Two immiscible fluids that have different densities and have an appropriate interfacial tension can be separated under flow conditions. Minute droplets are captured grown and separated in a process called coalescing.
The obvious candidates are liquids and air and oil and water but any two appropriate fluids can be separated.
In compressed air the use of such equipment to remove fluids is industry standard.
In liquid systems the removal of a small amount of water from a main flow of oil is called coalescing. Such applications include refineries, fuel oil , fuel storage, oil based hydraulic systems, aviation fuel and bunkerage. In some cases the process is enhanced using hydrophobic stripper cartridges.
The removal of a small amount of oil from a main flow of water is called oily water separation. Such applications include hydroelectric power, bilge water systems, environmental protection and tramp oil removal.
This process can be applied to any two non mixing liquids (within certain physical parameters) not just oil and water.
Total Filtration can meet all of these requirements