Additive-assisted continuous microfluidic formation of uniform micron-scale droplets

Microfluidic devices are effective for producing highly uniform droplets. However, a major current limitation in microfluidic droplet formation processes is that the smallest geometric feature in the device restricts the minimum droplet size that can be generated. In this paper we discuss two methods to overcome this bottleneck, using additives such as surfactants and polymers to form stable, thin threads. These threads lead to the continuous production of micron-scale droplets. The mechanism for forming thin threads depends on the additive. The presence of surfactants at the liquid-liquid interface leads to strong surface concentration gradients, which allows the liquid interface to form a highly sharpened cusp and tiny threads to be drawn from this tip. A small amount of soluble polymer in the dispersed phase liquid imparts elasticity to the bulk solution, which stabilizes the thread against breakup.