A laboratory study reports pinch-off in a downward-flowing water jet under femtosecond laser forcing at a spacing where classical linear theory predicts stability. Under high-energy forcing, the reported pinch-off occurred at a dimensionless wavenumber of 1.9 with pulse energy above 3 μJ, and droplets were produced at 0.2 MHz.
The study’s central question was whether finite-amplitude forcing could produce complete droplet pinch-off where infinitesimal perturbations are linearly stable, rather than merely causing a temporary deformation. A finite-amplitude disturbance is a deliberately sized initial ripple, not the vanishingly small disturbance used in linear stability analysis.
How the jet was controlled
The experiments used downward-flowing deionized-water jets 90 to 130 μm in diameter, typically about 100 μm, moving at 10 to 20 m/s. The protocol linked perturbation spacing to laser repetition rate and jet velocity, expressed that spacing relative to jet radius with a dimensionless wavenumber, and used pulse energy to set perturbation amplitude. Time-resolved pump-probe shadowgraphy followed the sequence from plasma formation to droplet fragmentation.
The images also separated the early shock stage from the lasting surface response. The reported shock stage showed no measurable long-lived deformation, while the first persistent deformation appeared on microsecond scales through cavitation.
The familiar rule appeared first
Before looking beyond the classical boundary, the measured growth spectrum closely followed the Rayleigh-Plateau curve. Growth was greatest near dimensionless wavenumber 0.7, and the reported stability cutoff was 1. That baseline matters because the experiment reproduced the transition between growing and decaying perturbations before reporting breakup beyond it.
As pulse energy increased, breakup length followed a fitted linear-log relation until a crossover at 2.18 μJ. Beyond that point, the length entered a saturation plateau. The report describes that plateau as consistent with strongly nonlinear seeding.
The contrast was clearest above the classical cutoff. At 2.4 μJ and a dimensionless wavenumber from 1.5 to 1.7, the jet did not break up and the surface perturbation decayed. Under the higher-energy condition above 3 μJ, however, the reported case at 1.9 pinched off and produced droplets at 0.2 MHz.
One setup, several breakup patterns
The reported breakup patterns varied with wavenumber. At 0.9, the report describes synchronized monodisperse droplets, meaning a single droplet-size population; at 0.26, it describes two-population breakup; and at 0.05, isolated primary droplets.
At the 0.9 condition, analysis of 40 successive droplets over approximately 47 seconds gave a reported major axis of 183 plus or minus 2 μm and a minor axis of 169 plus or minus 1 μm. The report also described about 1% shape variation and centroid stability within about 11 μm.
There was also an upper edge to discrete breakup. At sufficiently high wavenumber, when the perturbation wavelength was no more than about one jet diameter, successive deformations overlapped. The reported response in that overlap regime was irregular spraying rather than discrete pinch-off.
A result tied to one laboratory system
The pattern was not a single outcome across high-wavenumber conditions. At one higher-wavenumber condition the perturbation decayed, at another high-energy condition it pinched off, and at tighter spacing the jet sprayed. The supplied document is an arXiv v1 preprint dated 1 October 2026.
The authors acknowledged support from the Department of Atomic Energy, Government of India. They declared no competing interests and said the data needed to evaluate the conclusions are in the main and/or Supplementary sections, with additional data available on request.
Paper data and sources
Original title: Femtosecond laser-induced cavitation seeds liquid-jet breakup beyond the Rayleigh-Plateau stability limit
Authors: Sourabh Singh, Tamanna, S. Sree Harsha, Prashant Kumar Singh
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-10-01
DOI: Not available
Original paper · Full text