Researchers at the University of Konstanz have developed a gentle, contact-free method to collect liquids and remove them from microscopic surface structures. The method uses vapor condensation to generate surface currents that transport droplets off surfaces.
Many modern technologies rely on microscopic elements, such as microchips in smartphones. The manufacturing process for these elements requires their surfaces to be exposed to different types of liquids that must be completely removed afterwards. A research team led by Stefan Karpitschka from the University of Konstanz has now developed a new method that uses surface tension to efficiently transport these liquids off the finished product.

Tension is what's exciting
Every liquid has a natural surface tension that varies in strength from substance to substance. In the case of water, this tension enables small insects like the water strider to walk across the surface. The same tension makes soap bubbles round. Micro- and nanoscopic structures, however, can be easily damaged by even this little tension.
Manufacturing microchips takes hundreds of precise, complex steps, some of which require wet processing. "For example, in order to turn very thin silicon discs, known as silicon wafers, into finished microchips, several of the necessary steps require the material to be wet. Take transistors, for example. They must be etched in acid baths and dried afterwards", Karpitschka explains. Yet, because the objects are so small and sensitive, simply wiping off the remaining traces of the liquid would not be an option. "Boiling off the remaining liquid is not a viable option because tiny contaminates would remain on the object instead of being removed with the liquid", the physicist adds.

Steering fluid currents in the right direction
In their search to find a way to move water off objects, the team of researchers developed a contact-free method that is gentler on the respective objects. The method harnesses the so-called Marangoni force, which relies on differences in surface tension. "When adjacent areas of a surface have different surface tensions, this results in a kind of 'tug-of-war'. The stronger side wins and displaces the weaker one", Karpitschka says. "Liquids below the surface are pulled along in the desired direction."

A difference in surface tension is required to generate this current, and the researchers do something surprising to create it – they add even more liquid. In their experiment, they evaporate alcohol, for example, which has a lower surface tension than the water to be removed from a surface. The introduced vapor condenses on the liquid already present – resulting in the desired difference in tension. "We steer the ensuing currents across the entire surface to gather the tiny amounts of remaining liquid into larger and larger drops", Karpitschka explains. The result is similar to what happens when raindrops collect on a windowpane, drip downwards and grow in size – except, in this case, the researchers direct the droplets' path.

This new method can be used in a wide variety of fields that work with micropatterned surfaces. It makes it possible to dry small surface structures without damaging them, thus enabling a more efficient production of various micro- and nanomaterials.

• Original publication: Ze Xue, Raphael Saiseau, Olinka Ramírez Soto, Stefan Karpitschka (2026): Vapor-mediated wetting and imbibition control on micropatterned surfaces, Proc. Natl. Acad. Sci. U.S.A. 123 (1) e2519761122, DOI: 10.1073/pnas.2519761122
• Stefan Karpitschka is a professor of physics at the University of Konstanz. His research focuses on living and soft matter as well as fluid physics at the micro and macro levels.