The chromocapillary effect

20 November 2009 - Parlour games

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In another installment of that chemical parlour game we like to call "make it move" a French-Japanese team moves around droplets in the petri dish arena (Diguet et al. DOI). The droplet consists of oleic acid, the medium is water with dissolved in it a cationic photosensitive azobenzene trimethylammonium bromide surfactant. Illumination of the water surface with a precise wavelight (regular microscope) brings about a photochemical trans-cis isomerization of the azobenzene group resulting in a change in surface tension and hence autonomous motion - away from the illuminated area with light at 365nm and the reverse with light at 475 nm. By a two-color illumination setup the droplet can be moved around around at will, ideal for letter writing!