Dual-stream of monodisperse droplet generator

•A piezoelectric droplet generator characterized by two different-sized orifices is designed.•The diameter ratio betweentwo orifices in a plate has to be less than 2.4.•The properties of generated liquid streams are visualized by high-speed shadowgraphy. The generation of droplets with controllable sizes is important in various engineering applications. However, existing studies commonly focused on generating a single monodisperse droplet stream. The generation of dual-stream droplets with different controllable sizes and spacings has not been clearly investigated. Here we design a piezoelectric droplet generator characterized by two different-sized orifices in a metal plate. On the basis of Rayleigh breakup theory, the practical diameter ratio of the larger and smaller orifices is qualitied, with a value less than 2.4. Experiments are carried out on four different designed orifice plates. The ranges of excitation frequency that facilitate monodisperse dropletformation are identified. The formation process and transformation characteristics of the droplet streams are visually analysed with high speed shadowgraphy. Meanwhile, a non-dimensional number Rs is proposed to describe the volume flow rate fraction in the operation of orifice plates. Experimental results show that the design works with high stability.