Scientists have built a molecular motor modeled on the Archimedes screw that turns when an electric current passes through it and can switch its direction of rotation when the voltage's polarity is flipped. The work, reported by Science Daily, was carried out by researchers at the University of Copenhagen and the Weizmann Institute of Science, with theoretical support from the University of Augsburg and other institutions. According to Science Daily, it is the first molecular motor driven by electric current that is firmly attached to two metal electrodes and can rotate in a controlled direction.
Why it matters
Molecular machines are often described as a coming frontier in miniaturization. Living cells already contain molecular motors that convert energy into motion, and scientists have spent decades trying to reproduce such systems artificially; Science Daily notes that progress in the field was recognized with the 2016 Nobel Prize in Chemistry.
Electric-powered molecular motors date back to 2011, but those earlier versions sat on surfaces and were driven by the probe tip of a scanning tunneling microscope. The new design instead anchors a single molecule between two electrodes, which Science Daily describes as an important step toward working machines at the smallest scale. The researchers also suggest the findings could inform molecular electronics, spintronics — which exploits electron properties beyond charge — and microfluidic systems that handle tiny volumes of fluid.
A spiral that turns in the current
The motor's core is a spiral-shaped molecule developed by the group of Prof. Michael Pittelkow at the University of Copenhagen. It measures only a few millionths of a millimeter and binds to gold through sulfur atoms, according to Science Daily.
At the Weizmann Institute, Prof. Oren Tal and Dr. Anil Kumar Singh placed individual molecules between two gold tips sharpened to atomic dimensions. Adjusting the gap between the electrodes let the team stretch or compress the molecular screw. When a voltage was applied, the measurements showed the molecule rotating, and reversing the voltage's polarity reversed the direction of spin.
The mechanism echoes a wind turbine: electrons moving through the spiral hand angular momentum to the molecule, setting it turning. The spiral's geometry governs how it responds to that flow, much as right- and left-handed threads behave differently. Past a certain voltage threshold, the rotation becomes continuous.
An unexpected jump in conductivity
The experiments turned up a second effect. A rotating molecule carried electricity far better than a stationary one, and the team needed simulations to understand why. Prof. Dr. Fabian Pauly and his doctoral students Léon Martin and Yuta Ito at the University of Augsburg ran quantum mechanical calculations on the LiCCA supercomputer.
The simulations tied the molecule's conductivity closely to its rotational angle, with conductance rising by up to two orders of magnitude — roughly 100 times — as it turned. The explanation, Science Daily reports, is that some of the orientations that conduct best are ones the molecule avoids when it is at rest; rotation brings those arrangements within reach.
Martin described the result as counterintuitive, noting that the positions the molecule shuns while still are precisely the ones that carry current best, and that only rotation opens them up. The finding indicates that motion inside such a machine does more than draw power — it can shape how current flows through it.
The Augsburg team then worked with Per Hedegård of the Niels Bohr Institute in Copenhagen and Jing-Tao Lü of Huazhong University of Science and Technology in Wuhan to build a simplified theoretical model. That model reproduced both the observed rotation and the accompanying rise in current.
Ito, a doctoral student originally from Japan, said experiments alone reveal only the current, and that the calculations offered a view inside the nanocontact. He credited collaboration between experiment and theory across several countries for resolving the puzzle of the increased current.
Watch for follow-up work on whether such current-driven screws can be integrated into molecular electronic circuits or used to move fluids at microscopic scales.
