Dye-sensitized microswimmer : from reversible phototaxis to active assembly

Abstract

Self-assembly implies the process where small components organize into large aggregates such as patterns and structures. Such processes are commonly observed in nature at various scales, ranging from atoms, colloids, cells, to animals, civilizations, and even celestial bodies. The macroscopic properties of the assemblies, from a materials science perspective, are predominantly determined by the complex interplay between components, which usually involves the ordering structure, composition, and interfacial transportation. Particularly, when it comes to the study of kinetics and dynamics at atomic scale, direct observation and manipulation becomes more strenuous. As a part of matter at mesoscopic scale, colloidal particles may serve as proxies for studying self-assembly at the atomic level, given their capacity for direct observation under an optical microscope. Recent advances in both chemistry and physics have demonstrated the programmable properties of colloidal particles and their assemblies, as well as the use to study fundamental physical problems and the creation of novel materials with unprecedented characteristics. Governed by equilibrium thermodynamics, colloidal particles can self-assemble into static structures like colloidal molecules, one-dimensional chains, two-dimensional lattices and three-dimensional crystals. On the other hand, since the self-assembly of active colloidal particles encompasses more intricate dynamic processes, it has become an emerging field that arises more and more attentions. Driven by chemical reactions, electric fields, magnetic fields and so forth, active colloidal particles exhibit dynamic structures like flocking, swarming and more complex assemblies that is chaotic and even intelligent. More importantly, the interactions between active particles can be easily adjusted, and the assembly process can be manipulated, showcasing a more versatile and powerful tool compared to passive colloidal particles. Therefore, finding an active colloidal system that can both simulate equilibrium processes and achieve non-equilibrium behavior becomes a paramount goal. In this thesis, I present an active colloidal system where the pairwise interaction between particles can be conveniently tuned by visible light. This system is composed of micrometer-scale spherical titanium dioxide particles with dye-sensitizer loaded into it. The activity is achieved by immersing this particle into a solution with the presence of redox shuttle like ferrocene or hydroquinone and under visible light illumination. Under the illumination of light at the wavelength at absorption peak or off-absorption peak, a totally opposite direction of phototaxis is observed, which attributes to the so-called photonic nanojet effect. Due to the easy fabrication of these particles, a macroscopic phototactic migration can be achieved. Enabled by the similar effect, a reversible pairwise interaction is also observed, leading to attractive and repulsive interaction. By employing bidirectional illumination, we observed a zigzag pattern emerges after only a few seconds of illumination. Moreover, we utilize an additional blue light source positioned above, which causes repulsion between neighboring particles, to examine the stability of the zigzag pattern. Under different repulsion intensities, three distinct phases were observed. An electrostatic-like model employing Langevin dynamics is developed to simulate the hydrodynamic behavior. Our results effectively replicate the dynamic behaviors and capture the essence of the intricate hydrodynamic interactions among the particles. By further adjusting the illuminating condition, the tunable directional potential can be generated on spherical colloids, which allows the polymorphic assembly to all 2D Bravais lattices on demand. Such directional interaction potentials are confirmed under optical tweezers system. We reveal the rapid colloid phase transition, as controlled with light, can be used for living photonic devices to control the diffraction of the near-infrared laser. The dye-sensitized TiO2 system can also display self-trapping under focused visible light with relatively low light intensities, similar to optical tweezing effect. This active tweezing system is enabled by structure light, which can be used to manipulate and assemble the active particles. By incorporating optical tweezers and structure light, we observed a decreasing apparent temperature by increasing light intensities. Since the crystal lattices can be assembled on demand while the apparent temperature can be easily measured and tuned, this active tweezing system has the potential to study the phonon dispersion of various two-dimensional lattice structures. In connection with applications of this dye-sensitized TiO2 system, interfaces usually play pivotal roles for mass transportation, especially liquid-liquid interfaces. To ensure the performance of the dye-sensitized TiO2 particles in aqueous scenarios, a TiO2-oil hybrid active droplet is designed, with the TiO2 particles loading on the water-oil interface. By isolating the ferrocene redox shuttle in oil phase, a biphasic reaction has occurred which drives Marangoni flow on the surface of the oil droplet, creating an explosion-like phenomenon on the TiO2 particles. Surprisingly, by submerging this active droplet in ionic solution, it can display optically controlled wetting and dewetting. This work further demonstrates the versatility of this photoactive system.