Phototactic Flocking of Photochemical Micromotors

Abstract

Inspired by astonishing collective motions and tactic behaviors in nature, here we show phototactic flocking of synthetic photochemical micromotors. When enriched with hydroxyl groups, TiO₂ micromotors can spontaneously gather into flocks in aqueous media through electrolyte diffusiophoresis. Under light irradiation, due to the dominant nonelectrolyte diffusiophoretic interaction resulting from the overlap of asymmetric nonelectrolyte clouds around adjacent individuals, these flocks exhibit intriguing collective behaviors, such as dilatational negative phototaxis, high collective velocity, and adaptive group reconfiguration. Consequently, the micromotor flocks can migrate along pre-designed paths and actively bypass obstacles with reversible dilatation (expansion/contraction) under pulsed light navigation. Furthermore, owing to the enhanced driving force and rapid dilatational area covering, they are able to execute cooperative tasks that single micromotors cannot achieve, such as cooperative large-cargo transport and collective microenvironment mapping. Our discovery would promote the creation of reconfigurable microrobots, active materials, and intelligent synthetic systems.

Keywords: Catalysis; Chemistry; Nanoparticles.

Graphical abstract

Figure 1

Characterization and Spontaneous Clustering Behaviors of the Micromotors (A) Scanning electron microscopy (SEM) image, X-ray diffraction (XRD) pattern, and TG-DSC analysis of the TiO₂ micromotors. Scale bar: 200 nm. (B) TiO₂ micromotor flocks in the aqueous medium with 0.25 wt.% H₂O₂. Scale bar: 20 μm. (C) Numerical simulation of electric potential (ϕ) resulting from the different diffusivities of the secreted H⁺ and OH⁻ around three TiO₂ micromotors with an interparticle distance of 3 μm. Black triangles indicate the direction of E. (D) The simulated velocity in the X direction (u) and streamlines (black curves) of the converging hydrodynamic flow induced by the electroosmotic slip of electron double layer of the glass substrate and TiO₂ micromotors under the local E, indicating that TiO₂ micromotor 1 and 3 would move toward 2 along the converging electroosmotic fluid flow. (E) Clustering behaviors of the micromotors with different C_OH obtained at different temperatures, and those obtained at 650°C after alkaline hydrogen peroxide (AHP) treatment, respectively. Scale bar: 20 μm.

Figure 2

Phototactic Flocking of the Micromotors (A) Phototactic and Brownian motions of single TiO₂ micromotors when UV_Y is on and off, respectively. Images are taken from Video S2. Scale bar: 10 μm. (B) Optical microscopic images (Top panels) and trajectories (Bottom panels) of the TiO₂ micromotors in a flock under UV_Y irradiation. Scale bar: 20 μm. (C) Optical microscopic images (Top panels) and trajectories (Bottom panels) of the TiO₂ micromotors in a flock when UV_Y is turned off. Images in B and C are taken from Video S3. Scale bar: 20 μm. (D) Instantaneous velocity vectors of the TiO₂ micromotors in a flock when UV_Y is on. The length and the color of the arrows denote the magnitude of the velocity, with red and long arrows being the fastest, and dark and short arrows being the slowest. Scale bar: 20 μm. (E) Steady-state distribution of O₂ concentration (C) around three TiO₂ micromotors with an interparticle distance of 2 μm. Black triangles represent the gradient (∇C) of O₂ concentration. Purple arrows and curves represent the direction of incident UV light and the illuminated surfaces of the micromotors, respectively. (F) Simulated velocity in the X direction (u) and streamlines (black curves) of the hydrodynamic flow induced by ∇C along the surface of the micromotors.

Figure 3

Light-Controlled Motions of the Flocks (A) The size (D) versus time for a typical flock under pulsed UV_Y irradiation, indicating the reversible expansion and contraction. (B) The velocity (U) of the micromotor flock at different UV intensity (I, left panel) and fuel concentration (C_f, right panel). (C) The TiO₂ micromotor flock moves along a pre-designed path under the navigation of pulsed UV irradiation. Images are taken from Video S6. Scale bar: 20 μm.

Figure 4

Adaptive Reconfigurations of the Flocks (A) Reversible splitting and merging of the TiO₂ micromotor flock after overexpansion by regrouping the scattered individuals. Images are taken from Video S7. The intensities of UV_X and UV_Y are 0.25 and 1 W cm⁻², respectively. (B) Cordon, splitting, and re-joining of the TiO₂ micromotor flock when it bypasses a prism obstacle. (C) Elongation of the TiO₂ micromotor flock when it fits through a narrow channel. Images in B and C are taken from Video S8. The arrows represent the velocity vectors of the flocking micromotors. All scale bars are 20 μm.

Figure 5

Cooperative Cargo Transport of the Flocking TiO₂ Micromotors (A) A flock of TiO₂ micromotors transporting a large SiO₂ cargo (10 μm in size) in open space. The white dash line indicates the displacement of the SiO₂ cargo with the moving flock in the Y direction. (B) A flock of TiO₂ micromotors transporting a large SiO₂ cargo (10 μm in size) in a narrow channel. Images are taken from Video S10. Golden arrows represent directions of the flocks, and red curves are trajectories of the SiO₂ cargoes. Golden dots are the positions of the cargo at different time. All scale bars are 20 μm.