The impact of mass transfer and interfacial expansion rate on droplet size in membrane emulsification processes

In membrane emulsification, especially under conditions where droplets are forming with a narrow droplet size distribution, it is conjectured that the interfacial phenomena are determining the emulsification result. The process parameters of continuous phase flow and dispersed phase flux were analysed from the perspective of how they could be affecting the interfacial tension of the growing droplet. This work first reviews the applicability of current droplet formation models (force balance and spontaneous transformation based (STB)), describes the interfacial transport of surfactant molecules to an expanding oil-water interface, and models the flow of dispersed phase through a pore using MATLAB. The data from these calculations are then applied in a model to predict the final size of the droplets, which includes dynamic effects of mass transfer and expansion rate. The droplet detachment mechanism in membrane emulsification was modelled from the point of view of Gibbs free energy. An interactive finite element program called the surface evolver was used to test the model. A program was written and run in the surface evolver, which allows the user to track the droplet shape as it grows, to identify the point of instability due to free energy, and thus predict the maximum stable volume (MSV) attached to the pore. The final droplet size was found by applying a pressure pinch constraint (PPC), which is based on the division of the surface into two volumes, a droplet and a segment, which remains attached to the pore mouth. The relative size of these two volumes is such that the resulting radii of curvature of the droplet will maintain an equal Laplace pressure across the surface of both volumes. Predicted droplet sizes were compared to experimental data from the literature. It was found that changes in surfactant coverage caused by mass transfer coupled to the expansion rate of the oil-water interface have a significant and predictable effect on the final droplet size in membrane emulsification. The extent of the influence of the dispersed phase flux on dynamic interfacial tension was quantified using a dimensionless parameter, the mass transfer expansion ratio (MER). The MER can be used to predict the effect of increasing the depletion of surfactant on the relative final droplet size in membrane emulsification. This new insight into the role mass transfer and surface expansion play in membrane emulsification allows us to now predict a priori the final droplet size that would form for a particular set of conditions, and can lead to better process design methods in the future

PIV and CFD measurements of internal velocity in a forming drop in a liquid-liquid system

A PIV method has been used to determine the internal motion in an oil drop during formation and to validate a numerical simulation of the drop formation process. The PIV system included a microscope attached to the camera, which gave a focal depth of 50 µm and a possibility to measure the velocity in the centre cross section of the drop. Oil was forced through a capillary with a diameter of 200 µm into a channel with a cross-flowing continuous phase that induced a shear at the interface of the forming drop, which resulted in a rotational motion inside the drop. The angular velocity in the drop reached a maximum after 1/4 of the drop formation time and approached a steady state before drop detachment when a neck was formed above the capillary opening. The velocity of oil out of the capillary was also investigated. A clear dependence on the pressure inside the forming drop was obtained

CFD modelling of drop formation in a liquid-liquid system

The formation of an oil drop from a single capillary in a continuous phase flowing perpendicular to the capillary opening has been studied numerically. The shear stress at the interface of the forming drop, the angular velocity inside the drop and the pressure field around the drop have been determined in a cross section of the drop formation area. The results show a maximum pressure in the continuous phase near the stagnation point and a maximum shear stress in both phases nearer the top of the forming drop. The lowest pressures were found behind the top of the drop, where the surrounding flow starts to separate from the interface of the drop. The shear stress outside the drop causes a drag which, together with the drag originated from the pressure field around the drop, promotes drop detachment

Optical Spectroscopy of Single Nanowires

This thesis describes optical spectroscopy on III-V semiconductor nanowires. The nanowires were grown by metal-organic vapor phase epitaxy (MOVPE) and chemical beam epitaxy (CBE). Photoluminescence and photocurrent spectroscopy are used as tools to investigate issues such as the size of the band gap, the effects of surface states, and the charge carrier transport in core-shell nanowires. The band gap of InAs1-xPx nanowires with wurtzite crystal structure is measured as a function of the composition for 0.15<x<0.48. The band gap is measured using photocurrent spectroscopy on single InAs nanowires with a centrally placed InAs1-xPx segment. The wurtzite band gap is found to be about 120 meV larger than the corresponding zinc blende band gap over the entire composition range. The photocurrent spectrum is measured for excitation polarized parallel and perpendicular to the nanowire axis. The nanowires are found to have a large polarization dependence of the photocurrent, which is explained by the difference in dielectric constant of the nanowire and the surrounding air. The large polarization dependence in combination with the tunable band gap and the low dark current due to the band edge offset in the heterostructure, makes such nanowires possible candidates for polarization-sensitive photodetectors in the infrared. The effect on the optical properties of the crystal structure is further investigated by comparing the spectral excitation power dependence of InP nanowires with zinc blende crystal structure and InP nanowires with a high density of rotational twins. The difference in excitation power dependence is explained by interpreting the rotational twins as monolayer thick wurtzite segments. The rotationally twinned structure responds to the light as a type II heterostructure due to the type II offset between the zinc blende and wurtzite energy bands. p- and n-doped InP nanowires are studied with photoluminescence spectroscopy. The radial band bending caused by the Fermi level pinning at the surface, causes the electrons and holes to be separated radially and this is observed as a lowering of the photoluminescence energy. This is further investigated by applying a gate voltage on the nanowire sample to change the band bending, and observe the changes in the photoluminescence signal. This could potentially be used for investigating the doping concentration in such nanowires. Core-shell nanowires with GaAs core and a larger band gap GaxIn1-xP shell are studied by photoluminescence and time-resolved photoluminescence spectroscopy. It is observed that the photoluminescence decay is fast, indicating that the decay is dominated by non-radiative recombination also with a passivating shell on the nanowire. The charge carrier transport from the shell to the core is partially hindered at the low temperatures used (10~K). The photoluminescence decay is modelled by simple rate equations, with qualitative agreement with the experiments. It is also studied how the strain from the lattice mismatched shell, and the choice of substrate (Si or GaP) affects the photoluminescence intensity and decay time. It is found that the maximum PL intensity is obtained for unstrained nanowires. A smaller part of the thesis describes photoluminescence measurements on the conjugated polymer MEH-PPV (poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene]). The measurements are performed on single polymer chains dispersed in a PMMA matrix. The polymer spectra acquired at room temperature and 20~K are compared to obtain information about the conformational dynamics of the polymer chain. It is observed that at 20 K, the photoluminescence spectrum has a narrow line width and there is a large spread in the distribution of the spectral maxima. This was explained by assuming that at this low temperature, the thermal energy was not enough to allow conformational changes, and each single chain is frozen in a specific conformation. At room temperature conformational changes are possible, resulting in the single chain spectra being broad with only small inhomogeneous broadening of the ensemble spectrum

On the chemotropism of insects and its significance for Economic Entomology

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Membrane emulsification modelling: how can we get from characterisation to design?

There has been an increasing interest in a new technique for making emulsions known as membrane emulsification, which uses a microporous membrane operated in cross-flow. The continuous phase is pumped along the membrane and sweeps away dispersed phase droplets forming from pore openings as shown in Fig. 1. The effects ofprocess parameters in membrane emulsification have been studied, especially on a quantitative level. However, the physical mechanisms of droplet formation are still under investigation to better elucidate the roles of operating parameters, and finally model the process. This work reviews current developments and deficiencies in the modelling membrane emulsification processes

Adding attenuation corrected images in myocardial perfusion imaging reduces the need for a rest study.

The American Society of Nuclear Cardiology and the Society of Nuclear Medicine conclude that incorporation of attenuation corrected (AC) images in myocardial perfusion scintigraphy (MPS) will improve diagnostic accuracy. The aim was to investigate the value of adding AC stress-only images for the decision whether a rest study is necessary or not