A treatment process for an aqueous phase which contains oil droplets, possibly of 10-50 nm diameter, in aqueous flow from a hydrocyclone separator, comprises bringing the water into contact with a surface subdivided into areas of differing surface energy and affinity for oil and such that when the surface is submerged in an aqueous phase, oil droplets adhere to it with an apparent contact angle in a range from 90 to 150 degrees. Areas of the surface may reduce their affinity for oil in response to an external stimulus causing controlled release of droplets adhering to the surface. The process may be used to remove oil droplets from water produced by an oil or gas well, after downhole oil water separation or after production at a at a well head, or used to coalesce droplets in such water to a larger size to enable conventional separation.