  Understanding the shell rheology of ultrasound contrast agent microbubbles is
vital for anticipating their bioeffects in clinical practice. Past studies
using sophisticated acoustic and optical techniques have made enormous progress
in this direction, enabling the development of shell models that adequately
reproduce the nonlinear behaviour of the coated microbubble under acoustic
excitation. However, there have also been puzzling discrepancies and missing
physical explanations for the dependency of shell viscosity on the equilibrium
bubble radius, which demands further experimental investigations. In this
study, we aim to unravel the cause of such behaviour by performing a refined
characterisation of the shell viscosity. We use ultra-high-speed microscopy
imaging, optical trapping and wide-field fluorescence to accurately record the
individual microbubble response upon ultrasound driving across a range of
bubble sizes. An advanced model of bubble dynamics is validated and employed to
infer the shell viscosity of single bubbles from their radial time evolution.
The resulting values reveal a prominent variability of the shell viscosity of
about an order of magnitude and no dependency on the bubble size, which is
contrary to previous studies. We find that the method called bubble
spectroscopy, which has been used extensively in the past to determine the
shell viscosity, is highly sensitive to methodology inaccuracies, and we
demonstrate through analytical arguments that the previously reported
unphysical trends are an artifact of these biases. We also show the importance
of correct bubble sizing, as errors in this aspect can also lead to unphysical
trends in shell viscosity, when estimated through a nonlinear fitting from the
time response of the bubble.
