| Spontaneous translational motion of a mm-sized droplet is generated by a transfer of surfactant from one phase to the other phase even in an isotropic condition. Such motion can be realized because the increase of the surfactant concentration results in the change of the wettability, interfacial tension, and rheology. Interestingly, macroscopic regular motion is caused through the spontaneous symmetry breaking. In this talk, we show the examples of such droplet motion induced by wettability difference[1, 2], and difference in rheology [3-6]. The system, in which a droplet moves by wettability difference, is composed of an aqueous phase and an oil droplet set on a glass surface. A cationic surfactant, stearyltrimethylammonium chloride (STAC), is dissolved into the aqueous phase. Iodine solution of nitrobenzene saturated with potassium iodide is used as the oil droplet. In this system, STAC initially dissolved in the surrounding aqueous phase continuously flows into the oil droplet through an oil-water interface and a glass surface. When STAC molecules adhere on the glass surface, the surface becomes more hydrophobic. If the oil droplet pass through the glass surface, the adhered STAC molecules dissolves into the oil droplet results in the hydrophilic surface. Thus, the surface underneath the oil droplet is always more hydrophilic than the outside of the oil droplet. Consequently, the oil droplet propels itself with infinitesimal perturbation. Since the droplet prefers to travel at a fixed velocity, the droplet shows regular rotational motion under the attractive centrifugal force. A droplet can move by the generation of a surfactant aggregate around the oil droplet. The system composed of an aqueous phase containing STAC, and an oil droplet floating on the aqueous phase. The oil droplet is composed of palmitic acid solution of tetradecane. In this system, palmitic acid dissolved in the oil droplet flows into aqueous phase. Palmitic acid in the aqueous phase forms aggregate with STAC, which possesses polarized lamellar structure. Continuous formation of such surfactant aggregate on the oil-water interface, results in the blebbing-like instability of oil-water interface. The correlation between size of the instability and the size of the oil droplet is explained with a simple mathematical model, adopting the elastic nature of the aggregate. References [1] Y. Sumino et al., Phys. Rev. Lett. 94, 068301 (2005). [2] Y. Sumino, K. Yoshikawa, Chaos 18, 026106 (2008). [3] Y. Sumino et al., Phys. Rev. E 76, 055202 (2007). [4] Y. Sumino et al., J. Phys. Chem B 113, 15709-15714 (2007). [5] Y. Sumino et al., Softmatter 7, 3204-3212 (2011). [6] Y. Sumino et al., Langmuir 28, 3378-3384 (2012). |
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