Researchers in Germany have shown that a sheet of intense laser light can act as a trampoline for small droplets of ink. Michael Esseling and co-workers from Westfälische Wilhelms-Universität Münster observed the trajectory of strongly absorbing 50-μm-diameter particles of liquid ink falling vertically through a sheet of green laser light. When the light was sufficiently intense — a peak intensity of more than 100 μW μm−2 — the falling droplets were repelled from the sheet when in close proximity. The mechanism for the interaction is thought to be the photophoretic force — a force resulting from a light-induced thermal gradient on an object's surface that pushes it away from regions of high light intensity. Because the direction of the force depends on the intensity distribution of the light field, the researchers were able to alter the droplet trajectories by using a cylindrical lens to change the angular orientation of the light sheet. The interaction was also not limited to a single event; the researchers observed up to three bounces for a single drop. The ability to control liquid droplets in this manner may prove useful in the fields of optofluidics, biomedicine and chemistry.
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Graydon, O. The photonic trampoline. Nature Photon 6, 800 (2012). https://doi.org/10.1038/nphoton.2012.317
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DOI: https://doi.org/10.1038/nphoton.2012.317
