mixtures of oil and water are classified based on the size of oil droplet (doil) – free oil if doil > 150 μm, dispersed oil if 20 μm < doil < 150 μm and emulsified oil if doil < 20 μm. we have developed a novel solution for the separation of free oil, dispersed oil, and oil-water emulsions based on the design of hygro-responsive (from the greek word ‘hygra’ meaning liquid) membranes. these membranes, counter-intuitively, are wet by water, but are still able to repel low surface tension oils like rapeseed oil or hexadecane. this makes these porous surfaces ideal for gravity-based separation of oil and water as they allow the higher density liquid (water) to flow through while preventing the flow of the lower density liquid (oil). as the separation is solely gravity driven, it is expected to be one of the most energy efficient technologies for oil-water separation. we have developed a setup based on these membranes that allows for the continuous separation of oil–water emulsions for over 100 hours without a decrease in flux, using only gravity. the demonstrated separation methodology is expected to have numerous applications, including the clean-up of oil spills, wastewater treatment, fuel purification and the separation of numerous commercially relevant emulsions.