with respect to topography and surface energy 34–36 . Geared towards applications in microfluidics, it was shown
that a Marangoni-type flow can be induced by cis-trans isomerization of azobenzene solutes at the liquid-air or liquid-liquidinterface; this effect has been used to steer oil droplets on an aqueous subphase 37 .
In contrast, working with particle based systems, we have discovered a peculiar phenomenon related to col- loids located at a liquid-solid interface immersed into aqueous solution of photosensitive surfactant. Here we report on how small particles trapped at a solid/liquidinterface can be moved by diffusioosmotic focusing ini- tiated by the photoisomerization process. We will provide a theoretical account of how the local liquid flows emerge. It will turn out that the phenomenon is best understood as light-driven diffusioosmosis. In this way, the surfactant becomes a photo-soap in a new sense: not only particle-surface interaction is reduced, but also control over the “rinsing” of contaminants can be gained.
The term coalescence describes the confluence of two dispersed fluid particles after collision as shown in a high speed image sequence in Figure 1. Coalescence is an important phenomenon in many everyday events, technical applications and products. Raindrops for example would not flow together to puddles and ponds but accumulate on the ground. In industrial processes (e.g., extraction columns, separators or stirred tanks) it is essential to control and influence the coa- lescence behaviour of dispersions. On the one hand, coalescence is the essential phenomenon in separation processes. Additionally, coalescence determines, together with the opposing mecha- nism of drop breakage, the drop size distribution within a dispersed system. On the other hand, coalescence is not always desired: for example in cosmetic products like lotions and creams where water and oil are finely dispersed. Also in food and nutrition products such as milk where the fat is homogenised and in supplements where active ingredients are encapsulated in (dou- ble) emulsions coalescence has to be hindered. In these cases, the coalescence is significantly inhibited by additives which accumulate at the liquid/liquidinterface (surfactants). Hence, the probability of two drops to coalescence might and often should vary significantly from applica- tion to application and, thus, its influencing parameters need to be understood. In the last dec- ades, considerable research was done but the mechanisms and influencing parameters are still not understood in detail. A reason for this is the high spatial and temporal resolution of coales- cence and its preceding steps: the decisive events of interface approach, rupture and confluence occur in order of magnitudes of nanometres and microseconds. Particularly, film rupture is de- termined by small scale fluctuations which prohibit deterministic descriptions of the whole coa- lescence process up to now. Therefore, coalescence is described by statistical methods in availa- ble modelling approaches. Modern digital high speed imaging offers the possibility to establish an extensive database with high spatial and temporal resolution. With the help of this technique the influence of droplet properties (e.g., drop sizes, relative collision velocity), physical proper- ties (e.g., viscosity, density, interfacial tension), interfacial characteristics (e.g., adsorption of surfactants and ions, Marangoni convection) and system conditions (e.g., energy dissipation rate, flow pattern, geometry) on coalescence can be determined. Based on these findings the descrip- tion of coalescence in technical applications can be enhanced.
Chapter 7 Summary
The state of-the-art modeling techniques for designing liquid-liquid extraction columns focus on the properties of single droplets and the processes in which they are involved, like sedimentation, coalescence and mass transfer. The semi-empirical models that are used in the calculations are based on fitted parameters that require experimental measurements. In addition, the phenomena observed in the exper- iments are very complex in nature, thus making the interpretation of the results and, thereby, the reliable design of extraction apparatuses, a non-trivial matter. The liquid-liquidinterface often plays a very dominant role in describing the behavior of droplets, but it has not yet been fully investigated. This work is a follow-up of the work by Groß-Hardt (2007) and aimed in developing and testing an interfacial tension model for simulating the effect of surface-active impurities on the internal flow of single droplets. The project was part of the Collaborative Research Center CRC-540 of the RWTH Aachen University. The simulations were performed with the CFD-program “DROPS”. For validating the interfacial model, the experimental setup proposed by Groß-Hardt (2007) was used. The software and the experimental results were respectively provided by partner-projects of the Chair of Numerical Mathematics and the Institute for Macromolecular Chemistry.
The coalescence of drops can be hindered by steric obstruction but also massively by electrochemi- cal effects (Rambhau et al., 1977; Watanabe, 1984) up to a formation of a stable emulsion. A charge separation induces an electrostatic potential at the interface of the droplets which again is damp- ened by the solute ions in the continuous phase (Lyklema, 2000). Altogether, an electrostatic repul- sive force results between two approaching droplets. However, this effect is not as easy to be im- plemented in liquid/liquid systems as in the case of solid particles in an electrolyte solution. In the latter case, the charged particle surface is surrounded by the electrical double layer: one fixed layer of ions with opposing charge bound to the particle surface and a diffusive layer of ions dampening the excess charge of the covered surface (Delgado et al., 2007; Israelachvili, 1991). Although the reality is often more complex than this simple picture, the situation at a liquid/liquidinterface is even more complicated. A fixed layer of ions at a mobile interface seems to be quite unlikely. Con- sequently, mobile ions at the surface will lead to a locally inhomogeneous charge due to movement of the surface (Carroll, 1976). Additionally, it is known that the solubility of ions in oil is small though non-zero (Izutsu, 2002; Wagner et al., 1998), inevitably a partition of the ions between the two phases occurs and two electric ‘double’ layers are formed at both sides of the interphase (Derjaguin et al., 1987). Although the induced potentials were measured by several authors, the origin of the electrostatic potential at the interface is discussed in a controversial manner in litera- ture. Some authors argue solely by means of dissimilar partition coefficients of the present ions in the oil and water phase (Overbeek, 1952; Pfennig and Schwerin, 1998). Assuming different parti- tion coefficients of the ions and applying the Albertsson model (Pfennig and Schwerin, 1998), it was shown that only the relative magnitude of the partition coefficients may determine the potential difference at the surface. Another widely used assumption is the preferred accumulation or adsorp- tion of specific ions at the interface (especially hydroxide ions (Beattie and Djerdjev, 2004; Beattie and Gray-Weale, 2012; Beattie et al., 2005; Creux et al., 2009; Franks et al., 2005; Gray-Weale and Beattie, 2009; Marinova et al., 1996)), whether it is described by Gibbs (Lyklema, 2000) or Lang- muir monolayers (Marinova et al., 1996; Tian and Shen, 2009). Other research groups doubt this concept of ion adsorption and attribute the created potential to the immobilization and orientation of water dipoles at the surface (Chibowski et al., 2005; Vacha et al., 2011) or to the deprotonation of fatty acid impurities in the oil (Roger and Cabane, 2012a, 2012b).
Nanoparticles can be applied as emulsifying agents to stabi- lize liquid-liquid systems towards coalescence (Pickering emulsions) [1, 2]. Due to the adsorption of nanoparticles at the liquid-liquidinterface, coalescence is hindered or arrested, and smaller drop size distributions and a higher in- terfacial area can be achieved [3–5]. The higher interfacial area promotes mass transfer so that nanoparticles can be used as innovative additives for liquid-liquid reactions [6, 7]. However, the particles can also lead to an additional mass transfer resistance if they are densely packed at the liquid-liq- uid interface. It is crucial to understand the impact of particle characteristics such as shape, size and surface modification on their spatial arrangement at the interface, the drop size distributions and the resulting mass transfer . To achieve a long-term stabilization of Pickering emulsions against coa- lescence, small droplet sizes are needed . These are often realized by using high-energy dispersion units for emulsion preparation, such as ultrasonication or rotor-stator homoge- nizers. The formulation parameters and the corresponding final emulsion properties have been investigated by various authors [6, 7, 9–12]. However, for large-scale industrial appli- cations with Pickering emulsions as innovative reaction sys- tems, high energy consumption during emulsion preparation should be avoided in order to achieve an economically viable process. Furthermore, there still is a gap of knowledge con- cerning the detailed emulsification mechanisms .
catalysts, it can serve as a host for the hydrophobic reagents. As collapsing of the microgel-catalysts is driven by enhanced hydrophobicity of the network, an acceleration of the aldol reaction could be observed. The detailed mechanism behind this phenomenon is demonstrated on the molecular level with computer simulations based on dissipative particle dynamics (DPD). We demonstrated that in the heterogeneous reaction mixture, the microgels adsorb at the liquid–liquidinterface between water and the hydrophobic reagents (depicted here as the oil phase). Further temperature increase forces the microgels to immerse more into the hydrophobic reagent phase due to the temperature-responsiveness of PNIPAM and consequently
This thesis deals with the investigation of silica-supported vanadia-based Supported Liquid Phase (SLP) catalysts in Oxidative Dehydrogenation of Propane (ODP). The aim is a better understanding of the structural requirements of oxidation catalysts. The approach is based on the observation that high-performance catalysts in selective oxidation exhibit a thin surface layer, which is chemically and structurally different from the bulk. Furthermore, it has been shown that strain is responsible for high activity. SLP catalysts provide a unique opportunity to study structure-activity relationships, since they can be operated in both solid and molten states. The main criterion, which should be fulfilled, concerns the melting point of the coating, which should be in the range of the target reaction. The potassium vanadate system meets this requirement. Varying the ratio of K:V, mixed as well as single phase compounds with different melting points were formed on the surface of silica (Aerosil) as support.
At this point, it is clear how the alignment of the director can be technically realized. For application in tunable components, the RF signal and the control (bias) must be discriminated. For magnetic alignment, the bias field and the RF signal’s E-field are decorrelated by definition. This is different for both other cases, where the electric field is used to reach at least one state. The only difference in RF signal and bias E-field is the frequency. The minimization of overall system’s energy according to Equation ( 8 ) must hold also in this case. The temporal behavior of the LC volume is encapsulated in the elastic term as this includes the mechanical materials properties. Due to the viscosity, the director cannot follow an electric field with fast changing amplitude. Additionally, the dielectric contrast required for LC steering is much higher at DC than at RF frequencies. Hence, the influence of the RF field can be neglected. This is also relevant for the linearity of the components. The electric tuning of the material leads automatically to a non-linear behavior of the components, as the RF field will also change the state of LC. There are just few measurements of LC’s linearity published. In 2006, IP3 measurements of two different nematic liquid crystals have been published in [ 7 ]. Measurements of a phase shifter showed that the IP3 of this device is in the range of 60 dBm. Since then, no other measurements have been published. The excellent large signal characteristics obtained in this measurement already fulfill the requirements of many RF applications. Furthermore, the achieved performance makes linearity measurements of LC very difficult, as standard measurement equipment is at its boundaries. Hence principles for measuring passive intermodulation (PIM) must be adapted for LC non-linear characterizations. This specific topic will gain more interest in future as commercialization advances. Up to now, no devices showed significant non-linearity.
reported evidence for a wall effect. Diffusion coefficients calculated for samples held in capillaries of 0.83 mm I. D. were considerably lower than those calculated for samples held in 1.60 mm capillaries. These data are shown in Fig. 1. In the course of studies underway in our laboratory, it became necessary to know ac curately the self-diffusivity in liquid indium. Con sequently an investigation over a limited temperature range was undertaken by use of techniques different from those employed in Ref. 1 and 2. The method used was analogous to the standard “thin layer” slicing method in the solid state. Following L a r s s o n and L o d d i n g ’s technique3 of preparing samples, a small
Drop size distribution depends on physical properties, e.g., viscosity and interfacial tension, and operating parame- ters such as the energy input into the system through the prior mixing process . For both pure liquid compo- nents, the absolute difference in density and the viscosity of MeOH, the main component of the continuous phase, decrease over temperature (Fig. 5). Since stirring velocity stays constant over the whole temperature range, different drop size distributions must result. Miscibility effects of dissolved components must be considered though to predict sedimentation behavior by physicochemical substance properties. With the increase in temperature, density differ- ence between the continuous and dispersed phase decreases due to higher miscibility of MeOH in Dec. While only 5.6 mol % MeOH dissolve in Dec at T = 290.24 K, the solubility of MeOH in Dec increases to 25.44 mol % at T = 334.95 K . In contrast, the solubility of Dec in MeOH is not heavily temperature-dependent in the temper- ature interval considered. The solubility increases only from 2.19 mol % Dec dissolved in MeOH at T = 290.98 K to 5.13 mol % at T = 333.67 K . This leads to the slower sedimentation velocity for T 3 . As the sedimentation velocity
Bedingt durch die intensive industrielle Nutzung und stetige Entwicklung innovativer Nanoma- terialien (NM) ist auch eine Weiterentwicklung von Methoden zur toxikologischen Beurteilung dieser Materialien erforderlich. Die Erfassung des toxikologischen Potentials nanoskaliger Ver- bindungen in vitro beruht im Wesentlichen auf submersen Expositionen in Zellkultursystemen. Jedoch spiegelt diese Kultivierungsmethode keine physiologischen Bedingungen des Respirations- trakts wider. Insbesondere im Alveolarbereich der Lunge, den für die Nanotoxikologie relevan- testen Lungenabschnitts, werden Epithelzellen lediglich von einem Flüssigkeitsﬁlm bedeckt und liegen an einer Grenzﬂäche zwischen Luft (Atmosphäre) und Flüssigkeit (Blut) vor. Um realitätsnä- here Bedingungen zu simulieren ist es möglich, Zellen an einem air-liquidinterface (ALI) zu kulti- vieren und anschließend über ein ALI-Expositionssystem einem NP-haltigen Aerosol auszusetzen. Diese Methode bietet im Gegensatz zur submersen Behandlung mit NP neben physiologischeren Bedingungen auch eine verlässliche Technik zur Erfassung der applizierten Dosis durch die Ver- wendung einer Quarzkristall-Mikrowaage (QCM). Sie liefert damit einen elementaren Baustein zur toxikologischen Beurteilung der inhalationstoxischen Wirkung von NP.
strong electrostatic interaction between the two compounds. Since more surfactant molecules than polyelectrolyte segments are present in the solution, all charges of the polymer chain are complexed by surfactant molecules, which expose their hydrophobic alkyl chains to the surrounding. The release of the counterions further increases the entropy of the system, which also enhances the complexation between the compounds. Additionally, the hydrophobic backbone of the polyelectrolyte can be adsorbed at the surface. Altogether, this leads to a strong decrease in surface tension already at quite low polyelectrolyte concentrations. With further increase of the PSS concentration, the surface tension is reduced until a minimum at the nominal IEP. The surface tension measurements in this concentration regime show that PSS60 adsorbs slightly stronger at the surface than the longer polyelectrolyte. This indicates that the length of the polyelectrolyte chain indeed has an influence on the adsorption. As described earlier, a change in surface tension gives a hint that the polyelectrolyte does not adsorb flat to the interface. 126 In that case, no change in surface tension would be visible since the
Stirred liquid-liquid systems are encountered in a large variety of technical processes, with major importance for chemical, pharmaceutical, mining, petroleum, and food in- dustries. In these systems, two immiscible liquids are stirred so that one liquid disperses into the other one by building drops. The size distribution of these drops, resulting from the opposed phenomena of turbulent drop breakage and coalescence, plays an important role in the overall performance of many technical processes. Thus, the aim is to control the mean drop size and drop size distribution. In the production of polystyrene foam, for example, all drops in the system should have a certain, pre-deﬁned drop size. For liquid-liquid systems with chemical reactions taking place at the interface between the two phases, the ratio between the surface area and the volume of the drops should be maximal, which would be achieved by the smallest possible drop size. However, one usually has the restriction that the drops still have to be so large that the phases can be separated after the reaction.
germanium nanostructures also have gained a lot of attention due to their high specific capacity (4200 mA h g 1 for Li 4.4 Si,
1600 mA h g 1 for Li 4.4 Ge). 6,7 Both silicon and germanium
undergo a large volume change during lithium intercalation. To reduce these volume changes nanostructures especially in the form of nanowires and nanotubes are beneficial. 6 However, cycle life greater than a few hundred cycles has not yet been demon- strated which is also due to the instability of the solid electrolyte interface (SEI). 8,9 Presently, carbonate electrolytes with LiPF 6 or
In order to justify a correlation between the performance of cells on O/N and O/N/O interlayer stack and the front-side passivation quality determined in section 6.3.1 the interlayer stacks should not influence the LPC-Si bulk quality. For example, the interlayer stack should not influence the grain size because the grain size affects the open-circuit voltage if the grain size is smaller than the absorber thickness [ 18 , 19 ]. Figures 5.1 (b) and (c) show glass/IL/LPC-Si samples after crystallization and texturing on the O/N and the O/N/O interlayer stack, respectively. Large laterally grown grains of several centimetres in scanning direction and a few millimetres in width (which is larger than the absorber thickness) are seen which are typically observed after liquid-phase crystallization of the thin silicon films on the glass substrates. No difference in the grain size is observed and thus, the similar performance of test cells on O/N and O/N/O interlayer stack is related to the similar passivation quality at the front-side of the test cells.
The drop swarm analysis in the agitated vessel showed clearly that a reproduction of the experimental data by means of the population balance can be achieved already with common models from literature. One con- straint for such a successful implementation is the use of a 2-zone model to consider spatial inhomogeneities of the energy dissipation and velocities. It was shown in this work, that with one pair of adjusted parameters, using the model of Coulaloglou and Tavlarides (1977), a variation of geometry factors and operating parame- ters (e.g. stirrer speed, reactor size or type of agitator) is possible without further parameter adaptation. Even severe changes, like the one from a single to a quadruple impeller system with parallel four times increasing liquid level with constant reactor diameter, could be predicted with deviations smaller 10%.
The present study starts the development of the numerical model with the method/code validation for single air-bubble rising in a viscous quiescent liquid. In terms of bubble terminal velocity and instantaneous bubble break-up process by a solid cylinder, the numerical model-delivered results are found to be in good match with experimental data from literature. Then the numerical model is applied to predict rising process of an air bubble through a representative domain of a POCS filled with stagnant water. Solid surface wettability (i.e. equilibrium contact angle) is an input parameter to the numerical model and is varied in a series of simulations to reveal its effect on bubble interacting behavior with the solid structures. The present results provide a clear evidence that bubble deformation and rising path largely depend on the wettability. In the circumstances of industrial application of POCS, therefore, the use of structures with high surface wettability (i.e. low equilibrium contact angle) is expected to favor mass transfer enhancement and catalytic gas-liquid reactions.