Numerical and theoretical study on the forced breakup of capillary liquid jets in ambient gas

The forced breakup of liquid jets in ambient gas surroundings is studied systematically through numerical simulations and theoretical analyses, with particular emphasis on characterising the response modes of jet breakup across wide ranges of perturbation frequency and amplitude. Simulations reveal that the breakup of liquid jet can be effectively synchronised with external actuation within a frequency range encompassing the natural breakup frequency, thereby enabling the generation of highly uniform droplets. As the perturbation frequency exceeds an upper critical value, the external perturbation cannot dominate the jet breakup, while below a lower critical frequency, the jet breaks up with multiple droplets generated within one period. A high perturbation amplitude can result in liquid accumulation, leading to the formation of a pancake-shaped jet configuration. Through spectrum analyses, the development of jet interface perturbations under different response modes is elucidated, revealing the competition between the natural frequency and the external frequency. A linear instability analysis of a liquid jet is performed, which successfully predicts the synchronised frequency range by comparing the breakup time between the free liquid jet and the actuated jet, along with the variation tendencies of jet breakup length with varying perturbation frequency, amplitude and jet velocity. Quantitative numerical results demonstrate that in the case of multiple droplet generation under low perturbation frequency, the rear droplet maintains a higher velocity than its leading counterpart and the emergence of a high-pressure zone at the leading edge of a droplet train facilitates the droplet coalescence. Furthermore, the study introduces an innovative approach by superimposing periodic pulses onto the sinusoidal perturbation waveform, enabling active modulation of multiple droplet merging dynamics. This fundamental study is intended to offer valuable guidance for the on-demand generation of droplets in various industrial applications. © The Author(s), 2025. Published by Cambridge University Press