Particles, Drops, and Bubbles in Gradient Fields

Particles, drops, and bubbles submerged in a host liquid are omnipresent in nature and industry. Often, they are subjected to gradients in concentration or temperature (or both). These gradients might change locally or evolve over time, placing the system far from its equilibrium. This gives rise to extraordinary rich physics at the intersection of fluid dynamics, chemical engineering, and colloid and interface science. In this thesis, we investigate the behaviour of particles, drops, and bubbles in applied gradient fields. We focus on small-scale, idealized table-top experiments closely combined with theoretical and numerical modelling to study these objects under conditions that are far from equilibrium. For the latter, we consider particles, drops, and bubbles at a water-ice interface during unidirectional solidification (Part I) and immiscible drops in density stratified ethanol-water mixtures (Part II).In Part I we deal with the freezing of suspensions and oil-in-water emulsions in order to study the interaction between different objects and an approaching water-ice solidification front. To do so in a controlled manner, we apply a thermal gradient over our sample and ensure slow, uni-directional freezing. We then change the type of object near the front to add more and more complexity to the system, starting with solid particles (chapter 1) before moving on to drops (chapter 1-4) and eventually bubbles (chapter 5).In Part II we study the dynamics of immiscible drops in a density stratified ethanol-water mixture. These studies further investigate the peculiar observation that these drops can show continuous bouncing, against gravity, caused by an oscillatory solutal Marangoni flow around the drop. In chapter 6 we look in depth into the onset of the bouncing instability and extend the experimental parameter space by changing the viscosity of the oil, in order to determine the different mechanisms that trigger it. Finally, in chapter 7, we dive further into the characteristics of the bouncing cycle through well-performed experiments and numerical simulations, aiming for a one-to-one comparison.<br/

Freezing-induced topological transition of double-emulsion

Solidification of complex liquids is pertinent to numerous natural and industrial processes. Here, we examine the freezing of a W/O/W double-emulsion, i.e., water-in-oil compound droplets dispersed in water. We show that the solidification of such hierarchical emulsions can trigger a topological transition; for example, in our case, we observe the transition from the stable W/O/W state to a (frozen) O/W single-emulsion configuration. Strikingly, this transition is characterised by sudden expulsion of the inner water drop from the encapsulating oil droplet. We propose that this topological transition is triggered by the freezing of the encapsulating oil droplet from the outside in, putting tension on the inner water drop thus, destabilizing the W/O/W configuration. Using high-speed imaging we characterize the destabilization process. Interestingly, we find that below a critical size of the inner drop, R in,crit ˜ 19 µm, the topological transition does not occur any more and the double-emulsion remains stable, in line with our interpretation.</p

Radical Polymers as Anodic Charge Extraction Layers in Small Molecule Organic Photovoltaic Devices

Organic photovoltaic (OPV) devices based on the copper (II) phthalocyanine(CuPc)/ fullerene(C60) system are an innovative photovoltaic technology optimal for situations requiring low-cost, transparent, and flexible devices. Furthermore, the high degree of reproducibility of this system allows for the ready study of new OPV technologies. Here, we have used this system to elucidate systematic structure-property-performance relationships for a new OPV anode modifier. The addition of interfacial modifier materials between the organic CuPc/C60 layers and the metallic anode drastically can improve efficiency. Radical polymers are a class of polymers with aliphatic backbones and pendent stabilized radical groups. Here, we utilize poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA) to examine the feasibility of radical polymers as anode modifiers. OPV devices utilizing a PTMA thin film deposited onto an ITO substrate (anode) with subsequent CuPc and C60 active layers followed by a BCP cathode modifier and an aluminum layer (cathode) were fabricated using thermal evaporation. Device performance was evaluated by measuring current density as a function of voltage during simulated solar radiation. Addition of a thin layer of PTMA between the ITO and CuPc layers increased device power conversion efficiency to approximately 0.95% from a control of 0.57%, likely due to enhancement of the crystal structure of the CuPc layer. The addition of interfacial modifiers significantly increases the overall efficiency, and consequently, viability of CuPc/C60 OPV devices, and this logic should be extendable to a myriad of other polymer based solar cell designs

An Examination of adherence to and departure from the reference and user services association's guidelines for reference transactions in chat services interactions

This study examines the adherence to and departures from the prescribed pattern of reference transactions as established by the Reference and User Services Association (RUSA) of the American Library Association. 300 chat reference transactions collected from the reference department of the Walter Royal Davis Library at the University of North Carolina at Chapel Hill were examined for patterns indicating the situations in which divergences in particular occurred and the nature of those divergences. This study reveals that these divergences, generally, stem from the text-based communication medium of chat reference, and certain adaptations, to account for this text-based medium, have been created instinctively by both librarians and library users

On the rising and sinking motion of bouncing oil drops in strongly stratified liquids

When an immiscible oil drop is immersed in a stably stratified ethanol-water mixture, the Marangoni flow on the surface of the drop can experience an oscillatory instability, so that the drop undergoes a transition from levitating to bouncing. The onset of the instability and its mechanisms have been studied previously, yet the bouncing motion of the drop itself, which is a completely different problem, has not yet been investigated. Here we study how the bouncing characteristics (jumping height, rising and sinking time) depend on the control parameters (drop radius, stratification strength, drop viscosity). We first record experimentally the bouncing trajectories of drops of different viscosities in different stratifications. Then a simplified dynamical analysis is performed to get the scaling relations of the jumping height and the rising and sinking times. The rising and sinking time scales are found to depend on the drag coefficient of the drop $C_D^S$ in the stratified liquid, which is determined empirically for the current parameter space. For low viscosity (5 cSt) oil drops the results on the drag coefficient match the ones from the literature. For high viscosity (100 cSt) oil drops the parameter space had not been explored and the drag coefficients are not readily available. Numerical simulations are therefore performed to provide external verification for the drag coefficients, which well match with the experimental results.Comment: 21 pages, 11 figure

Low-voltage waveguide Ge APD based high sensitivity 10 Gb/s Si photonic receiver

We demonstrate low-voltage Ge waveguide avalanche photodetectors (APDs) with gain-bandwidth product over 100GHz. A 5.8dB avalanche sensitivity improvement (1x10(-12) bit error ratio at 10Gb/s) is obtained for the wire-bonded optical receiver at -5.9V APD bias

Regulation of P-Glycoprotein in Renal Proximal Tubule Epithelial Cells by LPS and TNF-α

During endotoxemia, the ATP-dependent drug efflux pump P-glycoprotein (Abcb1/P-gp) is upregulated in kidney proximal tubule epithelial cells. The signaling pathway through which lipopolysaccharide (LPS) or tumor necrosis factor-α (TNF-α) regulates P-gp expression and activity was investigated further in the present study. Exposure of rat kidney proximal tubule cells to TNF-α alone or TNF-α and LPS increased P-gp gene and protein expression levels and efflux activity, suggesting de novo P-gp synthesis. Upon exposure to TNF-α in combination with LPS, P-gp activity in renal proximal tubule cells is increased under influence of nitric oxide (NO) produced by inducible NO synthase. Upon exposure to TNF-α alone, P-gp upregulation seems to involve TLR4 activation and nuclear factor kappaB (NF-κB) translocation, a pathway that is likely independent of NO. These findings indicate that at least two pathways regulate P-gp expression in the kidney during endotoxemia