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 70kPa-300kPa. 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(P2), P3, and two different P4 oscillations at low excitation amplitude. Despite the reduced damping of the uncoated bubble system, such enhanced nonlinear oscillations has not been observed and can not be theoretically explained for the uncoated bubble. To investigate the mechanism of the enhanced nonlinearity, 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 causes the intensified non-linearity at low excitations. The simulated scattered pressure by single MBs are in good agreement with the experimental signals.
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