From One to
Many: Dynamic Assembly and Collective
Behavior of Self-Propelled Colloidal Motors
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Abstract
ConspectusThe 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 <i>active matter.</i>In this Account, we focus on the powered assembly and collective
behavior of <i>self-propelled colloids</i>. These nano-
and microparticles, also called <i>nano-</i> and <i>micromotors</i> or <i>microswimmers</i>, 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