A type of non-axisymmetric oscillations of acoustically levitated drops is excited by modulating the ultrasound field at proper frequencies. These oscillations are recorded by a high speed camera and analyzed with a digital image processing method. They are demonstrated to be the third mode sectorial oscillations, and their frequencies are found to decrease with the increase of equatorial radius of the drops, which can be described by a modified Rayleigh equation. These oscillations decay exponentially after the cessation of ultrasound field modulation. The decaying rates agree reasonably with Lamb’s prediction. The rotating rate of the drops accompanying the shape oscillations is found to be less than 1.5 rounds per second. The surface tension of aqueous ethanol has been measured according to the modified Rayleigh equation. The results agree well with previous reports, which demonstrates the possible application of this kind of sectorial oscillations in noncontact measurement of liquid surface tension.
SHEN ChangLe, XIE WenJun & WEI BingBo Department of Applied Physics, Northwestern Polytechnical University, Xi’an 710072, China
The dynamic solidification of ternary Ag38.5Cu33.4Ge28.1 eutectic alloy within a 35 kHz ultrasonic field is investigated and compared with both its equilibrium solidification by DSC method and its rapid solidification in drop tube. The volume fractions of the primary （Ge） phase and pseudobinary （Ag＋ε2） eutectic solidified within ultrasonic field are larger than those formed under equilibrium state, whereas that of ternary （Ag＋ε2＋Ge） eutectic exhibits the reverse trend. During rapid solidification, the liquid alloy droplet directly solidifies into ternary （Ag＋ε2＋Ge） eutectic if its diameter is smaller than 350 um. The ultrasound stimulates the nucleation of alloy melt and prevents the bulk undercooling. With the increase of sound intensity, the primary （Ge） phase transfers from faceted dendrites to nonfaceted blocks with blunt edges, and its grain size is remarkably reduced. Both pseudobinary （Ag＋ε2） and ternary （Ag＋ε2＋Ge） eutectics experience a morphological transition from regular to anomalous structures. This indicates that their cooperative growth mode is replaced by independent growth of eutectic phases under the combined effects of cavitation and acoustic streaming. The ultrasound also shows a prominent coarsening effect to the pseudobinary （Ag＋ε2） and ternary （Ag＋ε2＋Ge） eutectics.
