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Understanding the shell rheology of ultrasound contrast agent microbubbles isvital for anticipating their bioeffects in clinical practice. Past studiesusing sophisticated acoustic and optical techniques have made enormous progressin this direction, enabling the development of shell models that adequatelyreproduce the nonlinear behaviour of the coated microbubble under acousticexcitation. However, there have also been puzzling discrepancies and missingphysical explanations for the dependency of shell viscosity on the equilibriumbubble radius, which demands further experimental investigations. In thisstudy, we aim to unravel the cause of such behaviour by performing a refinedcharacterisation of the shell viscosity. We use ultra-high-speed microscopyimaging, optical trapping and wide-field fluorescence to accurately record theindividual microbubble response upon ultrasound driving across a range ofbubble sizes. An advanced model of bubble dynamics is validated and employed toinfer the shell viscosity of single bubbles from their radial time evolution.The resulting values reveal a prominent variability of the shell viscosity ofabout an order of magnitude and no dependency on the bubble size, which iscontrary to previous studies. We find that the method called bubblespectroscopy, which has been used extensively in the past to determine theshell viscosity, is highly sensitive to methodology inaccuracies, and wedemonstrate through analytical arguments that the previously reportedunphysical trends are an artifact of these biases. We also show the importanceof correct bubble sizing, as errors in this aspect can also lead to unphysicaltrends in shell viscosity, when estimated through a nonlinear fitting from thetime response of the bubble.

Shell viscosity estimation of lipid-coated microbubbles