A lipid coated bubble (LCB) oscillator is a very interesting non-smooth oscillator with many important applications ranging from industry and chemistry to medicine. However, due to the complex behavior of the coating intermixed with the nonlinear behavior of the bubble itself, the dynamics of the LCB are not well understood. In this work, lipid coated Definity^® microbubbles (MBs) were sonicated with 25 MHz 30 cycle pulses with pressure amplitudes between 70 and 300 kPa. Here, we report higher order subharmonics in the scattered signals of single MBs at low-amplitude high-frequency ultrasound excitations. Experimental observations reveal the generation of period 2, period 3, and two different period 4 oscillations at low excitation amplitudes. Despite the reduced damping of the uncoated bubble system, such enhanced nonlinear oscillations have not been observed and cannot be theoretically explained for the uncoated bubble. To investigate the mechanism of the enhanced non-linearity, the bifurcation structure of the lipid coated MBs is studied for a wide range of MBs sizes and shell parameters. Consistent with the experimental results, we show that this unique oscillator can exhibit chaotic oscillations and higher order subharmonics at excitation amplitudes considerably below those predicted by the uncoated oscillator. Buckling or rupture of the shell and the dynamic variation of the shell elasticity cause the intensified non-linearity at low excitation pressure amplitudes. The simulated scattered pressure by single MBs is in good agreement with the experimental signals.

中文翻译：

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微观尺度上增强非线性的实验和数值证据：脂质涂层声泡

脂质涂层气泡 (LCB) 振荡器是一种非常有趣的非光滑振荡器，具有从工业和化学到医学的许多重要应用。然而，由于涂层的复杂行为与气泡本身的非线性行为混合在一起，LCB 的动力学还不是很清楚。在这项工作中，脂质涂层 Definity ^®微泡 (MB) 用 25 MHz 30 周期脉冲进行超声处理，压力幅度在 70 到 300 kPa 之间。在这里，我们报告了在低振幅高频超声激励下单个 MB 的散射信号中的高阶次谐波。实验观察揭示了在低激励幅度下产生了第 2 期、第 3 期和两个不同的第 4 期振荡。尽管未涂层气泡系统的阻尼降低，但尚未观察到这种增强的非线性振荡，并且无法从理论上解释未涂层气泡。为了研究增强非线性的机制，研究了脂质包覆 MB 的分叉结构，适用于各种 MB 尺寸和壳参数。与实验结果一致，我们表明，这种独特的振荡器可以在大大低于未涂层振荡器预测的激发幅度下表现出混沌振荡和高阶次谐波。壳的屈曲或破裂以及壳弹性的动态变化导致在低激发压力振幅下增强的非线性。单个 MB 模拟的散射压力与实验信号非常吻合。