Continuous inkjets, consisting of a stream of ink broken into regularly spaced droplets, are often used in industrial applications to write batch numbers and expiration dates on manufactured items. A continuous jet of liquid is subject to instability as first described by Lord Rayleigh, who showed that axisymmetric jets of liquid are unstable to disturbances whose wave-length exceeds the circumference of the jet.
To add some measure of control and reproducibility to continuous inkjets, a prescribed perturbation is typically applied at the jet origin that insures a breakup of the jet into a series of regularly spaced droplets. The accompanying figure shows a FLOW-3D simulation of a continuous jet having an initial diameter of 60μm and a mean axial speed of 10m/s. The jet liquid has the properties of water. A sinusoidal perturbation of frequency 12.57e4rad/s in velocity at the jet origin results in the breakdown of the jet into droplets.