It has to be stored in a cold and dry place. in any case, our experience is that after few months the particles are not as fluorescent and appear much darker in images.

Have you notice any density changes? We store it at ambient temperature (around 20 C), in dark flask (or foil covered).

No, we didn't notice density changes. Typically our seeding particles all settle at some specific rate, e.g. In a couple of hours they'll settle in the experimental facility, if not flowing. It's difficult to notice if there is a minute change in density.

It should be a dark cool place (eg fridge), better remove the bulb from the refrigerator. After some time, the particles can stick together, you can use a washer with ultrasound to separate particles.

I didn't notice a big change of fluorescence over a year, but we are using PE and not PMMA particles. I always keep them in the darkness although there are not particular prescription.

For what concern the density you should ask yourself which is the density of the particles and which is the density of the fluid in which you store them. Finally you should consider also hydrophobicity, which can play an important role: if the particles are not treated with a surfactant and you put them in water, you will never see them in the media. They normally try to excape, and going at the interfaces, either walls or free surface.