Propulsion and dynamic assembly of polyhedral metal-organic framework colloids under AC electric fields

Abstract

Active particles can autonomously propel and have the tendency to organize into high-order ensembles and phases that evolve and reconfigure. They have emerged as a focused subject in contemporary colloid science, holding great promise in advancing fields such as cargo delivery, sensing, micromachinery and microrobotics, and materials science. Realization of the full potentials of active particles requires delicate control of their dynamics in propulsion and assembly, which is challenging due to the out-of-equilibrium nature of such systems. Recently, systematically engineered colloidal shapes have been exploited as an effective means to tune and even program the dynamic behaviors of active particles. Various anisotropic particles, with controlled geometries and possessing either homogeneous or heterogeneous composition, have been fabricated, regulating how particles actively propel, interact, and assemble under several chemical and physical stimuli. In this dissertation, I present the first study on utilizing polyhedral particles as active colloids, showcasing their symmetry breaking, propulsion, and collective dynamics under AC electric fields. Although polyhedral particles have been intensively studied previously, they are mainly used as anisotropic building blocks for self-assembly of colloidal superlattices in equilibrium. Conventional thinking was that they are of high symmetry and are unqualified for active colloids. We demonstrate herein that polyhedral particles of certain types are truly active and show a wide range of dynamic behaviors (i.e., propulsion and self-organization behaviors). The dissertation is structured as follows. In Chapter 1, I first provide an introduction of active colloids and an overview of the critical role of the shape on regulating active colloids. The effects of shape on propulsion and assembly are separately reviewed. In Chapter 2, before investigating polyhedral particles, I describe the active assembly of ellipsoidal particles under AC electric fields. As the simplest shape deviation from spheres, the ellipsoidal particles can already form many colloidal assemblies under AC electric fields, some of which are active due to their structure asymmetry. They serve as active colloids and promote dynamic assembly. Chapter 3 focuses on polyhedral particles, made from microcrystal of metal-organic frameworks (MOFs). I address the challenge of symmetry breaking and particle propulsion. By tuning the competition between the electric force and the gravitational force, a particle orientation with broken symmetry can arise, which induces unbalanced electrohydrodynamic flows that drive the particles moving forward. The mechanism is applicable to all tested particle shapes and their ability to propel depends on the emergence of a bilaterally asymmetric orientation. Building on the ability to self-propel, Chapter 4 investigates the collective behaviors of the polyhedral MIL-96 particles. These particles display controllable, anisotropic flow-based interactions. When applied for particle assembly, various collective structures and phenomena are obtained, including spinning dimers, transverse chains, nucleation, vortical and sweeping swarms, as well as phase separation. The diversity of these structures highlights the immense potential of active polyhedral assembly and advances our understanding of active colloids. In Chapter 5, I discuss the challenges and future perspectives of the field.