Individual behaviors and dynamic self-assembly of active colloids

Abstract

In recent years, there is mounting interesting in the study of active matters that self-propel by consuming energy and therefore lie beyond thermal equilibrium. Examples include fish schools, bird flocks, bacteria colonies, and tissues made of live cells, to name a few. Mobility of the individual components of these groups, coupled with communications among group members following some simple rules, often gives rise to fascinating and complex emergent behaviors. At the microscale, bacteria are often studied as model systems to understand how individual motion leads to collective behaviors. More recently, synthetic colloids that demonstrate autonomous motion and interesting dynamic assembly behaviors have been developed. Owing to their ease of fabrication, uniformity and functionalizability, these synthetic colloids have attracted much attention in extending our capability in the study of active matters. In this review, we highlight the recent development in the field of active colloids, which include both natural microorganisms as well as synthetic colloids. We begin with a short overview of the research on bacteria. When isolated, bacteria convert nutrients in the environment into directional motion randomized by Brownian motion, at a force scale around pN. A densely populated bacteria bath, however, demonstrate complicated collective behaviors such as swarming and the formation of biofilm. This is mostly attributed to hydrodynamic interaction and volume-exclusion effect at short time scales, while at longer times the chemicals excreted by bacteria start to significantly contribute to inter-bacteria communication. We then turn our attention to synthetic active colloids, especially their experimental aspects. A few popular synthetic systems are first described, focusing on their individual propulsion mechanisms. These include colloids propelled by self-generated gradients, such as self-electrophoresis, self-diffusiophoresis and self-thermophoresis. A few other popular mechanisms are also briefly introduced, including bubble-propulsion, and colloids driven by external fields (electromagnetic and ultrasound). Synthetic microswimmers when moving close by are found to communicate and interact with each other in a variety of ways, displaying collective behaviors ranging from clustering, schooling, and phase separation. We discuss the recent progress in the dynamic assembly of these active colloids, highlighting the complicated interplay among chemical gradients, electrostatic interactions, van der Waals forces and hydrodynamic interaction. It is common to find an active colloid that interacts with its neighbors by a combination of these mechanisms, especially for those driven by chemical gradients where chemical reactions, electric fields and hydrodynamic flows are intricately coupled. Although the behaviors of active colloids often bear striking resemblance to living microorganisms, the nature of their interactions reminds us of how fundamentally different these two types of swimmers are. Understanding their behaviors requires the collective wisdom of experimentalists, theorists and modeling experts. A well-defined model system that promises simple fabrication, ease of functionalization, tunability of parameters, and good reproducibility is also highly desired. At the end of this review, we present the possible directions for future studies in this field. These include: (1) The pursuit of new propulsion mechanisms for active colloids, particularly those that are biocompatible and/or suitable for high population density studies; (2) better understanding of the dynamic clustering effect of active colloids; (3) further exploration of behaviors of active colloids at high number densities; (4) the pursuit of appropriate thermodynamic parameters to describe active colloid systems.