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This improbable membrane can trap flies in a jar—and odor in a toilet

We all know how a filter works: by trapping large particles and letting the little ones through. But in what might seem like science fiction, a team of researchers has engineered a new kind of filter that does the exact opposite—trapping small particles and letting the large ones through.

This reverse filter, held together by surface tension, is a transparent liquid membrane made of sodium dodecyl sulfate and water. Instead of sorting particles by size, it sorts them by kinetic energy—larger objects with more force break through, but lighter, slower objects do not. Once broken, the puncture self-heals instantaneously.

To understand the filter’s properties, the researchers dropped beads from different heights and recorded whether they passed through the membrane or got stuck on top. They repeated the experiment with membranes of different surface tensions and sizes, as well as beads of different sizes and materials like glass or plastic. They even tested their membrane’s ability to trap living creatures like fruit flies. Together, their observations gave them enough data for a mathematical description of the membrane’s function, they report today in Science Advances.

This reverse filter was inspired by cellular phagocytosis: When a cell engulfs a large foreign object, its membrane allows the object to pass through without taking in extraneous small particles or dispersing the contents inside the cell. The team envisions several applications for their reverse filter membrane: as a surgical membrane that blocks dust, particles, and germs from a surgical site; a membrane to trap odors in a waterless toilet; and a barrier against infection-transmitting insects.