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How the swimming droplets came to life is in itself an adventure ....
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Self-propulsion of a drop of water in the oil can be induced by the Marangoni effect by generating an asymmetry in the concentration field of the surfactant which stabilizes the drop. A particular case of this technique consists in reacting the surfactant with a chemical compound produced within the drop. There is then a double surfactant system, the original system and the one that has reacted. The two surfactants do not develop the same surface tension. An instability leads to a heterogeneous distribution of these two surfactants on the surface of the drop. The induced surface tension gradient ensures the propulsion of the drop ... as long as the chemical reaction continues. In order to better control this type of system, we chose a catalyzed chemical reaction whose catalyst could only be active under UV. And ... what was not our surprise to find that our droplets were swimming even when protected from the UV! In order to have it clear, we removed any chemical compound from the inside of the drop: drops of water in the oil, charged with surfactant, and ... they swim !!! A drop of typically 100 microns in diameter travels typically 15 microns / s. The key to the mystery lies in the fact that the drops also decrease in size, very slowly but inexorably ... For more information, read the article by Ziane Izri and Marjolein van der Linden, who elucidated this case in collaboration with Sébastien Michelin Ladhyx). |
The mechanism is not based on a chemical reaction and is extremely stable: we could therefore use these drops to transport colloids, but also cells!
Since then, we study the swimming of these droplets in different geometries, particularly in the geometries of micro-fluidic channels where they could be very useful as autonomous carriers.