Filtration usually removes large particles from the mixture, and if the particles are small enough, they can be passed through the filter. What we are going to see today is an operation with exactly the opposite effect: large particles pass through, but small particles are intercepted.

what can produce this reverse filtration effect is no stranger: it is a soap film made of a surfactant solution (the liquid film in the picture contains water, glycerin and sodium dodecyl sulfate). Recently, in Scientific Progress, there is a paper discussing the possibility of liquid soap film for particle screening. The picture above is a demonstration in the paper. The researchers threw polytetrafluoroethylene balls of different diameters on the liquid film, and the larger ones passed through, while the smaller ones landed on a trampoline and stopped on the liquid film after two bounces.

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this phenomenon can be explained by the additional pressure on the curved liquid surface. When the falling object bends the liquid film downward, the liquid film which tends to retract will produce an upward force to hinder the downward movement of the object under the action of surface tension. The effect of this force depends on the size of the body, and the smaller the body is, the more it bends the liquid film locally, and the greater the hindrance to the object. As a result, small objects are blocked by liquid membranes and cannot pass through.

the authors believe that this may be a good way to separate particles and can be used to stop dust, odors or bugs. Although the soap film gives the impression that it is easy to break, by replenishing the liquid film and adjusting the composition of the solution, the researchers have also made a liquid film that can be used for several hours.

the following is the full version of the demonstration video provided by the author of the paper. The idea is very interesting, the device is not complex, whether it is easy to use depends on further verification.

original thesis: Birgitt Boschitsch Stogin, Luke Gockowski, Hannah Feldstein, Houston Claure, Jing Wang, Tak-Sing Wong. Free-standing liquid membranes as unusual particle separators. Science Advances, 2018; 4 (8): eaat3276 DOI: 10.1126/sciadv.aat3276