Monodisperse emulsions are a special class of emulsion where all droplets are of uniform size. The properties of emulsions (rheology, appearance, stability, reaction kinetics) are determined by the properties of both continuous and dispersed phases but also by the characteristics of the droplets themselves. It is for this reason that monodisperse emulsions are often sought after, as droplet size can have such a large influence upon emulsion behaviour. Having a uniform emulsion results in more predictability and allows for easier design in emulsion properties. Monodisperse emulsions find uses in many academic and industrial fields including pharmaceuticals, food science, paints, and coatings. This work covers two broad approaches to monodisperse emulsion creation; microfluidics and controlled shear. Microfluidics is a rapidly emerging technology where liquid flows are constrained to sub-millimetre channel sizes thus creating highly laminar and controllable flows. The methods are used in various lab on chip, and droplet creating applications. A study is undertaken on the nature of buoyancy-driven formation of drops from microchannels, attempting to further understand the fundamental principles of monodisperse drop generation at nozzles. Droplet producing microfluidic devices often suffer, however, from jetting when the desired emulsions are viscous or have low interfacial tensions, resulting in polydispersity. This work introduces two methods to overcome this, surfactant shielding and core-shell templating. Surfactant shielding is a method by which the nozzle of a droplet producing capillary tip is protected from surfactants by a tertiary, pure continuous phase thus limiting the reduction of interfacial tension at the point of droplet creation. Core-shell templating is a method of introducing water droplets into the stream of a would-be jetting system. These water droplets introduce regular instabilities which have the effect of forcing the system into a quasi-dripping regime and thus create highly monodisperse viscous emulsions. Controlled shear is another method for creating monodisperse emulsions whereby a coarse emulsion is subjected to a uniform shear stress resulting in a smaller more monodisperse emulsion. The work investigates two geometries for doing this, a cylinder-curved plate and a cylinder-flat plate. Both these designs are shown to have higher throughput rates than conventional shear methods. In the final part of this work, microfluidics and controlled shear are combined in an attempt to utilise the contrasting benefits found in both techniques. A study is undertaken in the possibility of shearing monodisperse precursor emulsions created via microfluidic techniques, to obtain uniform emulsions of much smaller size. A microfluidic shear cell is also introduced which aims to combine the benefits of a shear device (increased throughput rates, ability to handle viscous fluids) with the benefits of microfluidics (no moving parts, more control).
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:677188 |
| Date | January 2015 |
| Creators | Josephides, Dimitris Noel |
| Contributors | Sajjadi-Emami, Shahriar ; Suhling, Klaus |
| Publisher | King's College London (University of London) |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://kclpure.kcl.ac.uk/portal/en/theses/optimising-monodisperse-emulsion-creation(399fe892-a5e6-4dab-941a-457e620d651f).html |
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