The key phrase is "specific gravity", which is really just a way of expressing density w.r.t. that of water. SG > liquid = sink, SG < liquid = float.
You can adjust SG by adding things to water, i.e. salt water has higher SG than pure distilled. You can also try glycol, or experiment with oil solutions.
I suspect it is not unlike a lava lamp or the thermometers that have the various SG liquids with temperature tags attached...as the environment increases or decreases in temperature, the density (specific gravity) of the various liquids change due to thermal expansion (contraction) and "float" to their rightful place.
thanks a lot for the good answers. I have another question. What is the influence of the water agitation in the separation?. Is agitation better for this process?
Light agitation that dislodges bubbles from the objects you are trying to sink helps. Heavy agitation that aerates the liquid will introduce a source of new air bubbles, which won't help sinking things stay sunk.
Heavy agitation can also create currents that will cause particles to "float" that wouldn't otherwise. Think of a kite with and without wind.
This is a different effect than btrueblood's bubbles.
Depending on what you are doing you may want to stir and skim. Then settle and skim. Then repeat. Light stuff can get trapped under heavy stuff. If you stir it gets free and floats to the top.
Be aware that it can take weeks for everything to settle out. Smaller particles settle more slowly.
Depends on how many kinds of plastics you have. I wouldn't think you could do it using mixed household plastics.
Do you know about surface tension and buoyancy? Don't forget about gold panning.
