Phospholipid-coated microbubbles: the acoustic signature of monolayer buckling

In medical ultrasound imaging, the echo of the blood pool is enhanced using ultrasound contrast agents. The contrast agent suspension consists of microbubbles (1 to 5 $\mu$m in radius) of an inert gas coated with a phospholipid monolayer. We characterize the changes in microbubble dynamics due to the coating, through combined micromanipulation by means of optical tweezers and ultra-high speed imaging at 15 million frames per second with the Brandaris 128 camera. The experiments reveal that buckling of the phospholipid monolayer increases the non-linear response of the contrast agent bubbles at low acoustic pressure.