This problem is quite common in oil and gas industry. During the oil refinery process, waste material usually consists of water and oil mixed together. Water and oil cannot mix, then causes oil droplets to float above water. It would certainly be nice to be able to assign two different liquid materials that interacts with each other as the simulation runs. Perhaps simulation of oil suction away from the "clean" water can be performed once this feature is in place.
Picture below shows concept of oil water separator.
