Parametric instability of dielectric, slightly viscous liquid jets under ac electric fields

The ac electric fields longitudinally tangential to a dielectric liquid column are able to excite capillary waves on the cylindrical surface. The phenomenon is theoretically predicted using a Lagrangian formulation of the dynamics of small amplitude perturbations of the free surface. As in the de case, dielectric forces tend to stabilize those perturbations affected by the capillary instability, but if the field is periodic parametric resonance takes place for perturbations of shorter wavelength, otherwise stable with de fields. A detailed description of the stability spectrum is included and the role of field intensity and frequency in the parametric resonance phenomenon is discussed. Non axisymmetric modes are also excited but they are always dominated by the axisymmetric one. The effect of a small viscosity is to diminish or even suppress unstable regions. The field necessary to observe parametric resonance is calculated. (C) 1997 American Institute of Physics.