Metal-Ion-Dependent Motion of Self-Propelled Droplets Due to the Marangoni Effect

Abstract

Chemically driven self-propulsion of soft matter is useful for various applications because it can move toward a desired location, without external power fields, in response to chemical signals in environmental media. We have developed a suitable steering mechanism to maintain the orientation of self-propelled droplets loaded with surfactant in fluidic environments. A spatial gradient of alkaline-earth metal ions induces directional sensing. These metal ions can be arranged in descending order of directional sensing as Ba(2+) ∼ Sr(2+) > Ca(2+) > Mg(2+). On the other hand, the affinity between metal ions and di-(2-ethylhexyl)phosphoric acid (DEHPA) decreases in the order as Ca(2+) > Ba(2+) > Sr(2+) > Mg(2+). To clarify the difference between the order of directional sensing and that of affinity, we investigated the effect of metal ions on the driving force to create asymmetric convection. We found that changes in the interfacial tension under nonequilibrium conditions play an important role in directional sensing.

Phototaxis of Oil Droplets Comprising a Caged Fatty Acid Tightly Linked to Internal Convection

“…demonstrated that droplets emitting surface‐active chemicals are self‐propelled to seek a source of acid placed at one exit of a maze, finding the shortest path through the maze . In addition to these examples, self‐driven movements in response to various external fields, such as an electric field, an optical field, or a concentration gradient of metal ions, have been reported. These results are attributed to the characteristics of soft matter samples, such as their viscoelasticity, utilizing the kinetic energy created from the direct chemical gradient or gradients caused by physical fields.…”

“…Such movements of mesoscaled molecular assemblies have drawn attention, not only because of their characteristic nonlinear dynamics but also from the viewpoint of substance transport in a restricted space. Typical examples are directed movements of oil droplets on a surfactant‐coated substrate or at an air–water interface . For example, a nitrobenzene droplet containing iodine moves spontaneously on a glass surface coated with surfactants, absorbing the surfactants in front of the droplet .…”

“…Typical examples are directed movementsofoil droplets on as urfactant-coated substrate or at an air-water interface. [2][3][4][5][6] For example, an itrobenzene droplet containing iodine moves spontaneously on ag lass surface coated with surfactants, absorbing the surfactants in fronto ft he droplet. [2] Grzybowski et al demonstrated that droplets emitting surfaceactive chemicals are self-propelled to seek as ource of acid placed at one exit of amaze, finding the shortestpath through the maze.…”

Evolution of Self‐Propelled Objects: From the Viewpoint of Nonlinear Science

“…[170] Here, we will give some examples of artificial self-propelled objects that could "sense" their environmental physicochemical conditions. [175][176][177][178][179][180] As elf-propelled dropletd riven by the Marangonif low (section 2.2.3) can also response to an external gradiento fp H, [177] rare earth metal concentration, [178,179] surfactant concentration, [180] or light intensity. The object can spontaneously move even though its environment is homogeneous, in which its movingd irection is random.…”

Negative/positive chemotaxis of a droplet: Dynamic response to a stimulant gas

“…The spontaneous motion of self-propelled droplets has recently attracted considerable attention in relation to energy transduction by living organisms, i.e., chemo-mechanical energy transduction. [1][2][3][4][5][6][7][8] Various kinds of droplet motion driven by a gradient in interfacial tension have been reported, as well as the propelled motion caused by diffusiophoresis. 9,10 In a related system, an oilwater system composed of an organic phase with potassium iodide and iodine and an aqueous phase containing stearyltrimethylammonium chloride (STAC) exhibits self-agitation at the oil-water interface, accompanied by spatio-temporal nonequilibrium fluctuation of the interfacial tension.…”