Droplet break-up with negative momentum Fluid Dynamics Videos

TL;DR

This paper investigates droplet break-up at low Weber numbers with negative momentum, revealing that droplets can reverse direction and return to the nozzle, supported by experimental high-speed imaging and CFD simulations.

Contribution

It presents novel experimental and simulation evidence of droplet break-up dynamics at low Weber numbers with negative momentum, a phenomenon not widely documented before.

Findings

Droplets can break up and then return to the nozzle due to flow inversion.

Flow velocity inside the droplet is slowed and inverted by surface tension.

CFD simulations effectively illustrate the break-up and return process.

Abstract

The ejection of liquid droplets from a nozzle is highly important for physics of fluid. The Weber number describes how much kinetic energy is needed to overcome the surface tension and create a free-flying droplet. According to literature Weber numbers above 12 assure the creation and safe break up of a liquid droplet. However, even when this number goes down below 8, it is still possible to observe droplet break-up but sometimes with particular effects. We present here a fluid dynamics video showing experimental results and CFD simulations for droplet break-up at low Weber number where the droplet is generated with negative momentum. Such droplet generation is characterized by the droplet breaking up and then returning back into the nozzle. This is due to the fact that during the droplet formation the surface tension begins to slow down the flow velocity inside the droplet and then finally inverts the flow direction, while the droplet tail still breaks off from the nozzle. Thus after the break up the droplet momentum is oriented toward the nozzle. It is therefore possible to observe the droplet returning into the bulk fluid. High-speed images of this particular phenomenon are shown and simulation results are presented to illustrate the break up dynamics and the local velocities in the droplet.