Light-activated nonequilibrium colloidal assembly : from binary phases of active and passive spheres to cooperative crystallization of self-propelled particles

Abstract

The self-assembly of colloidal particles has appealed broad scientific interests, either in the bottom-up fabrication of nano/microscale materials or as fundamental models for atomic/molecular systems. For instance, entropy or surface-encoded interactions can direct colloidal suspension to form thermodynamic equilibrium crystalline phases. Beyond equilibrium, the recently emerged active colloids are energy-consuming systems with force-generating particles and often exhibit exotic collective behaviors and dynamic phases different from the equilibrium counterparts. However, such systems are very rare and far from rational design. In addition, due to the complex, out-of-equilibrium nature, the understanding toward active colloids and their guiding principles for making structures and materials are still limited. The work described in this thesis is based on the introduction a model system of dye-sensitized TiO2 microspheres, which induce diffusiophoretic/osmotic interactions through photocatalytic reactions. The complex nonequilibrium interactions are quantitatively measured and can be controlled with various parameters (light intensity, particle size, etc.) to tune the collective particle organizations. When integrated in various systems, we demonstrate a wide variety of nonequilibrium structures and dynamics and we also show the control by tailoring the colloidal activity, shape engineering, etc. In the first work, the active spheres are co-assembled with passive polymeric particles, forming diverse binary phases by light-controlled phase separation. They also form binary crystalline compounds when the size ratio and the interaction between active and passive particles are properly tuned. In the second work, we develop a new colloidal synthetic method for shape-tunable micromotors with various propulsion modes. The micromotors are prepared by specifically growing the photoactive TiO2 sphere on the end of a SiO2 rod, producing TiO2-SiO2 matchstick-like particles in large quantity. The shape of the micromotors can be tuned by introducing a bent angle during the growth of silica rods. We demonstrate that the propulsion modes of the micromotor, from linear propulsion to steering and spinning, can be easily modulated by tuning the bent angle of the silica rod. In the third work, we fabricate the active-passive heterodimer micromotor consisting of a TiO2 sphere and a polymer sphere. We demonstrate the crystallization of the active colloids via a cooperative mechanism, where the particles serve distinct roles in the crystal nucleation and growth process. Through the interparticle cooperation, the micromotors first assemble to clusters as nuclei consisting of one “standing particle” in the center and six “pushing particles” in the periphery. The subsequent crystal growth features a consecutive “role switching” process, where the original pushing particles reconfigure as standing particles, giving spaces for the incoming assembled motors. We further control the activity of the micromotors (i.e., velocity and interaction) by light intensity, thus creating membrane and chain-like crystalline structures. Taken together, our system tune and employ nonequilibrium forces to construct the colloidal materials with distinct structural diversities and intelligent cooperative dynamics.