Phototactic behavior of self-propelled micrometer-sized oil droplets in a surfactant solution

Abstract

We demonstrate the phototactic behavior of self-propelled micrometer-sized oil droplets in the presence of azobenzene-containing surfactants. These droplets respond sensitively to UV light irradiation due to a variation in the interfacial tension at the droplet surface induced by the molecular conversion of the azobenzene-containing surfactants.

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

“…[11,12,[82][83][84] As ar esult,t he speed of motion, its direction, and/or the surface area of the dropletcan be changed. [11,12,[82][83][84] As ar esult,t he speed of motion, its direction, and/or the surface area of the dropletcan be changed.…”

“…Although there are severalr eports about chemotaxis of nano-or molecular-sized objectsw hich are based on enzymatic reaction and self-diffusiophoresis, [171][172][173][174] we will focus on only tactic behavior of self-propelled objectsd riven by interfacial tension explained above (section2). [84] Once the external gradient generates inhomogeneity on the interface of the droplet, the Marangoni flow is induced from low to high interfacial tensionr egions, and thus the dropleti sp ropelled toward the direction with low interfacial tension( Figure 5). Once the environment becomes asymmetrical, the force balance is broken, and the object tends to move toward specific directions determined by the gradientoft he environmental condition.…”

“…Amphiphilic molecules including azobenzene are well known to create photo-regulated motion because their surface activities, such as surface tension and hydrophobicity at adsorbed interfaces, are changed under UV or visible light irradiation. [11,12,[82][83][84] As ar esult,t he speed of motion, its direction, and/or the surface area of the dropletcan be changed.…”

“…[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. [84] Once the external gradient generates inhomogeneity on the interface of the droplet, the Marangoni flow is induced from low to high interfacial tensionr egions, and thus the dropleti sp ropelled toward the direction with low interfacial tension( Figure 5).…”

Directional and velocity control of active droplets using a rigid-frame

“…We have focused on the mechanisms combining hard and so materials that are unique to biological organisms, [13][14][15][16] and have developed a new control method by combining chemical reactions with these mechanisms. In this research, we examine self-propelled oil droplets, [17][18][19][20][21][22][23][24][25][26][27][28][29][30] and apply an exoskeleton to a self-propelled droplet for the controllable droplet locomotion. Additionally, we exploit the controllability of the exoskeleton to develop a milli-size transporter on water.…”