Self-propelled droplets for extracting rare-earth metal ions

Abstract

We have developed self-propelled droplets having the abilities to detect a chemical gradient, to move toward a higher concentration of a specific metal ion (particularly the dysprosium ion), and to extract it. Such abilities rely on the high surface activity of di(2-ethylhexyl) phosphoric acid (DEHPA) in response to pH and the affinity of DEHPA for the dysprosium ion. We used two external stimuli as chemical signals to control droplet motion: a pH signal to induce motility and metal ions to induce directional sensing. The oil droplets loaded with DEHPA spontaneously move around beyond the threshold of pH even in a homogeneous pH field. In the presence of a gel block containing metal ions, the droplets show directional sensing and their motility is biased toward higher concentrations. The metal ions investigated can be arranged in decreasing order of directional sensing as Dy(3+)≫ Nd(3+) > Y(3+) > Gd(3+). Furthermore, the analysis of components by using an atomic absorption spectrophotometer reveals that the metal ions can be extracted from the environmental media to the interiors of the droplets. This system may offer alternative self-propelled nano/microscale machines to bubble thrust engines powered by asymmetrical catalysts.

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.…”

“…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. [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).…”

“…The object can spontaneously move even though its environment is homogeneous, in which its moving direction is random. Once the environment becomes asymmetrical, the force balance is broken, and the object tends to move toward specific directions determined by the gradient of the environmental condition …”

Self-propelled droplets

“…Several examples of chemical reaction schemes can be found in literature [9][10][11][24][25][26][27][28][29][30][31]. The common feature of all these systems is that the used surfactants are affected by a chemical reaction and the surface tension of an interface covered with the pristine surfactant differs from the surface tension of an interface covered with surfactant after the chemical reaction.…”

Stimuli-Controlled Fluid Control and Microvehicle Movement in Microfluidic Channels

“…However, the movement of the droplet was random and the direction was solely based on how the droplet was placed on top of the solution. Using the DEPHA surfactant, Ban et al [135] demonstrated self-propelled droplets which moved in the direction of a higher concentration of heavy metal ions. In this study, Ban and coworkers again used an oil droplet which contained the DEPHA surfactant.…”