Abstract
Production of droplets is a very important process in the chemical and liquid-liquid extraction industries. In order to produce monodispersed droplets, high electric fields have been commonly used in liquid spraying, yet the fundamental aspects are not fully understood. In the work reported in this thesis, various fundamental knowledge relating to the applications of DC and pulsed DC electric fields have been advanced. Various spraying modes (such as dripping mode, cone jet mode and smooth jet mode) have been investigated with respect to highly viscous polyvinyl alcohol aqueous solutions. In order to produce monodispersed droplets, the smooth jet should be used together with pulsating DC field. An optimum frequency has been observed where the droplet production frequency follows linearly the applied pulse frequency until a limit, beyond which the liner relationship does not hold. The optimum frequency is a strong function of the liquid viscosity. An analytical force balance model has been developed for dripping mode. The model allows the estimation of the produced droplet diameter as a function of the applied DC potential. It has been experimentally shown that this model can be applied to a wider range of applied potential compared to other force balance models in the literature. However, as it is a simple model, it does not describe the actual mechanism behind the droplet formation. A number of liquid spraying geometries have been investigated, where optimum electrode geometry has been developed and tested. In this geometry, an optimum distance between the nozzle tip and the earthed electrode has been used such that satellite formation is inhibited, thus producing monodispersed droplets. This is especially beneficial in the spraying of highly viscous and conductive liquids. State-of-the-art high-speed video photography and image-processing software have been used for the quantification of the above phenomena. A macro has been specifically written to automatically process a large number of images. The above monodispersed droplet production technique, together with the high-speed video photography and the macro developed for quantification, can be used commercially and/or industrially to generate monodispersed droplets for various functions.