Drop formation from a wettable nozzle

The process of drop formation from a nozzle can be seen in many natural systems and engineering applications. However, previous research focuses on the pinch-off mechanism of drops from a non-wettable nozzle. Here we investigate the formation of a liquid droplet from a wettable nozzle. In the experiments, drops forming from a wettable nozzle initially climb the outer walls of the nozzle due to surface tension. Then, when the weight of the drops gradually increases, they eventually fall due to gravity. By changing the parameters such as the nozzle size and fluid flow rate, we have observed different behaviors of the droplets. Such oscillatory behavior is characterized by an equation that consists of capillary force, viscous drag, and gravity. Two asymptotic solutions in the initial and later stages of drop formation are obtained and show good agreement with the experimental observations.

► We study the capillary effect of drop formation in varying fluxes and nozzle sizes.
► We develop a mathematical model to describe the drop motion.
► A single drop moves upward due to surface tension as it forms.
► A single drop falls down due to gravity after moving upward.