Microtechnology Micromotor with Water Drive

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Karlsruher Institut für Technologie (KIT) KIT-Campus Süd

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Researchers from the Karlsruhe Institute of Technology (KIT) (Germany) and partners from China demonstrate how flows alone on a water surface can control the rotation of a floating object in one direction.

Directed rotational movements are widespread in nature and technology. However, at a small scale, they are difficult to produce specifically, according to researchers from KIT. Chemical drives, for instance, become depleted, and methods using electric or magnetic fields require complex technology.

Researchers have now succeeded in assembling ultrathin fibers into ordered bundles—for example, for low-loss high-frequency cables or surgical sutures. A team from the Institute of Microstructure Technology (IMT) at KIT and the Suzhou Institute of Nanotech and Nanobionics (Sinano) of the Chinese Academy of Sciences has demonstrated that the flow on a water surface alone is sufficient to rotate a floating object in a fixed direction—all at a miniature scale and entirely without the use of chemicals, electricity, or magnetic fields for control. This was achieved solely through the forces on a water surface. Such precise assembly of the finest structures is now possible, says Professor Jan G. Korvink from IMT at KIT. The research has been published for the first time in Science Advances.

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From Pure Pendulum Motion to Directed Rotation

The core of the setup is a 3D-printed component with a spiral-shaped channel. It holds a tiny object at the water surface without touching it. Moving the component slowly up and down causes the object to simply oscillate back and forth without any residual rotation. However, at higher speeds, small vortices form that disrupt this balance. The object then rotates step by step in the same direction—much like a ratchet, the researchers compare, so that the rotation gradually accumulates. 

This process could also be visualized at KIT through flow simulations. In the simulation, researchers were able to precisely observe how the flow at high speed breaks the symmetry of the movement. This exact breaking transforms the back-and-forth motion into a directed rotation.

Finest Fiber Bundles for Cables, Medicine, and Artificial Muscles

According to the scientists, the effect can be harnessed in a targeted manner. The component behaves like a tiny motor driven solely by the water surface. Its torque is about 10⁻⁸ newton meters—far below that of an electric motor, but significantly higher than that of biological motors. For instance, it has been possible to combine silk fibers with a diameter of ten to 20 micrometers (0.00039–0.00079 inches)  step by step into multilayered, intertwined bundles. Such structures are also typical for litz wires or surgical sutures. 

Possible applications could include low-loss cables in data centers, multifunctional suture material, and artificial muscles. Traditional braiding machines fail in this size range because the fibers simply break under tension. The new approach, on the other hand, avoids mechanical contact entirely, offering a new way to precisely produce even the finest spiral structures.

The German Research Foundation (DFG), the European Research Council (ERC), and Sinano supported the work.