Oily water is widely generated during global industrialization processes and seriously harms the environment and people’s health in a wide range of ways.^1,2  Accidents involving oil spills, from the Lakeview Gusher oil spill in 1910 to the Deepwater Horizon oil spill in 2010, release millions of tonnes of crude oil into the environment, especially the marine areas, every year and produce large quantities of oily water.^3,4  Besides the petroleum industry, oily water is also widely generated by diverse industrial technologies, such as the food, textile, leather, steel and metal finishing industries etc.¹  In a broader sense, oily water can be produced by the releasing of organic reagents into water as well.⁵  The direct discharge of oily water kills large numbers of creatures, seriously damages people’s physical health, and even further aggravates the shortage crisis of drinkable freshwater.^6–8  The unsuitable treatment of oil pollution, such as in situ burning, will give rise to a fire hazard and finally bring about dire air pollution.⁴  Separating oil from water is widely recognized as a right and sustainable strategy and has become a new worldwide subject.⁹  However, this subject is always tough and challenging as oily water is prone to be present in multiple forms under differing conditions: free layered oil/water mixtures and oil-in-water or water-in-oil emulsions by formation, surfactant-free emulsions and surfactant-stabilized emulsions by components, micrometre-scale emulsions and nanometre-scale emulsions by oil droplet size.¹⁰