Physicists at the University of Liège have succeeded in sculpting the surface of water by exploiting surface tension. Using 3D printing of closely spaced spines, they have combined menisci to create programmed liquid reliefs, capable of guiding particles under the action of gravity alone. This is a promising advance for microscopic transport and sorting, as well as marine pollution control.
Have you ever tried tilting a liquid in a glass? It's completely impossible. If you tilt the glass, the surface of the liquid will automatically return to the horizontal ... except for a small - barely visible - curvature that forms near the edge of the glass. This curvature is called a meniscus. And this meniscus is due to capillarity, a force acting on a millimetre scale and resulting from the surface tension of the liquid. What would happen if we could create lots of little menisci over a large surface? What if these small reliefs could add up to form slopes, valleys, or even entire landscapes ... liquid? This is exactly what scientists from the GRASP laboratory at the University of Liège, in collaboration with Brown University (USA), have succeeded in doing.
Drawing on its experience in the field of liquids, and more specifically of liquid interfaces, and with access to cutting-edge 3D printing equipment, the GRASP team set about printing several 'models', several playgrounds, in an attempt to validate their theory: 3D printing conical spines close enough together to deform the surface of water on a large scale. As we know, each spike creates a meniscus around itself," explains physicist Megan Delens . Following this logic, this means that if we align them well and they are close enough together, we should see a sort of giant meniscus appear, resulting from the superposition and addition of each individual meniscus." The team found that ' by modifying each spine individually, the surface of the liquid no longer remains flat, but forms a kind of "programmed" liquid landscape. "Programmed" because it is by modifying the height or distance between the spines that the researchers have been able to design liquid interfaces that follow all sorts of topographies: inclined planes, hemispheres, but that also draw much more complex shapes. For example, they have succeeded in creating the Atomium in Brussels in liquid relief!*
A motorway for bubbles and microparticles
But that's not all. This method also offers a new way of moving and sorting floating objects such as marbles, droplets or plastic particles," explains Professor Nicolas Vandewalle , physicist and director of the lab. When the liquid surface slopes, the lighter objects rise thanks to Archimedes' thrust , and the denser ones sink under the action of their own weight, as if they were sliding down a hill of water". This completely passive approach could be used in micromanipulation, particle sorting or even cleaning liquid surfaces, for example, to capture microplastics or oil droplets on the surface of water.
Future research could look at more advanced ways of making the small tips move, for example by using materials that react to magnetic fields or that can change shape." The idea would be to be able to control the shape of the liquid surface in real time. These advances would make this method even more useful for developing innovative new technologies in microfluidics," concludes Megan Delens."
