The measurement of growth rates in capillary jets

The growth of perturbations on a capillary jet issuing from a circular nozzle in the Rayleigh regime is experimentally investigated. Electrohydrodynamic sinusoidal stimulation is employed to this end, along with two independent methods to obtain growth rates of the linear regime with the best accuracy so far. The first method exploits the correlation between the stimulation voltage and the breakup time measured with the help of stroboscopic images of the jet. The second method is an analysis of the spatial evolution of perturbations through a local jet-shadow-width photometry, with careful avoidance of the initial transient and the final nonlinear stages. Experiments conducted with ink allow the application of both methods, as the liquid is opaque. They give consistent results, with very small statistical errors, with respect to the expected theoretical dispersion relation, once the dynamic surface tension is adjusted. The adjusted value is in accordance with an estimate made from drop-dynamics experiments also reported here. By dealing with a simpler liquid (aqueous solution of NaNO3), we are able to compare results from the first method against the theoretical predictions without adjustment of any parameter. The agreement is again excellent. Possible sources of systematic errors in this kind of measurements are identified and procedures for avoiding them are designed.