Collective behavior of driven Janus colloids in external fields

[摘要] Colloids are micron-sized particles widely used in industrial processes and everyday products. Conventional interest has focused on their equilibrium self-assembly structures by engineering static interactions between the colloids. The introduction of the fourth dimension, time, gives rise to a new generation of dynamic structures with novel functions. In this thesis, magnetic and electric fields are employed to inject energy into Janus colloids, particles with two sides of different properties. Many interesting dynamics are observed for single particles as well as the whole system. Meanwhile, a plethora of dynamic structures and patterns are discovered. Starting from hexagonal crystals in a rotating magnetic field, Janus particles are observed to undergo a transition to bound dumbbells and further self-assemble into dynamics superlattices. In a precessing field, a single Janus sphere performs curious nutation-like motion with adjustable phases. By synchronizing their dynamics, these magnetic Janus spheres self-assembles into microtubular structures that further synchronize with the constituent particles. Upon the introduction of AC electric fields that drive the particles to swim, non-equilibrium phase segregation is observed from two species that spontaneously move in opposite directions. In the absence of magnetic fields, millions of Janus spheres form coherent swarming patterns, such as moving bands and vortexes, by aligning with each other through electric dipole-dipole interactions. Also explored is the effect of anisotropy in the building blocks, as well as directional interactions when particles are confined on regular lattices. The phenomena and principles studied in this thesis vastly enrich our knowledge about colloidal suspensions under external driving forces, and point to many potential applications such as intelligent materials that can shift properties with external triggers.

[效力级别] Active Matter

[关键词] Colloids;Active Matter;Magnetic materials;Self-assembly;Electric fields;Collective behavior