Physics of Nano- Micro-Bubbles and their Ultrasound Response

Nano-Biotechnology is coming more important in a medical application. Nanomicro-bubble could have a significant role in this field, like delivery system of drug, enzymes gene and so on. Micro-bubble is utilized as a contrast agent for ultrasound imaging and there are many commercial agents like Levovist, Sonovue, Optison, Sonazoid and so forth. Nano-bubbles can be also used for such contrast agents if the ultrasound frequency is high enough. However, those tiny bubbles have short life time, for example, that of 1μm air bubble is about 10ms in pure water due to the internal high gas pressure caused by the surface tension. For the utilization of nano-bubble, we need some mechanism to stabilize the bubbles to elongate their life time. A surfactant, electrolyte or lipid, which covers the bubble surface, could reduce the surface tension and the diffusion of the internal gas to the surrounding liquid.