Term of solid phase floating on surface of liquid phase

I am looking for what to call the solid or dust particles that reside at the surface of a liquid after density separation by flotation.

I had the term supernatant in mind, but I looked it up and it seems the supernatant is the liquid resting on top of another liquid or on top of a solid phase (the pellet) after centrifugation or similar procedure. What do you call the solid phase that floats at the surface of the supernatant after centrifugation?

Example

During density separation by flotation, there are particles heavier than the fluid, and those will sink to the ground. Those particles can be called the pellet. However, there are particles, that are lighter than the fluid, and those particles will float. Assume you have tiny bits of candle wax in water. Those bits are so small that -- instead of floating right away -- they remain in suspension in the water. After sufficient centrifugation, the wax will float, since it is lighter than the water. What do you call the wax floating on top of the water? (Wax, obviously, but that's not what we're looking for in this example.)

I know I am more than two years too late, but I'll still give it a shot!

I believe that the word you may be looking for is "scum."

"Scum" is defined as: