Two-faced channel recovers both oil and water from emulsions

Wastewater from many industries, restaurants, and households, contains stable oil-water emulsions that are challenging to break apart. Membranes can do this separation without using chemicals or large amounts of energy. But traditional membranes filter out only oil or water. Now researchers report a simple technique to efficiently recover oil and water at the same time (Science 2024, DOI: 10.1126/science.adq6329).

Inspired by the two-faced Roman god Janus, the team at Zhejiang University put a hydrophilic and a hydrophobic membrane next to each other to form a 4 mm-wide channel. As they pump a lab-mixed, surfactant-stabilized, oil-water emulsion into the channel, water quickly passes through the hydrophilic membrane on one side, leaving behind an increasingly oil-rich mix.

In the narrow channel, the tiny oil droplets collide and form larger drops, which stick more easily to the hydrophobic membrane and then pass through for collection. The Janus channel setup recovered almost 100% pure oil and water.

The team used commercially available polypropylene membranes, but the setup doesn’t require a specific type of membrane. Instead, the channel width is key, says Hao-Cheng Yang, a polymer scientist at Zhejiang. As the researchers narrowed the width from 125 to 4 mm, oil recovery in the setup increased from 5 to 97%, and water recovery went from 19 to 75%. A multistage device with several Janus channels could further increase efficiency, Yang says. He adds that the team now plans to partner with wastewater treatment companies to scale up and test the technology.

Xing Yang, a chemical engineering professor at KU Leuven, says the concept could be applied to other complex mixtures, such as filtering water or glycerol from biofuels or separating cells with oil or water solubilities from biological samples. “This is a very smart idea. It’s a simple design but with profound outcomes,” she says.