We report on the ninth-mode sectorial oscillation of acoustically levitated drops excited by actively modulating sound pressure. A numerical computation based on the level set method was performed to model drop shape evolution by solving an incompressible two-phase flow problem. The calculated shapes of the oscillating drop are in good agreement with experimental observations. The relationship between the oscillation frequency and parameters describing the flattened drops is studied both experimentally and numerically. The frequency of the ninth-mode sectorial oscillation decreases with increasing equatorial radius and can be well-described by a modified Rayleigh equation. Drop oscillation has been attracting considerable attention in various fields for over a century. In the last few decades, a series of studies have been performed to explore the oscillatory dynamics of liquid drops with both experimental and numerical techniques [1][2][3][4][5][6][7][8][9][10]. Many experimental studies involving axisymmetric oscillations were performed using electromagnetic levitation, acoustic levitation, electrostatic levitation and aerodynamic levitation in recent years [4,[6][7][8]11]. However, only few studies have been attempted on non-axisymmetric oscillations of an isolated drop [12][13][14]. Understanding the nature of the non-axisymmetric oscillations is desirable, not only from a scientific viewpoint but also for practical aspects related to material processing under containerless conditions. The suspension of liquid drops provides a preferable boundary condition for the investigations of various free surface phenomena. Acoustic levitation is an important containerless processing technique for in situ investigations of free-drop dynamics that require special physical environments [15][16][17][18]. It has the particular advantage of not being constrained by electromagnetic properties of the samples.Sectorial oscillations are a non-axisymmetric type of oscillation and can be characterized by either top-view or bottom-view images. Thus far, single-mode sectorial oscillations of acoustically levitated water drops have been observed up to the eighth mode. In this work, we report the ninth-mode sectorial oscillation obtained using an active modulation method. A numerical computation incorporating level set method is employed to model the observed drop shape evolution. Both experimental measurement and numerical prediction are used to investigate the relationship between the oscillation frequency and parameters describing the flattened drops.
Experimental methodThe experiments are performed using a single-axis acoustic levitator, which is actuated by a piezoelectric (PZT) transducer at a frequency of 22 kHz. In the experiments, the acoustic radiation force suspends a water drop with diameter of 5 mm. To excite the levitated water drop into an oscillating state, we actively modulated the electric voltage V applied to the PZT transducer. The form of this modulation is V = V 0 (1+ cos m t)cos ac t, where m and ac ar...