Investigation of capillary standing waves on Ultrasonic Electric Propulsion system

As a novel space propulsion technology, Ultrasonic Electric Propulsions system mainly employs crests of capillary standing waves as emitters to produce charged droplets with higher impulse and thrust density. In this work, the formation mechanism of capillary standing waves is investigated by experimental and numerical methods. Different morphologies of capillary standing waves are captured by an imaging system and their wavelengths at vibration frequencies 120 kHz, 60 kHz and 25 kHz are measured and compared with theoretical corresponding values, respectively, which are in a good consistency. Irregular patterns of capillary standing waves on emission surface are first found in experiments, which are characterized by partial coverage and alterable boundaries. In order to further explore their micro characteristics, capillary standing waves are also simulated with different vibration frequencies and amplitudes. It is found that multiple factors such as amplitude, vibration frequency and thickness of liquid film affect the formation and condition of capillary standing waves, and a higher vibration frequency with great amplitude more easily forms atomization on emission surface, which emits droplets mainly by ultrasonic vibration rather than electrostatic force. A larger thickness of liquid film needs higher vibration intensity to form capillary standing waves.