(95b) Response of Liquid Jets to Internal Modulated Ultrasonic Radiation Pressure

Liquid jet excitation by internal pressure pulsation is a well-known technique to modify the jet dynamics. In this case the jet responds at the frequency of the pressure oscillation and the disturbance is localized near the nozzle. If one wishes to study the jet response over a range of frequencies this direct method of excitation has the disadvantage that it is difficult to maintain a constant amplitude as the frequency is changed owing to such things as mechanical resonances of the drive system. One way to avoid this problem is to use a second-order pressure effect, that of acoustic radiation pressure. The jet is driven by modulation of the ultrasound amplitude while the ultrasound frequency is kept constant to avoid problems with mechanical resonances. If the ultrasound frequency is high enough, the jet acts as a waveguide so that the radiation pressure is applied to the surface of the jet over a large region beyond the nozzle. Modulated ultrasound was applied to the supply reservoir for a vertical low-velocity jet of several millimeter diameter and the response was monitored at different heights below the nozzle using a light extinction method. The jet profile oscillates at the frequency of the radiation pressure modulation and where the response is small, the amplitude was found to increase in proportion to the square of the transducer voltage as previously predicted and demonstrated for oscillating levitated drops. The length of the liquid jet prior to breakup into drops can be greatly reduced by adjusting the modulation frequency.