Beim Stoffübergang einer grenzflächenaktiven Substanz in einem flüssigen Zweiphasensystem kann solutale Marangoni-Instabilität einsetzen. Die weitere nichtlineare Entwicklung der Marangoni-Instabilität geht mit einer enormen Vielfalt von Strömungsmustern einher. In der Literatur wird dieser Aspekt häufig unter dem unscharfen Ausdruck „Grenzflächenturbulenz“ zusammengefasst. Diese Arbeit stellt heraus, dass drei grundlegende Strukturformen existieren: Rollzellen, Relaxationsoszillationen und Relaxationsoszillationswellen. Ein großer Teil der Komplexität der Strömungsmuster ist dadurch begründet, dass die Grundstrukturen unterschiedliche Hierarchieebenen aufweisen. Es werden die zugrunde liegenden Bedingungen für das Auftreten der jeweiligen Strukturtypen, ihre transiente Natur und die Bildung der hierarchischen Strömungsmuster untersucht. Des Weiteren betrachtet diese Arbeit die Wechselwirkungen mit Dichteeffekten, die sowohl die Charakteristik der Strukturen als auch ihre zeitliche Entwicklung beeinflussen.

This dissertation aims to better understand the evolution of single hydrogen gas bubbles evolved during the water electrolysis at microelectrodes. In particular, the growth and detachment processes were studied in detail experimentally by means of electrochemical and optical methods in terrestrial, micro-, and hypergravity conditions. The combination of microelectrode and sulfuric acid promoting the bubble coalescence results in a periodical growth and the detachment of single bubbles. This provides a systematic view on the phenomena under study. A shadowgraphy system was used to provide general insight into the bubble behaviour, while Particle Tracking Velocimetry (PTV) was used for the flow velocity measurements around the growing hydrogen bubble. By applying high electric potentials considerably exceeding that in industrial electrolysers, it is possible to analyse the evolution of hydrogen bubbles under extreme conditions and for a wide range of electrolyte concentrations, overall shedding more light on bubble dynamics in general, and especially the underlying balance of forces. The growth of single hydrogen bubbles at micro-electrodes was studied in an acidic electrolyte over a wide range of concentrations and cathodic potentials. New bubble growth regimes were identified which differ in terms of whether the bubble evolution proceeds in the presence of a monotonic or oscillatory variation in the electric current and a carpet of microbubbles underneath the bubble. Key features such as the growth law of the bubble radius, the dynamics of the microbubble carpet, the onset time of the oscillations and the oscillation frequencies were characterised as a function of the concentration and electric potential. Furthermore, the system's response to jumps in the cathodic potential was studied. The electrode, tilted to the horizon, promotes faster growth and, therefore, earlier detachment at the smaller volume of the bubble. During its evolution, the bubble moves laterally from the electrode centre, releasing the electrode area and enabling higher electric current, therefore faster hydrogen generation and bubble-bubble coalescence rates. The duration of the bubble position oscillations found on the horizontal electrode gradually reduces upon tilt angle increase, with an almost complete disappearance at 5°. Based on the analysis of the forces involved and their scaling with the concentration, potential and electric current, a sound hypothesis was formulated regarding the mechanisms underlying the micro-bubble carpet and oscillations. A detailed look was also taken on the dynamics of single hydrogen bubbles in microgravity during parabolic flights. Three bubble evolution scenarios were identified depending on the electric potential applied and the acid concentration. The dominant scenario, characterised by lateral detachment of the grown bubble, was studied in detail. For that purpose, the evolution of the bubble radius, electric current and bubble trajectories as well as the bubble lifetime were comprehensively addressed for different potentials and electrolyte concentrations. The bubble-bubble coalescence events, which are responsible for reversals of the direction of bubble motion, were particularly analysed. Finally, as parabolic flights also permit hypergravity conditions, a detailed comparison of the characteristic bubble phenomena at various levels of gravity was drawn. Finally, the Marangoni convection at the foot of hydrogen gas bubbles mainly induced by the thermocapillary effect is systematically studied during the bubble evolution, the bubble position oscillations, at horizontal and tilted electrodes both in terrestrial and hyper-g environments. The flow structure progressively modifies with the bubble evolution or during the bubble position oscillations, i.e. as per electric current and bubble geometry variation. The velocity increases both with the bubble size and the electric current magnitude. It reaches up to 50 mm/s and 125 mm/s shortly before the bubble detachment at horizontal and tilted electrodes, correspondingly. The bubble position oscillations characterised by the large variation of the electric current govern the velocity of around ~80 mm/s at the highest and ~40 mm/s at the lowest positions. In the case of tilted electrodes, both in terrestrial and hyper-g environments, the lateral movement of the bubble enables higher values of the current and, therefore, stronger convection. The non-homogeneous distribution of the electric current lines at the tilted electrode results in the asymmetrical Marangoni convection around the bubble. There is a certain limitation in terms of the maximal magnitude of the velocity at different tilt angles, governed by the optimal size of the bubble and electric current. At last, the effects of the particles and laser used for PTV measurements were shown to reduce the duration of the oscillations and to retard the bubble evolution. Both effects were considered during the measurements.

Present work is focused on the numerical study of evaporation of sessile liquid droplets on top of smooth solid substrates. The process of evaporation of a sessile liquid droplet has lots of different applications both in industry and research area. This process has been under study for many years, and still it is an actual problem, solution of which can give answers on some fundamental and practical questions. Instantaneous distribution of mass and heat fluxes inside and outside of an evaporating sessile droplet is studied in this research using computer simulations. The deduced dependences of instantaneous fluxes are applied for self-consistent calculations of time evolution of evaporating sessile droplets. The proposed theory of evaporating sessile droplets of liquid has been validated against available experimental data, and has shown a good agreement. Evaporation of surfactant solution droplets is studied experimentally. The theory, proposed for two stages of evaporation, fits experimental data well. An additional evaporation stage, specific for surfactant solutions, is observed and described. Mathematical modelling of this stage requires further research on surfactant adsorption and its influence on the value of receding contact angle. Numerical study of the evaporation of microdroplets is conducted in order to evaluate the significance of different evaporation mechanisms (diffusive and kinetic models of evaporation) and different physical phenomena (Kelvin s equation, latent heat of vaporization, thermal Marangoni convection, Stefan flow).

No description available.

Marangoni flows are free-surface flows driven by gradients of surface tension. Because surface tension depends on chemical composition, Marangoni flows may be generated by the uneven distribution of surface-active species at an interface. The primary goal of this thesis is to develop a rigorous computational framework for the simulation of the fluid dynamical and interfacial phenomena underlying the physics of Marangoni flows. The focus is on characterizing the different dynamical regimes generated by the presence of surface-active species (surfactants) at an interface. The computational framework was developed using direct numerical simulation, that is, by simultaneously solving the full system of partial differential equations governing the free-surface flow and the surfactant transport on a continually deforming interface. Results from the simulations enabled detailed examination of the interfacial mechanisms of surfactant transport and provided a comprehensive picture of the free-surface flow. Analysis of the results established limits of applicability of scaling solutions previously proposed in the literature, calculated the necessary corrections, and also lead to the discovery of previously unobserved scaling laws in viscous Marangoni flows. New findings from this research not only enhance the fundamental understanding of the physics of Marangoni flows, but also the ability to accurately predict the behaviour of Marangoni flows and the associated transport of surface-active species, which is critical to the understanding of important natural and biomedical processes, ranging from the surfactant-driven propulsion of insects and microorganisms to the spreading of drugs and natural surfactants (proteins) in the eye and lungs. Controlled Marangoni transport of chemical species is also relevant to a wide range of environmental and technological processes, with applications ranging from cleaning of oil spills to coating of microfluidic devices.

<div>The evaporation of a liquid drop containing particles resting on a substrate have diverse industrial applications including inkjet printing, spray coating, fabrication of functional nanomaterials, disease diagnosis, among others. In addition to these wide ranging practical applications, the sessile drop evaporation can be observed in everyday life with dew drops, coffee spills, and the dry patterns of other beverages.</div><div><br></div><div>The self-assembly of particles during drop evaporation is a process that is affected by various factors, such as contact line (CL) behaviors, microfluidic flows, short-range interactions of particle-interface and particle-particle. Each of these factors are complicated enough to study, let alone the total effects on the process. The primary goal of this work is to investigate the influence of microfluidic flows and the particle-interface interaction, viz. the evaporation process was subject to a pinned CL and the particle-particle interaction was neglected under dilute particle concentration. </div><div>To accomplish this goal, the Galerkin/Finite Element Method (G/FEM) is used to solve for the flow, the temperature and the particle concentration profiles. </div><div><br></div><div><br></div><div>The complexity of the problems comes from various surface phenomena, one of which is the surface tension. The surface tension brings capillary force in the normal direction and capillary flow toward the CL, which results in the well-known coffee-ring effect. Moreover, the surface tension changes with temperature, surfactant concentration, etc. resulting in Marangoni stresses in the tangential direction. The Marangoni stress on the surface leads to circulations of flow inside the drop and the circulation can be either clockwise or counterclockwise depending on the direction of the stress. </div><div><br></div><div>When the Marangoni stress is merely caused by temperature change, the circulation direction changes not only in time but also in space. At late stage of evaporation, i.e. with a small contact angle (CA), multi-circulation flow profiles emerge. This flow profiles are featured with stagnation points and transition points. The stagnation points can be further categorized into capillary-induced stagnation points and Marangoni-induced stagnation points. By introducing the concept of capillary-induced stagnation points, the simulations reached agreement with experiments in terms of the radial location of the observed stagnation points.</div><div><br></div><div><br></div><div>The multi-circulation flow profiles implied regional segregation inside the drop. When a large circulation is observed in most part of the drop and a small circulation exists near the CL, particle concentrations are relatively uniform in each individual region but differs significantly across the two regions. Transition points are used to characterize the location of the regional segregation, which can be adjusted by Marangoni stress.</div><div><br></div><div><br></div><div>Marangoni circulations in different directions revealed distinct influences on particle distribution and deposition. First, while both directions facilitate even distribution of particles, a clockwise circulation strengthens CL accumulation for a small Marangoni stress. Second, a counterclockwise circulation with a small Marangoni stress impedes the deposition rate of particles, while a clockwise circulation facilities the deposition no matter how small the Marangoni stress is. This results is under a condition of a strong adsorption between particles and substrates. </div><div><br></div><div>The analysis and understanding of the above results are crucial to elucidating and controlling the final deposition patterns of particles. Thus, the focus of this research is to understand the combined effect of Marangoni stress and capillary flow on particle deposition during sessile drop evaporation.</div><div><br></div>

This thesis examines the mechanisms of magnetic particle deposition and retention in human airways for magnetically targeted drug delivery. As this is a novel application, fundamental studies were performed to establish the necessary background knowledge for further development. Magnetic particle deposition from an aerosol in simulated airway conditions was studied using numerical and experimental models. The model results showed qualitative agreement; discrepancies were due to particle aggregation, which enhances deposition. Aerosol flow rate had a limited effect; the main factor in effective deposition was the proximity of the particle trajectories to the magnets. This spatial bias shows the importance of particle distribution in the flow as well as magnetic field geometry. These studies demonstrated the feasibility of capturing magnet particles from aerosol in airway conditions. For retention, clearance of particles due to motion of the mucus lining of the airways must be overcome. Particle retention was studied in vitro using various liquids to simulate mucus and identify relevant parameters. An ex vivo animal tissue model was used to demonstrate feasibility. Retention of 3-5 m diameter iron particles was achieved at reduced liquid/mucus viscosities. Larger (~100 m) particles were retained at normal mucus viscosities. The size dependence shows that particle aggregation after deposition is crucial for effective retention. In vitro retention experiments showed aggregate size is correlated with liquid viscosity, i.e. formation of aggregates is limited by forces opposing particle motion along the mucus layer interface. To determine these forces, particle motion on various air-liquid interfaces, chosen to simulate different mucus properties in isolation, was studied. When surfactants are present, as in the mucus layer, particle motion is limited by a velocity-dependent surface tension gradient as well as viscous drag. Pulling particles through the mucus layer into the tissue beneath was also considered as a potential retention strategy. The force required to pull particles through the mucus layer was also studied using various liquids to simulate mucus properties. In addition to the surface tension force holding the particles at the interface, hydrodynamic forces must be overcome to pull particles into or out of a liquid film such as the mucus layer.

The molten pool shape and thermocapillary convection during melting or welding of metals or alloys are self-consistently predicted from scale analysis. Determination of the molten pool shape and transport variables is crucial due to its close relationship with the strength and properties of the fusion zone. In this work, surface tension coefficient is considered to be negative, indicating an outward surface flow, whereas high Prandtl number represents a thinner thickness of the thermal boundary layer than that of momentum boundary layer. Since Marangoni number is usually very high, the domain of scaling is divided into the hot, intermediate and cold corner regions, boundary layers on the solid-liquid interface and ahead of the melting front. The results find that the width and depth of the pool, peak and secondary surface velocity, and maximum temperatures in the hot and cold corner regions can be explicitly and separately determined as functions of working variables or Marangoni, Prandtl, Peclet, Stefan, and beam power numbers. The scaled results agree with numerical data, different combinations among scaled equations, and available experimental data.

The molten pool shape and thermocapillary convection in melting or welding of metals or alloys having negative surface tension coefficients and Prandtl number greater than unity are determined from a scale analysis. Negative surface tension coefficient indicates that the surface flow is in outward direction, while Prandtl number greater than unity represents that boundary layer thickness of conduction is less than that of momentum. Determination of the molten pool shape is crucial due to its close relationship with the strength, microstructure and properties of the fusion zone. Since Marangoni and Reynolds number are usually greater than ten thousands, transport processes can be determined by scale analysis. In this work, the molten pool is divided into the hot, intermediate and cold corner regions on the flat free surface, boundary layers on the solid-liquid interface and ahead of the melting front for analysis. The results find that the pool shape, surface speed and temperature profiles can be self-consistently evaluated as functions of Marangoni, Prandtl, Peclet, Stefan, and beam power numbers. The predictions agree with numerical computations and experimental data in the literature.

The Steam-Assisted Gravity Drainage (SAGD) Process involves the injection of steam through a horizontal well and the production of heavy oil through a lower horizontal well. Several authors have tried to model this process using analytical, semi-analytical and fully numerical means. In this dissertation, we improve the predictive ability of previous models by accounting for the effect of anisotropy, the effect of heat transfer on capillarity and the effect of water-in-oil (W/O) emulsion formation and transport which serves to enhance heat transfer during SAGD. We account for the effect of anisotropy during SAGD by performing elliptical transformation of the resultant gravity head and resultant oil drainage vectors on to a space described by the vertical and horizontal permeabilities. Our results, show that unlike for the isotropic case, the effect of anisotropy is time dependent and there exists a given time beyond which it ceases to have any effect on SAGD rates. This result will impact well spacing design and optimization during SAGD. Butler et al. (1981) derived their classical SAGD model by solving a 1-D heat conservation equation for single phase flow. This model has excellent predictive capability at experimental scales but performs poorly at field scales. By assuming a linear saturation -- temperature relationship, Sharma and Gates (2010b) developed a model that accounts for multiphase flow ahead of the steam chamber interface. In this work, by decomposing capillary pressure into its saturation and temperature components, we coupled the mass and energy conservation equations and showed that the multi-scale, multiphase flow phenomenon occurring during SAGD is the classical Marangoni (or thermo-capillary) effect which can be characterized by the Marangoni number. At low Marangoni numbers (typical of experimental scales) we get the Butler solution while at high Marangoni numbers (typical of field scales), we approximate the Sharma and Gates solution. The Marangoni flow concept was extended to the Expanding Solvent SAGD (ES-SAGD) process and our results show that there exists a given Marangoni number threshold below which the ES-SAGD process will not fare better than the SAGD process. Experimental results presented in Sasaki et al. (2002) demonstrate the existence of water-in-oil emulsions adjacent to the steam chamber wall during SAGD. In this work we show that these emulsions enhanced heat transfer at the chamber wall and hence oil recovery. We postulate that these W/O emulsions are principally hot water droplets that carry convective heat energy. We perform calculations to show that their presence can practically double the effective heat transfer coefficient across the steam chamber interface which overcomes the effect of reduced oil rates due to the increased emulsified phase viscosity. Our results also compared well with published experimental data. The SAGD (and ES-SAGD) process is a short length-scaled process and hence, short length-scaled phenomena (typically ignored in other EOR or conventional processes) such as thermo-capillarity and in-situ emulsification should not be ignored in predicting SAGD recoveries. This work will find unique application in predictive models used as fast proxies for predicting SAGD recovery and for history matching purposes. / text