(407b) Drop Formation in Co-Flowing Fluid Streams

Production of droplets of controllable size in emulsions plays an important role in a variety of applications such as inkjet printing, DNA arraying, food, cosmetics, and pharmaceuticals. The co-flowing geometry helps to control the drop size by introducing a co-flowing stream of immiscible fluid. Surfactants are present or added intentionally to applications involving drop formation in emulsions to influence the sizes of detaching primary drops and satellite drops. To investigate the surfactant effects on drop formation in a co-flowing system, we conduct a numerical study using a hybrid volume-of-fluid (VOF) numerical method combined with a front-tracking scheme. The surfactants are modeled using a Langmuir adsorption framework. The non-uniform distribution of surfactants at the interface in the dynamic process of drop formation gives rise to Marangoni stresses that affect necking and breakup. We consider the effect of soluble surfactants in the adsorption-desorption limit on the drop formation process. A drop in the co-flowing geometry typically breaks up at the primary neck (close to the primary drop) in the absence of surfactants. At high surfactant coverage, the primary neck formation slows down due to strong Marangoni stresses and breakup occurs at the secondary neck, close to the remnant cone or thread. Increasing the outer co-flowing stream weakens the effect of high surfactant coverage leading to breakup again at the primary neck. The detached drop volume shows non-linear behavior with Biot number in the absence and presence of an outer flow. We will also present our results on the effect on a non-Newtonian fluid rheology on the drop formation process.