Bubbles are delicate little things, which are incredibly sensitive to what goes on around them. And when you hit one with a sound wave, it expands and contacts, creating beautiful patterns in the fluid that surrounds it. A bubble is (usually) a small, spherical pocket of gas in a another gas or liquid. When sound -- which is really just the name we give to changes in pressure in a fluid -- hits it, the pressure of the gas inside the bubble changes too. As the pressure changes, the volume changes, so the surface of the bubble moves in and out in interesting ways. That motion pushes and pulls the fluid surrounding the bubble, creating flow patterns.
This image, created by Dr Dario Carugo from the University of Oxford's Biomedical Ultrasonics, Biotherapy and Biopharmaceuticals Laboratory, viusalises those flows. To create it, he placed fluorescent microparticles into the fluid, hit a bubble with ultrasound, and then captured the motion of the particles using a microscope. The bubble itself sits in the middle of the distinctive dumbbell shape. The image was one of the winners of the Engineering and Physical Science Research Council's annual photography competition.
The flow around the bubble doesn't just move in and out as the bubble expands uniformly. Instead, the bubble moves in weird and wonderful ways: Back and forth, in and out and then in a series of higher order oscillations, all of which combine to create the pattern you see in the image. Carugo is experimenting with different ways to use those flow patters to move drugs around the human body.
Image by Dario Carugo/EPSRC