Excitation of a vertical liquid capillary jet waveguide by modulated ultrasonic radiation pressure

Abstract

The excitation of a liquid capillary jet issuing from a nozzle is investigated using internally applied modulated ultrasonic radiation pressure. The transducer used here is more efficient than one used in prior studies [J. B. Lonzaga et. al., J. Acoust. Soc. Am. 116, 2598 (2004)] and is suitable for nozzle velocities as low as 25 cm/s. At low velocities, the liquid jet is significantly tapered. As a consequence of the taper, the acoustic cutoff frequency increases with increasing distance from the nozzle. For ultrasound propagation down the jet from the nozzle, finite‐element calculations of the radiation pressure show that the radial stress at the cutoff location is significantly larger than at any other region of the jet. Two distinct capillary modes exist and are conjectured to be excited near the cutoff location by modulated radiation pressure: one traveling upward and the other traveling downward. In a certain range of modulation frequencies, the latter is an exponentially growing mode and leads to the forced disintegration of the jet into liquid drops. Excitation of the growing mode at the nozzle is achieved for low‐speed jets by using carrier frequencies close to or below the nozzle cutoff frequency. [Work supported by NASA.]