Some separators use hydrophobic and/or oleophilic media. These media repel water and attract oil either due to their absorptive characteristics, or electrostatic and surface tension effects. However, these media will not attract oil that is in the bulk of the flow. They will retain the oil particles that impinge upon them or at a distance where microscopic molecular attraction is possible. The media are supposed to allow the oil to rise to the surface, but in many instances, the molecular attraction is so great that the oil ends up saturating the media and re-entrainment occurs.