Wetting controls of droplet formation in step emulsification

Significance Step emulsification is one of the few methods that enables upscaled production of droplets with a monodispersity desired for many industrial processes. However, the physical mechanisms of the droplet formation process are poorly understood. We study the droplet breakup by experiments and simulations and find a simple theory predicting the transition from dripping to jetting and the minimal contact angle of α = 2π/3 = 120° for which droplets form. These results have important consequences for the development and design of novel microfluidic systems and reactors that address the growing demand for tools to manipulate fluids at the submillimeter scale.