Pneumatic drop generator: Liquid pinch-off and velocity of single droplets

Abstract

A chamber was designed for a pneumatic drop generator with interchangeable nozzles and capable of producing single droplets with different sizes and from various liquids. Using high speed imaging along with image processing, velocity of droplets was investigated for a range of liquid viscosities and surface tensions that has not been studied to date. It was shown that in optimal working conditions, pressure pulse applied to the liquid column in a drop on demand system does not give an initial velocity to the generated droplet; while in higher pressures, the drop is pushed out of the nozzle with an initial velocity and satellite droplet(s) are also generated. However, if the pressure is just above the optimum level, instead of the usual one pinch-off, two pinch-offs were observed leading to generation of one tiny satellite droplet. Nevertheless, the satellite droplet either merged into the main drop or moved upward and merged with the meniscus in the nozzle to ultimately generate a single droplet. In this case the droplet still had a negligible initial velocity.