Spontaneous motion of an oil droplet driven by nonequilibrium chemical conditions is reported. It is shown that the droplet undergoes regular rhythmic motion under appropriately designed boundary conditions, whereas it exhibits random motion in an isotropic environment. This study is a novel manifestation on the direct energy transformation of chemical energy into regular spatial motion under isothermal conditions. A simple mathematical equation including noise reproduces the essential feature of the transition from irregularity into periodic regular motion. Our results will inspire the theoretical study on the mechanism of molecular motors in living matter, working under significant influence of thermal fluctuation.

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The generation of convective flow by a chemical wave was studied experimentally on a mm-sized droplet of Belousov-Zhabotinsky ͑BZ͒ reaction medium. A propagating chemical wave causes a transient increase in interfacial tension, and this local change in interfacial tension induces convection. The observed flow profile was reproduced with a numerical simulation by introducing the transient increase in interfacial tension to a modified Navier-Stokes equation coupled with a chemical kinetic equation; a modified Oregonator. We also observed the periodic motion of a BZ droplet floating on an oil phase. Such periodic motion is attributed to the rhythmic change in interfacial tension. The observed periodic convective motion coupled with a chemical reaction is discussed in relation to chemo-mechanical energy transduction under isothermal conditions.

Herz Ass: Flüssigkeitströpfchen lassen sich entlang Trajektorien mit beliebiger Form bewegen (wie das abgebildete Herz), indem man durch Bestrahlen mit Licht einen wellenlängenabhängigen Gradienten der Grenzflächenspannung an einer Flüssig‐flüssig‐Grenzfläche erzeugt. Bei diesem neuartigen Phänomen („Farbkapillareffekt“) führt ein Fluss durch die Grenzfläche zu einer Tröpfchenbewegung in Gegenrichtung zum Gradienten.

Self-movement in an oil/water system generated by chemically driven Marangoni instability was studied. Repetitive changes in the interfacial tension were monitored for a two-phase system, in which the oil phase was a nitrobenzene solution of iodine and potassium iodide and the aqueous phase contained cationic surfactant. Our findings show that inversion of the contact angle induced by a sudden increase in tension is essential for inducing marked acceleration in the self-motion of the oil droplet. The self-motion continues for 30−60 min and then terminates. The movement starts again with the addition of iodine to the oil droplet after an induction period necessary for “digestion” of the nutrient, i.e., the iodine.

Directed motion of an oil droplet floating in an aqueous solution is generated by using a laser beam. Interestingly, the direction of the droplet motion can be switched between forward and backward by changing the optical path of the laser through the droplet. This motion is caused above a certain critical power of the laser, and above this value the velocity increases almost linearly with the power. The mechanism of this directed motion is explained as follows: the oil droplet is locally heated by a narrow laser beam, this local heating induces a specific mode of convection inside the droplet, and this generated convective motion produces translational directed motion of the droplet.

Chemical control of the spontaneous motion of a reactive oil droplet moving on a glass substrate under an aqueous phase is reported. Experimental results show that the self-motion of an oil droplet is confined on an acid-treated glass surface. The transient behavior of oil-droplet motion is also observed with a high-speed video camera. A mathematical model that incorporates the effect of the glass surface charge is built based on the experimental observation of oil-droplet motion. A numerical simulation of this mathematical model reproduced the essential features concerning confinement of oil droplet motion within a certain chemical territory and also its transient behavior. Our results may shed light on physical aspects of reactive spreading and a chemotaxis in living things.