystal by itself, we introduce an achiral and chiral nematic into one labyrinth of channels with homeotropic anchoring. Complexly shaped channels induce both ordered and disordered structures of defects. Simulating the passive nematic flow in porous microchannels we study the formation of individual umbilic defects of various strength and umbilic defect lattices that arise as the consequence of complex velocity field containing both multiple peaks and saddles. We investigate the 3D active turbulence in droplets of active nematic with homeotropic and non slip boundary condition. The transition from the point defect to the active turbulence is studied by analysing both the topological defects and corresponding events as well as flow. More generally, this work is aimed at the development of novel functional soft matter, which can exhibit exciting and unusual material characteristics, including light guiding, topological defect states, photonic bandgaps, metamaterials and optical cloaking.V doktorskem delu smo razvili strukture polja v pasivnih in aktivnih nematskih tekočinah. Ti profili v polju so določeni z ograditvijo, delci, tokom in zunanjimi polji. Osrednji raziskovalni pristop je numerično modeliranje, ki temelji na minimizaciji proste energije z metodo končnih diferenc, in modeliranje toka s hibridno mrežno Boltzmannovo metodo. Ustvarjene strukture so rezultat kombinacije topoloških defektov, zunanje ograditve in koloidnih delcev. Preučevali smo urejanje podkvastih koloidnih delcev s planarnim sidranjem. Geometrijske parametre koloidnega delca smo optimizirali tako, da so delci medsebojno interagirali privlačno in so se lahko sestavili v 2D in tudi 3D koloidne kristale. Študirali smo tudi metamaterialni odziv tovrstnih podkvastih koloidov, ki se obnašajo kot resonatorji. Pokazali smo optično zakrivanje z ustvarjanjem polimernih struktur direktno v tekočekristalni celici, nastavljivi z električnim poljem. S pomočjo numeričnega modeliranja smo raziskali strukture v nematskem polju, ki se formirajo v okolici kompozitnih koloidnih delcev s posebnim koničnim sidranjem in ustvarjajo višje multipolne momente. Predstavimo tudi preprosto pravilo, s katerim lahko napovemo multipolni moment samo z opazovanjem defektnih struktur. V enega od obeh prepletov kanalov, v giroidni strukturi, uvedemo kiralni in nekiralni nematski tekoči kristal. Kompleksna oblika kanalov povzroči nastanek tako urejenih, kot tudi neurejenih defektnih struktur. Simuliramo pasivni nematski tok v poroznih mikrokanalih in študiramo nastanek umbiličnih defektov različnih moči ter regularnih mrež umbiličnih defektov, ki nastanejo zaradi sedelnih in ekstremalnih točk v toku. Preučimo 3D aktivno turbulenco v kapljicah aktivnega nematika s homeotropnimi robnimi pogoji. Študiramo prehod iz točkastega defekta v topološko turbulenco z analizo topoloških defektov in topoloških dogodkov, kot tudi z analizo samega toka. To delo je torej namenjeno razvoju nove funkcionalne mehke snovi, ki ima zanimive lastnosti, kot so na primer vodenje svetlobe, topološka defektna stanja, fotonske reže, metamateriali in optično zakrivanje

Microfluidics and Nanofluidics Handbook

The Microfluidics and Nanofluidics Handbook: Two-Volume Set comprehensively captures the cross-disciplinary breadth of the fields of micro- and nanofluidics, which encompass the biological sciences, chemistry, physics and engineering applications. To fill the knowledge gap between engineering and the basic sciences, the editors pulled together key individuals, well known in their respective areas, to author chapters that help graduate students, scientists, and practicing engineers understand the overall area of microfluidics and nanofluidics. Topics covered include Finite Volume Method for Numerical Simulation Lattice Boltzmann Method and Its Applications in Microfluidics Microparticle and Nanoparticle Manipulation Methane Solubility Enhancement in Water Confined to Nanoscale Pores Volume Two: Fabrication, Implementation, and Applications focuses on topics related to experimental and numerical methods. It also covers fabrication and applications in a variety of areas, from aerospace to biological systems. Reflecting the inherent nature of microfluidics and nanofluidics, the book includes as much interdisciplinary knowledge as possible. It provides the fundamental science background for newcomers and advanced techniques and concepts for experienced researchers and professionals

Microdroplets: fabrication of microdevices for interfacial phenomena studies

When fluids are confined on the length scales of microfluidic channels, typically in the range of tens and hundreds microns, their behavior may results significantly different with respect to the so called “bulk” proprieties. This is mainly due to the fact that the miniaturization is always characterized by a large surface to volume ratio, where the body forces can be normally neglected in favor of the surface forces. Notable example of this kind of systems is observable when two immiscible fluids are mixed to form droplets of emulsions. In the last ten years, the idea to use droplets in microfluidics has been inspired mainly because it allows to further scale down the typical size involved in these systems, bringing to a huge number of applications in chemistry, biology and physics. However, despite a large notoriety, microfluidic systems using droplets are not yet fully understood for the complexity of the interfacial phenomena that are involved. Aim of this thesis is to characterize the droplet systems commonly used in microfluidic devices. In detail, we worked with droplets in both open and closed microfluidic systems, focusing with the problem of their generation, control and manipulation with suitable microdevices, in presence of defects having different geometry and wettability. Regarding the open microfluidics, in Chapter 3,we first compared the shape of water droplets confined on posts having circular and square cross sections, observing that the pinning of the contact line is strongly influenced by the post shape. In particular, in the case of a circular profile, the contact line is pinned to the whole edge, confirming the Gibbs criteria, while on the square post, the contact line can spill along the vertical walls, because it is sustained by the corners. Then, in Chapter 4,we moved to investigate the change of morphological configuration from filament to bulge state, typical of liquid droplets confined on posts with rectangular cross section. This effect was already know in literature, but it was not quantify in term of post geometry and volume of the water droplets. Therefore, we realized rectangular posts with different aspect ratio ("l"), between length (L) and width (W). Changing the water volume on the posts, we observed that the morphological transition occurs for all the aspect ratios "l" and that, for "l">16, there is a bistability of the two states at the same volume. Furthermore, we started to investigate the dynamic of the transition, induced by oscillations, founding that, for posts with "l">16, it is possible to induce the transition by the oscillation, without change the volume. Next, in order to control the droplets motion, in Chapter 5, we studied the different behavior of sliding droplets on homogeneous and on chemically patterned surfaces. To do that we realized surfaces with hydrophilic and hydrophobic stripes by microcontact printing. On these surfaces, droplets show stick-slip motion, which causes the deformation of their shapes and introduces an extra friction imputable to the dissipation of energy at the contact line. With the aim to study generation and control of droplets in closed microfluidic channels, in Chapter 6, we focused our attention to define a reliable protocol for the production of droplets by T-junctions. Moreover, we investigated the swelling problem, which occurs using organic solvent into PDMS microchannels. We noticed that the swelling deformation is strongly connected with the geometry of the devices, being more evident when the aspect ratio (high to width) of the channel cross section is higher. Finally, in Chapter 7, we introduced a new method to change the wettability proprieties of thiolen resins, which are commonly used in microfluidics. In particular we worked with NOA, a commercial available resin, which shows a contact angle of 70°. Using chlorosilane chemistry, we changed its wettability to a more hydrophilic and to hydrophobic contact angles, showing that this technique can be used both to open and closed microfluidic device

Optical Deformation of Microdroplets at Ultralow Interfacial Tension

What is the shape of a droplet? Its interfacial tension dictates that it is very close to a perfect sphere. Herein, the interfacial tension is reduced to ultralow values (0.1 - 100 uN/m) by careful formulation of surfactant additives, such as for mixtures that form microemulsions. The droplet need not be spherical but can accommodate external forces of a similar magnitude. The control and precision of forces afforded simply by light - in the form of highly focused Nd:YAG laser beams - are exploited in this work to deform hydrocarbon oil-in-water emulsion droplets of 1-10 um diameter. To this end, a novel, integrated platform for microfluidic generation, optical deformation and 3D fluorescent imaging of droplets is presented. Previous attempts to characterise optically-controlled microdroplet shapes have been limited to 2D projections. Here, that ambiguity is resolved using 3D confocal laser scanning- and structured illumination microscopy. 2D and 3D arrays of up to four Gaussian point traps are generated by holograms and acousto-optics. A variety of regular, prolate, oblate and asymmetric shapes are produced and correlated with parameters such as optocapillary number, trap separation and capillary length. Exotic shapes exhibiting zero or negative mean and Gaussian curvatures are presented alongside their brightfield counterparts. The complex phase behaviour of emulsion droplets and their parent phases is observed to couple strongly to thermal absorption of the beams. The rich interfacial chemistry, its relation to the forces determining droplet shape and the surprising ability to create nanofluidic networks between droplets are investigated