Abstract: In ultrasonic aided electrospray thrusters, liquid films are destabilized and broken up under the effect of acoustic vibration and electrostatic field, then droplets emitted and thrust produced. In order to reveal the physical mechanism of initial instability of liquid films in ultrasonic aided electrospray thrusters, the linear instability of liquid films under the coupling effect of ultrasonic and electric field force is studied theoretically. Based on the Floquet theory, the linear instability of the planar liquid film under the coupling effect of acoustic vibration and electrostatic field is analyzed, and the dispersion relationship among the rate of disturbance growth, frequency and wavenumber is solved. According to the dispersion curves, the influence of electrical, oscillation and physical parameters on the linear instability is obtained. The results show that the effect of the liquid conductivity and dielectric constant on the instability can change with the ratio of the plate spacing to the liquid film thickness. When the growth rate of the parameter unstable region is large, the increase of the electric field force will promote the instability, and when the growth rate of the parameter unstable region is small, the increase of the electric field force will restrain the instability.

Key words: ultrasonic aided electrospray, leaky dielectric, electrostatic field, acoustic oscillations, parametric instability