Many a child has wondered how many licks it takes to get to the center of a lollipop. Physically, this is a problem of a solid body dissolving in a flow, and it’s one scientists are interested in for its geological, industrial, and pharmacological applications.
The animation above shows flow around a dissolving (candy!) body that was originally spherical. With both spheres and cylinders, the final shape the body takes is consistent – it has a front boundary with a curvature of nearly constant radius and a back face that is approximately flat. This creates a boundary layer of uniform thickness across the front face, and that uniform flow makes the surface dissolve steadily and evenly so that it maintains the same overall shape.
With their model and experiments, researchers have even been able to tackle the classic question of how many licks it takes:
“For candy of size 1 cm licked at a speed of 1 cm/s, we estimate a total of 1000 licks, a prediction that is notoriously difficult to test experimentally.”
(Image credit: J. Huang et al., source, pdf)