Finding an invisible sphere from the flow around it

Refractive index matching makes an acrylic sphere optically vanish in a sodium iodide solution, which is what allows our cameras to see the flow right up to its surface, and also what makes the sphere impossible to see. Jibu Tom Jose, Aviel Ben-Harosh and Omri Ram show how to find it anyway, from the hole it leaves in the tracer particles, the fluid it pushes and the vortices it sheds, and use the result to measure drag and lift on a freely rising sphere for the first time.

The paper is open access in the International Journal of Multiphase Flow (doi). The full story is on our research page.