Title Promoter Affiliations Abstract "Experimental study of the impact dynamics of rheologically complex repeated droplets on different substrates" "Patrick Anderson, Ruth Cardinaels" "Soft Matter, Rheology and Technology (SMaRT)" "Droplet impact dynamics for rheologically complex fluids onto different substrates is relevant in various areas, for instance for droplet-on-demand printing. However, current investigations mainly focus on the impact of non-repeated single droplets of Newtonian fluids under isothermal conditions. The research in this PhD project will investigate the impact dynamics and heat transfer characteristics of repeated single and repeated multiple droplet impacts on a range of different micro-/nanostructured surfaces using non-Newtonian fluids. Thereto, a combination of experiments and theoretical considerations will be utilized." "Development of a remote sensing technique for airborne droplets and particles using the glory and speckle patterns" "Steve Vanlanduit" "Faculty of Engineering, Applied Mechanics" "Since 1990, more than one hundred engine power-loss events have occurred, on both commuter and large transport airplanes, due to the presence of ice crystals in convective weather. These incidents could have been prevented if a reliable particle characterization instrument would have been available. In some cases, in order to avoid these hazardous conditions, pilots have been using a visual inspection of what is known as the glory optical phenomenon. This phenomenon is an interference pattern that appears when the liquid water droplets in the clouds scatter the sun light at an angle of 180° (full backscatter). During my Research Master thesis, different interference patterns that appear in the backscatter region were studied in order to detect ice formation within single water droplets. The glory phenomenon was used here to determine the diameter of single water droplets. Although the conclusions obtained by this study are still far from being applicable, the promising results motivate this project to further investigate these phenomena. The final aim of this project is thus to use the backscattered laser light to develop a quantitative and reliable measurement technique for particle characterization. The mentioned technique will allow us to measure size distribution and concentration of a wide range of particles (spherical and transparent droplets, irregular and opaque particles) in the size range from microns to several millimeters. Up to date, several optical characterization principles and even commercial instruments already exist (e.g. based on laser scattering, digital imaging and laser diffraction). These techniques are very useful at lab scale, but they have important limitations in case of in-situ and harsh environments. Therefore, the technique developed during this project, which is not limited to lab scale, is not only essential for aircraft safety, but can be used as well in many other applications, such as spray characterization for the pharmaceutical and agricultural industry, painting, liquid metals cooling, fire safety, etc. In order to develop a technique based on the interference patterns that appear in the backscattered light, i.e. the glory and speckle patterns, a systematic approach with increasing problem complexity is proposed. A preliminary study of these phenomena will be performed through the investigation of the interference patterns produced by single particles. Afterwards, the glory and the speckle pattern will be investigated for multiple droplets (first liquid water sprays and then flows of ice or sand particles). Finally, mixed phase conditions will be investigated. Common tasks exist for the different kind of particles, i.e.: the correct understanding of the phenomena, the development of a suitable inversion algorithm for particle size determination and the validation of the technique. This project will benefit from the collaboration of two laboratories: the Environmental and Applied Fluid Dynamics department of the von Karman Institute (VKI-EA) and the department of Mechanical Engineering in the Vrije Universiteit Brussel (VUB-MECH). The VKI has ample experience in optical techniques and in the development of new optical instrumentation (Particle Image Velocimetry in 1980, first precommercial Laser Doppler Velocimetry in 1972, Global rainbow instrument in 1999). Furthermore, it possesses several wind tunnels and optical facilities, which make the VKI an excellent environment to develop the project. In addition, VUB-MECH has vast experience in system identification and inverse problems. Through this collaboration, the project will benefit from all the necessary tools and knowledge needed to bring it to a successful end ." "The Effects of Particles on the Dynamics of Sheared Droplets" "Paula Moldenaers" "Soft Matter, Rheology and Technology (SMaRT)" "Polymer blending is a commonly used technique to create new polymer materials that combine the properties of their components. Polymer blends are multiphasic materials and hence, their final material properties not only depend on the type of polymers and concentration, but also on the final blend microstructure. This microstructure is formed during processing wherein a complex flow field is applied. Hence, understanding the relation between the applied flow during processing and the structure development in polymer blends is crucial to tailor and design their material properties. When dealing with blends that contain (nano)particles at the interface or in the bulk, new possibilities to tune their properties become feasible. Solid particles are therefore often added to polymer blends to improve their performance or introduce an additional functionality. The morphology of these blends can be substantially affected by the addition of particles and can be investigated by optical microscopy, e.g. in a counter-rotating device. In the long run, understanding and generating concentrated blends with controlled interfacial characteristics will be the ultimate challenge. Finally, rather than having a structured interface these materials can have a structured matrix or internal phase that imparts an additional degree of freedom for controlling the development of the microstructure. However, the intrinsic effects of particles on the structure development remains essentially unexplored and is still poorly understood. Therefore, the effects of particles on the structure development of blends are investigated in this dissertation by means of a fundamental study of the single droplet dynamics. The dynamics of single particle-filled and particle-covered droplets are studied microscopically with a counterrotating shear cell." "From Stable Water-based Titania Colloidal Suspension Preparation to Film Formation from Drying Droplets" "Wim Thielemans" "Chemical Engineering, Kulak Kortrijk Campus" "Despite the limitations associated with titania regarding its unsustainable production and potential toxicity, this material is still used in a wide range of applications thanks to its unique properties. While titania can be replaced by components such as zinc oxide in some applications, in others, titania is likely to prevail until suitable alternatives are found, if there ever are. In the meantime, one can still take advantage of the properties of titania in a safer and more sustainable manner. This begins with better understanding titania, the goal of this thesis.This thesis starts with a meta-analysis of literature data (Chapter 3) where key synthesis parameters for the most sustainable titania production approaches, namely sol-gel and hydrothermal methods, are identified, analyzed, and categorized. This resulted in ranking reaction parameters depending on their impact on the properties of the resulting titania nanoparticles. Chapter 4 takes a more pragmatic approach by focusing on the impact of acid catalyst type on the water-based sol-gel synthesis of these nanoparticles. This study enabled to identify the reaction conditions leading to the optimum trade-off of sample purity and nanoparticle crystallinity. The colloidal stability of the resulting titania nanoparticle suspensions is investigated in Chapter 5. This investigation was conducted using several methods, including synchrotron small angle X-ray scattering, leading to elucidating the shape of fractal aggregates formed inside the as-prepared aqueous titania suspensions as a function of their concentration. The added value of Chapter 5 is to explicate these fractal structures and the aggregation mechanism for titania nanoparticles in aqueous suspensions. Aware of this aggregation mechanism, Chapter 6 studied the drying behavior of titania nanoparticle aqueous suspensions. The most important result in this chapter is showing that the quasi-steady state diffusion based solute transfer models, which have largely been successfully applied to describe drying dynamics and pattern formation for highly stable nanoparticle suspensions, are also valid for aggregation-prone suspensions like aqueous suspensions of titania nanoparticles.Although titania has been extensively studied from various perspectives and for a broad range of applications, this thesis shows that there is still more left to be understood from this transition metal oxide. In the perspective of delivering more sustainable solutions for titania production, this thesis focused on water-based systems prepared at room temperature. Of course, working under such mild conditions has shortcomings like the mixed crystalline phases for the resulting titania nanoparticles, instead of the typical higher purity from less sustainable processes. However, for applications which do not require a high level of crystalline phase purity, these mild conditions could be an alternative to a more sustainable titania production. Regarding the safer alternatives, this dissertation dealt with never dried suspensions prepared in situ from titania precursor. This never dried aspect looks like an alternative to the common manner of preparing titania dispersions from dispersing titania powders in liquid formulations." "Magnetomobility of Paramagnetic Ions and Solution Droplets" "Jan Fransaer" "Sustainable Chemistry for Metals and Molecules, Surface and Interface Engineered Materials" "Separation of rare earth ions with strong magnetic fieldsRare earths elements (REEs) are a group of 17 elements in the periodic table, including the 15 lanthanides, scandium and yttrium. These elements are today critical because of their essential role in applications such as permanent magnets, lamp phosphors, catalysts, rechargeable batteries, etc. They occur in nature as mixtures and are very difficult to separate into the individual elements due to almost identical chemical properties. Traditional separation methods, based of chemical properties, demand fastidious steps and are not environmental friendly. Our objective is to explore magnetic properties of individual rare earth ions ions (paramagnetic - attracted to a magnetic field; and diamagnetic - repelled by a magnetic field) to develop an unconventional process of separation. During the 50’s, Noddack et al. obtained a partial separation of rare earth ions, however there is not much more literature on the separation of these elements.For the instance,  we obtained interesting results showing the magneto-migration of paramagnetic ions (ex.: dysprosium) towards the surface of a magnet (NdFeB) and the diamagnetic ions (ex.: yttrium) moving in the opposite direction of the magnet surface. A more complex scenario seems to take place when mixtures of paramagnetic and diamagnetic ions are exposed to a magnetic field gradient. Opposite to the expected, both ions migrate together in the direction of the magnetic field. It can be can be assumed as a strong indication that rare earth ions do not behave as individual ions when in solution and submitted to a magnetic field. Instead, a cooperative effect seems to exist and research is still need to understand this behavior. Even the fact rare-earth ions in solution are influenced by a magnetic field is already a counter intuitive observation. Theory predicts that rare-earth ions are too small to be influenced by external magnetic fields and results with these observations are very challenging to explain and demand deeper explanations." "Particles and droplets in viscoelastic fluids in confined flows: comparison between simulations and experiments." "Jan Vermant" "Soft Matter, Rheology and Technology Section" "Intensification of light-driven water resource recovery by using droplet millifluidics for the production of seed photogranules (Insight)" "Xevi Casadevall i Solvas" "Mechatronics, Biostatistics and Sensors (MeBioS), Process Engineering for Sustainable Systems (ProcESS), Chemical and Biochemical Reactor Engineering and Safety (CREaS)" "Over the last decade, stimulating bacteria to form spherical compact biofilms called granules instead of loose flocs has revolutionized wastewater treatment (WWT). Still, WWT technology with activated sludge granules faces some challenges such as high O2 demand and inevitable CO2 emissions. Recently, another type of granule, the photogranule, has been attracting attention due to its potential to achieve sustainable WWT and resource recovery. Photogranules are a mixture of heterotrophic and phototropic microorganisms. Phototrophs can autonomously produce O2 needed while consuming CO2 generated by heterotrophs, thereby eliminating the need for aeration of the aerobic granular technology. However, there are several problems that need to be solved to realize the full potential of this technology: 1) long start-up periods due to the non-optimized production of seed photogranules, 2) lack of fundamental studies on photogranulation and 3) poor characterization of photobioreactors (PBR) in terms of light fields. In this project I aim to address these issues by developing a droplet-based millifluidic platform to produce seed photogranules in a robust and controlled high-throughput manner. The same platform will enable me to identify the impact of environmental parameters (e.g., C/N ratio and light) on the microbial composition and the resource recovery potential of the photogranules. Finally, I will validate my results in a PBR intensified in terms of light fields." "How Nanofluids Influence Droplet impact on a Solid Substrate and Surface-to-Droplet Heat Transfer" "Maria Rosaria Vetrano" "Soft Matter, Rheology and Technology (SMaRT), Applied Mechanics and Energy conversion (TME)" "The aim of this thesis is to understand the effect of added nanoparticles on both the hydrodynamics and heat transfer of droplets impacting on solid substrate. Therefore, in the first part, nanofluid drop impact onto a smooth sapphire substrate is experimentally investigated over wide ranges of Reynolds (100 < $\Re$ < 10000) and Weber (50 < $\We$ < 500) numbers for three \Al~nanofluid mass concentrations of 0.01~wt.\%, 0.1~wt.\%, and 1~wt.\% using high-speed photography in order to study the effect of nanoparticles on droplet spreading and spreading-to-splashing transition. In the final part, another set of experiments is conducted with \Ti~nanofluids with the mass concentrations of 0.2~wt.\%, 0.5~wt.\%, and 1~wt.\% using both high-speed photography and high speed thermography. The nanofluid properties density, viscosity, surface tension, thermal conductivity, stability, and particle size distribution are experimentally characterized and used to describe and model both parts of this work.In the first part of the thesis, a new maximum spreading model based on energy balance is introduced with a new model of maximum spreading time incorporating fluid viscosity. A large portion in the $\We$ vs. $\Re$ map is covered with seven different concentrations of aqueous glycerol solutions. This new maximum spreading model is compared to others from the literature to evaluate the effect of nanoparticle existence in the corresponding base fluids. Afterwards, the effect on spreading-to-splashing transition is evaluated. It is demonstrated that even a small fraction of nanoparticles in fluids affects the splashing behavior of a droplet upon impact on a smooth surface. Nanofluids influence this transition boundary by promoting splashing at low Reynolds numbers. We explain this behavior through the increased lamella spreading speed and lift during the lamella spreading stage. Finally, we develop an empirical correlation that describes the splashing threshold dependence on nanoparticle concentration for the first time.In the second and final part, we investigate the effect of \Ti~nanofluids and their concentrations on the early stages of droplet spreading on a hot surface below boiling point (80~\deg). For this purpose, water and three water-based \Ti~nanofluids having different concentrations, i.e., 0.2~wt.\%, 0.5~wt.\%, and 1~wt.\%, are tested at four impact speeds on a smooth TiAlN-coated sapphire substrate. In order to achieve systematical results to see the effect of only nanoparticle existence in the fluid, the thermal and rheological properties of the nanofluids are experimentally characterized and considered in the calculations. Experimental results show that the nanofluids show marginally smaller spreading than in the case of water. The cooled area also does not change with the nanoparticle addition. Moreover, the temperature decrease at the impact point is also independent of both impact speed and nanoparticle concentration. Furthermore, the nanofluid with the largest concentration (1~wt.\%) yields less than 9~\% enhancement in the average cooling performance during the first 5~ms of the droplet spreading, which barely passes the uncertainty level ($\approx6~\%$) of the measurements." "Droplet generation and manipulation in microfluidic devices based on acoustic technology" "Simon Kuhn" "Process Engineering for Sustainable Systems (ProcESS)" "This project will investigate the effect of acoustic forces on droplet dynamics in microfluidic devices. We will focus on 2 important applications: i) acoustic assisted droplet generation in flow focussing geometries to generate nano-scale droplets, and ii) acoustically enhanced droplet coalescence. Experiments will reveal the underlying acoustofluidic effects, which will then be incorporated in a mathematical model. This approach will result in the design and optimization of novel sonicated microreactors which enable precise control of the droplet behavior." "Determining the relationship between Ultrasonic Spray Coating parameters, droplet characteristics and layer formation" "Wim DEFERME" "Engineering Materials and Applications" "Ultrasonic spray coating is a coating technology that – thanks to the ultrasonic vibrations at the nozzle – opens the road to deposit nanometre-thin homogeneous layers. Its industrial application, however, is limited due to the many possible parameters influencing the process. So far, there is insufficient knowledge and understanding on the interplay between coating formulation, droplet generation, transport and impact on the substrate. Developing a droplet visualization system will lead to scientific and fundamental breakthrough in understanding how the droplet evolves during flight. First the influence of several spray parameters on the droplet characteristic will be studied and in a second step these findings will be translated to the influence on layer formation. A dimensionless operating space for ultrasonic spray coating will be constructed as the final, scientific outcome. This will lead to an ultrasonic spray coat model from which the ideal set of spray parameters can be determined for any kind of formulations. These insights will lead to improvements in efficiency, stability and reproducibility of the process. We therefore state that the investigation performed in this PhD is a first step for broad integration of ultrasonic spray coating in research, development and industry."