Influence of Tween 20 on the Formation and Acoustic Properties of Monodisperse Microbubbles Produced by Flow-focusing

Monodisperse microbubbles (MDMBs) can be efficiently produced by flow-focusing microfluidic chips. Using lipid formulations with higher concentration can inhibit bubble coalescence at orifice of microfluidic chips. However, the low coating efficiency of lipids on bubble shell results in poor cost-effectiveness in MDMBs fabrication. Recent reports have shown MDMBs can be produced without coalescence under low lipid concentration, with the addition of surfactant. This study aimed to study how surfactant Tween 20 affects formation and acoustic properties of MDMBs. The coalescence rate of MDMBs was suppressed by Tween 20, and was eventually eliminated when Tween 20 was more than 2 mg/mL. MDMBs produced with lipid and Tween 20 mixture were stable, with a polydisperse index (PDI) of < 5% after produced. The ratio R<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</inf>/R<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</inf> (radius at orifice, radius in 24 h, respectively) increased with the increasing Tween 20, especially at lower lipid concentrations. The shell elasticity and viscosity increased from 0.22 N/m to 0.61 N/m and from 1.2×10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−8</sup> kg/s to 2.0×10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−8</sup> kg/s, respectively, which was larger than that of the control sample. Besides, under fixed Tween 20 concentration (20 mg/mL), when increasing the lipid concentration from 1 mg/mL to 3 mg/mL, both elasticity and viscosity decreased. These results indicates that Tween 20 was involved in the shell coating of MDMBs so that all the bubble radius, elasticity and viscosity were related to Tween 20.