Dependence of aggregate formation of microbubbles upon ultrasound condition and exposure time.

We have previously reported our attempts to control microbubbles (microcapsules) behavior in flow by primary Bjerknes force to increase the local concentration of the bubbles at a diseased part. However, there was a limitation in efficiency to propel bubbles of µm-order size. Thus we consider that forming aggregates of bubbles is effective to be propelled before entering into an ultrasound field by making use of secondary Bjerknes force under continuous ultrasound exposure. In this study, we observed the phenomena of aggregates formation by confirming variation of diameter and density of aggregates under various conditions of ultrasound exposure. Then we elucidated frequency dependence of the size of aggregates of micro-bubbles.