What Determines the Breakup Length of a Jet?

The breakup of a capillary jet into drops is believed to be governed by initial disturbances on the surface of the jet that grow exponentially. The disturbances are often assumed to be due to external sources of noise, to turbulence, or to imperfections of the nozzle. Here we demonstrate that the initial disturbances observed across a wide range of conditions are quantitatively consistent with thermal capillary waves, where the initiating disturbances must be of the order of an angstrom, suggesting that thermal noise can act as a primary driver of jet breakup under typical experimental conditions. Our experiments with a wide range of nozzles show no significant variation in breakup length linked to nozzle type, shape, or inner roughness. By systematically varying the jet diameter and velocity and the fluid properties, we validate our thermal disturbance model over 4 orders of magnitude in jet length, and 7 orders of magnitude when previous molecular dynamics simulations and stochastic hydrodynamics calculations of nanojets are included.