Continuous Drop Formation at a Capillary Tip and Drop Deformation in a Flow Channel

Description

Emulsions are meta-stable systems of liquids dispersed in another immiscible matrix fluid. They are encountered in a large variety of application areas including food, cosmetics, pharmaceutics and polymers. The mean size and the size distribution of the droplets represent important characteristics of an emulsion which affect the emulsion quality significantly. These physical properties are adjusted by a proper choice of the dispersing apparatus and the process conditions. Examples of well-established dispersing devices include rotor-stator systems or homogenizers. In these operation units droplets are subject to shear and elongational stresses and fragment into smaller droplets provided that the flow-generated stresses are supercritical. Generally, the dispersed droplets underlie a certain size distribution. Recently, the demand for almost monodisperse emulsions has been rising due to new advances in the production of microcapsules or specially structured multiphase systems.In this study an emulsification process was developed aiming to generate monodisperse emulsions. The disperse phase was injected via a capillary into a co-flowing matrix fluid. The capillary was positioned on the centerline of a flow channel where the velocity of the continuous phase is at a maximum. Two different drop formation mechanisms were distinguished: Either the drops break up close to the capillary - dripping - or they break up from an extended liquid jet - jetting. The effect of the various process parameters on the droplet size depends on the breakup mechanism and was investigated for each flow domain separately. Consequently, the transition point between the flow domains represents an important operating point which was determined experimentally by varying the material and process parameters. In the dripping mode the interplay of the counteracting forces, drag force of the continuous phase and interfacial tension force at the capillary, governs the drop breakup. Through the application of a well-defined flow field of the continuous phase the droplet size could be controlled externally. The effect of the process and material parameters on the mean droplet size and the size distribution in the dripping regime was studied. It could be shown that dropformation at a capillary tip represents a promising technique for the production of monodisperse droplets. In the jetting domain the drop breakup occurs due to the propagation of interfacial waves. Besides the droplet size and their size distribution, the jet length and jet diameters represented further parameters to be investigated. It was found that the droplet size distribution is not necessarily as narrow as in the dripping mode. Nevertheless, flow conditions were found where almost monodisperse droplets were generated in the jetting mode, too. The experimental results were compared with theoretical values obtained from stability theories. It is shown that under specific flow conditions the validity of the theory is limited.This work was embedded in the project "Structure engineering of emulsions by micro-machined elongational flow processing". The focus of this project was on generating, deforming and fixating droplets in order to manipulate the microstructure of a dispersion. Therefore, in addition to investigations on drop formation, this work involves studies on drop deformation in laminar channel flow. Droplets were injected eccentrically into a narrowing flow channel via a specially designed injection tool where monodisperse droplets were generated. Since the droplets left the injection tool on a well-defined streamline, they experienced same stresses and adopted identical shapes along the flow channel. The drop deformation could be correlated to the process and material parameters. Further, the possibility of imprinting large deformations on droplets in laminar channel flow is demonstrated. In summary, a flow device was constructed where droplets were both generated and deformed. This study provides new insight into the behavior of multiphase systems in flow. A comprehensive study on the drop formation in a co-flowing liquid-liquid system was still lacking although it is of relevance in a large variety of dispersing processes. This work may be regarded as afirst experimental step in printing out the effect of the various parameters on the drop breakup at a capillary in a co-flowing environment.