We report on our recent finding of self-propelled gel at air-water interface. Our gel is composed of an ionic liquid and a polymer, 1-ethyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide (EMIM-TFSA) and poly(vinylidene fluoride-co-hexafluoropropylene). When a piece of the gel with dimensions of approximately 0.5 x 1 x 8 mm is floated on distilled water in a petri dish, it immediately starts rotational motion. By the analyses of images taken by high-speed camera system (1000 frames per second), it becomes evident that the rotation center is a one end of the long gel piece and the rotational speed is about 8Hz. Thus the velocity of the other end of the gel piece reaches ~400 mm/s. This is, as far as we know, the fastest motion among the other self-propelled soft material thus far reported. The motion often accompanies the translation in addition to the rotation, especially at the initial stage of the motion.<br/>The rotation continues 500~1000 s with gradual deceleration, after which it still shows a slow and nonlinear motion. The rotation motion immediately stops when a small amount of surfactant is added on the distilled water surface. It does not move when the gel piece is fully immersed in distilled water (which means the specific gravity of the gel is greater than that of water). The gel immersed in a water overnight does not contain EMIM-TSFA, which is confirmed by Raman spectroscopy. Thus EMIM-TSFA is released into water, and probably participates in generating the force.<br/>We think that one possible explanation of the origin of self-propelled motion of our gel is Marangoni effect, known for driving “camphor boat”. In the boat system, camphor forms a monolayer at air-water interface on one side of the boat. It yields the difference in surface tension between the heading direction and the back direction, and yields a self-propelled motion. Camphor is readily sublimed, which is advantageous for its continuous motion. The material we used, however, is an ionic liquid. The ionic liquid is not sublimed and not reported to form monolayer at air-water interface. Further revision is necessary for the complete explanation of the mechanism.

Self-propelled ionic gel at air-water interface

253rd National Meeting (2017)

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