The new approach developed by Varanasi’s group uses membranes with hierarchical pore structures. The membranes combine a very thin layer of nanopores with a thicker layer of micropores to limit the passage of unwanted material while providing strength sufficient to withstand high pressure and throughput. The membranes can be made with contrasting wetting properties so their pores either attract oil and repel water, or vice versa.
“This allows one material to pass while blocking the other with little flow resistance,” Varanasi says. The choice of membrane, or a combination of both, could be based on which material predominates in a particular situation, he explains.