The diet soda will float, but the regular soda will sink, I think. I had the same problem and that was my answer.
Gasses contain highly energetic particles which tend to spread out to fill the room/ container they are in. Often when a smell rises, there is a high concentration of gas molecules in the rising smell. Here diffusion occurs where the molecules spread out randomly moving from an area of high concentration of gas molecules to areas of low gas molecules.
I think the answer is osmosis
Here's the order from smallest to largest tissues, cells, and organs
