  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.
