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If you set a bunch of toy tops spinning closely together, they will collide and stop spinning after a while. But according to the laws of physics, if there are many tops all spinning extremely rapidly and all in exactly the same way, something else might happen first: They could enter into a collective circular movement about a central point. Setting up this scenario with actual toy tops would be impractical, but new Weizmann Institute research suggests that it is quite feasible for molecules in a gas. Circulating molecular flow – a gas vortex – could have a number of uses in biomedical technology and nanoscience.

Rotating tops are said to have angular momentum, a measure of their tendency to continue spinning. Several years ago, Profs. Ilya Averbukh and Yehiam Prior and their teams in the Chemical Physics Department (Faculty of Chemistry) proposed that the angular momentum of molecules could be controlled and manipulated by very short laser pulses. In Prior’s lab, lasers that flash in femtosecond pulses – a millionth of a billionth of a second long – are used to get groups of molecules all spinning in the same direction.

Uri Steinitz, a student in Averbukh's group, wondered what happens afterward, as the spinning progresses. Do the molecules collide like tops, and how would this affect the spins? Physics tells us that angular momentum is always conserved. When molecules that are initially spinning more or less in tandem bump into one another, their rotation slows down, and the directions of their rotational axes become random. However, the total angular momentum should be maintained somewhere within the system. The question was: where?
Profs. Ilya Averbukh and Yehiam Prior
To find the answer, Steinitz, Prior and Averbukh considered a dense gas in which all of the molecules are spinning in the same direction, and calculated (using computer simulations) what happens when the molecules collide repeatedly.

The researchers found that after a certain number of collisions, the angular momentum of the individual molecules was lost, but it reappeared on a larger scale: The molecules in the gas system began to rotate together in a vortex around a central point. This vortex could be millions of times larger than the size of a single, spinning molecule, and it could theoretically reach rates of tens or even hundreds of thousands of revolutions a second. While the angular momentum eventually diffuses within the larger system, repeated spikes of the laser pulses could keep the gas stirred up indefinitely.

These results, says Steinitz, demonstrate a principle normally seen in much larger systems. Atmospheric cyclones, for instance, start out as much smaller eddies that collide and scale up to create large, well-formed vortices. On a practical level, molecular vortices might be useful for manipulating all sorts of particles. For example, as the vortex continues to spin, its momentum drags nearby molecules into its wake. Thus one could use the method to move delicate small particles – for instance biological molecules that are harmed by direct laser manipulation – without actually touching them. In addition, microfluidic devices used in biomedical and pharmaceutical research and industry, which today are operated by tiny channels and gates, could be designed to work efficiently with laser-controlled molecular vortices.

Prof. Ilya Averbukh is the incumbent of the Patricia Elman Bildner Professorial Chair of Solid State Chemistry.

Prof. Yehiam Prior’s research is supported by the Ilse Katz Institute for Material Sciences and Magnetic Resonance Research, which he heads; the Leona M. and Harry B. Helmsley Charitable Trust; the Carolito Stiftung; the Willner Family Leadership Institute for the Weizmann Institute of Science; and Mr. Luis Stuhlberger, Mexico. Prof. Prior is the incumbent of the Sherman Professorial Chair of Physical Chemistry.


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