超聲波懸浮是一種透過聲音輻射壓用來移動或固定需要高純度的樣品在半空中的非接觸式懸浮方法，藉由產生高強度聲音駐波，物體可以被懸浮在壓力節點上。和磁懸浮或電懸浮相比，因為他對於材料的限制較少，近年來有許多研究在探討其應用，包括懸浮極端環境的過冷水滴或融化的金屬液滴，或是利用時空上調制超聲波陣列的輸出強度來移動物體，但懸浮的穩定性一直是超聲波懸浮待解決的問題。 在這篇論文裡主要是探討水滴在不穩定的單節點聲音懸浮場中如何運動和其基本運動模態。透過降低聲場強度，實驗上觀察到水滴從穩定懸浮到不穩定的自發增長震盪包括準週期震盪、渾沌或是更加混亂的水滴破碎，而震盪增長和失穩是起源於小腔體裡區域聲場被水滴散射而導致的等效負阻尼力，當等效負阻尼力大於空氣阻力和水滴內部的黏滯力，水滴震盪的振幅將會增長進而導致非線性自發震盪的產生。 水滴自發的模態主要是由表面張力、重力、聲場強度和模態數所決定的。大顆的水滴的自發震盪是由振幅逐漸增長的垂直振盪模態跟緯向(zonal)模態所構成，當振幅過大時，大顆水滴將會掉落至底板。小顆的水滴則是由不同的扇形(sectorial)模態跟被誘發出來的緯向模態所組成，這些模態隨著時間演化，頻率跟振幅均互相調制而形成混亂的震盪模式。當這些模態同時激發時，因為表面張力無法抵抗劇烈的動量變化，水滴會在扇葉形狀的尖端擠出小水滴而導致水滴破碎。;Contactless handing of drops in air is an important technique for various basic researches and applications requiring high purity and less contamination. In recent years, acoustic levitation has attracted a great deal of interests due to its less restriction in materials. By using the acoustic radiation force, the drop can be suspended around the pressure node of an intensive acoustic standing wave. However, the stability of acoustic levitation is still one of the problem comparing with other techniques like electrostatic or magnetic levitation. In this work, the stability of mm sized water drops suspended in a single-node acoustic levitation system is investigated experimentally. The unstable self-excited oscillations, including quasi-periodic oscillation, chaotic oscillation, and shedding of the drop by decreasing the acoustic wave intensity below a threshold, are observed. The perturbation of the local acoustic field made by the drop motion in a small cavity could be a possible source for generating an effective negative damping force to sustain the growing amplitude of the oscillation and leads to the unstable of the levitation. The self-excited modes are determined by the interaction between surface tension, gravity, drop inertia, acoustic intensities, and mode numbers. The large drop shows self-excited quasiperiodic motion formed by a vertical oscillation mode and a zonal-like shape oscillation mode with growing coupling and amplitude, until falling to the ground. For small drops, chaotic shape oscillations are composed of several interacting spectrum-broadened sectorial modes and parametrically induced zonal modes. The high amplitude shape oscillation leads to droplet shedding at highly stretched lobes.
