From One to Many: Dynamic Assembly and Collective Behavior of Self-Propelled Colloidal Motors

Abstract

S Supporting Information CONSPECTUS: The assembly of complex structures from simpler, individual units is a hallmark of biology. Examples include the pairing of DNA strands, the assembly of protein chains into quaternary structures, the formation of tissues and organs from cells, and the self-organization of bacterial colonies, flocks of birds, and human beings in cities. While the individual behaviors of biomolecules, bacteria, birds, and humans are governed by relatively simple rules, groups assembled from many individuals exhibit complex collective behaviors and functions that do not exist in the absence of the hierarchically organized structure. Self-assembly is a familiar concept to chemists who study the formation and properties of monolayers, crystals, and supramolecular structures. In chemical self-assembly, disorder evolves to order as the system approaches equilibrium. In contrast, living assemblies are typically characterized by two additional features: (1) the system constantly dissipates energy and is not at thermodynamic equilibrium; (2) the structure is dynamic and can transform or disassemble in response to stimuli or changing conditions. To distinguish them from equilibrium self-assembled structures, living (or nonliving) assemblies of objects with these characteristics are referred to as active matter. In this Account, we focus on the powered assembly and collective behavior of self-propelled colloids. These nano-and microparticles, also called nano-and micromotors or microswimmers, autonomously convert energy available in the environment (in the form of chemical, electromagnetic, acoustic, or thermal energy) into mechanical motion. Collections of these colloids are a form of synthetic active matter. Because of the analogy to living swimmers of similar size such as bacteria, the dynamic interactions and collective behavior of self-propelled colloids are interesting in the context of understanding biological active matter and in the development of new applications. The progression from individual particle motion to pairwise interactions, and then to multiparticle behavior, can be studied systematically with colloidal particles. Colloidal particles are also amenable to designs (in terms of materials, shapes, and sizes) that are not readily available in, for example, microbial systems. We review here our efforts and those of other groups in studying these fundamental interactions and the collective behavior that emerges from them. Although this field is still very new, there are already unique and interesting applications in analysis, diagnostics, separations, and materials science that derive from our understanding of how powered colloids interact and assemble.

Emergent behavior in active colloids

“…First, they can help to understand the main underlying physical principles governing the collective motion of active microscopic individuals which share similar physical properties, namely self-propulsion, rotational diffusion, and low-Reynolds-number hydrodynamics, as well as short-range and phoretic interactions [5]. Second, the self-assembly of interacting active colloids offers the possibility of forming a new class of active materials with novel and tunable properties [35,286].…”

Engineering Contactless Particle–Particle Interactions in Active Microswimmers

“…[28] Currently, the behavior of individual active particles is well documented and mostly understood. However, as this field advances, greater emphasis is being placed upon investigating and engineering the interactions between swimmers, [4,[29][30][31][32][33] for the next wave of advanced applications. The majority of the available studies on the interactions between active colloids have demonstrated selfassembly via direct contact excluded volume interactions and through powered motility of the mobile entities, [34][35][36][37] e.g., the particles come into contact to form self-assembled clusters that translate and rotate.…”

“…We did not magnetize the particles before dispersal as magnetized Janus spheres were observed to rapidly self-assembled as a result of their permanent dipoles from the ferromagnetic Ni. However, as this field advances, greater emphasis is being placed upon investigating and engineering the interactions between swimmers, [4,[29][30][31][32][33] for the next wave of advanced applications. We note this alignment of the Janus director in the absence of a magnetic field is due to interactions between the particles and the solid boundary.…”

Dynamic and programmable self-assembly of micro-rafts at the air-water interface

“…Examples include supramolecular assemblies ( 2 ), various active colloids ( 3 , 4 ), millimeter spinning discs ( 5 , 6 ), and ferrofluid droplets ( 7 ). These works are driven mostly by two main quests.…”