Breakup of a Rectangular Laminar Jet in Immiscible Liquid-Liquid Systems
The breakup of a laminar liquid jet is the underling phenomena used to generate emulsions in micro-scale devices. Jet breakup is induced by the most unstable disturbance growing on the jet surface, and linear instability analysis can be utilised to predict breakup length and frequency and the resultant droplet size. Instability analysis has been extensively applied to axi-symmetric (circular) jets. However, this configuration is not really practical in micro devices, where rectangular geometries are most often used. This study investigates the influence of the rectangular shape on the jet instability for a water-in-oil system. The breakup of square (2.2 × 2.2 mm) and rectangular water jets (with different aspect ratios) were monitored using a high speed camera. In both cases, jet breakup was governed by symmetric disturbances. For the rectangular jets, the surface disturbance on the shorter side grew faster and controlled the breakup process. A two-dimensional linear analysis for a planar jet was developed to determine the most unstable infinitesimal disturbance. For the square orifice, it was found that the axi-symmetric linear stability model, based on the nozzle hydraulic diameter, was successful in predicting the resultant droplet size. However, the axi-symmetric model was not able to be applied to the rectangular jets and the planar model was required. The influence of rectangular length/width ratio on the droplet size and breakup frequency is discussed in relation to the water-in-oil emulsification process.