Active Colloids as Models, Materials, and Machines

Abstract

Active colloids use energy input at the particle level to propel persistent motion and direct dynamic assemblies. We consider three types of colloids animated by chemical reactions, time-varying magnetic fields, and electric currents. For each type, we review the basic propulsion mechanisms at the particle level and discuss their consequences for collective behaviors in particle ensembles. These microscopic systems provide useful experimental models of nonequilibrium many-body physics in which dissipative currents break time-reversal symmetry. Freed from the constraints of thermodynamic equilibrium, active colloids assemble to form materials that move, reconfigure, heal, and adapt. Colloidal machines based on engineered particles and their assemblies provide a basis for mobile robots with increasing levels of autonomy. This review provides a conceptual framework for understanding and applying active colloids to create material systems that mimic the functions of living matter. We highlight opportunities for chemical engineers to contribute to this growing field. Expected final online publication date for the Annual Review of Chemical and Biomolecular Engineering, Volume 14 is June 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Nanorobotic System with Fine Control over Multiple Modes of Motion

“…Furthermore, for the nanorobot when undergoing rotational motion under the effect of a magnetic field, the Reynolds number is evaluated by eq : where d is the diameter of the equivalent volume sphere and Ω is the rotational frequency taken as 2 Hz. The Reynolds number is hence calculated to be 1.4 × 10 –6 which is also in the low Reynolds number regime.…”

“…Indeed, acoustic propulsion is attributed to the nonreciprocal, low Reynolds number viscous flapping of the cylindrical metallic rods within the levitation plane. 33 Furthermore, for the nanorobot when undergoing rotational motion under the effect of a magnetic field, the Reynolds number is evaluated by eq 3: 27 d Re…”

“…While multiple approaches of powering nanorobots have been developed including chemical, − electrical, − magnetic, − acoustic, − and electromagnetic, − most of these methods, coupled with a particular nanorobot design, yield either primarily a translational mode of motion or a rotational one. Recognizing the need to expand nanorobot range of motion to increase their functionality has led to an increasing effort directed at using dual actuation methods. − Previous reports on dually actuated motors have used acoustic propulsion or chemical propulsion as one of the methods of propulsion. − Reports, where one of the actuation methods is acoustic propulsion, have primarily focused on the ability to control the direction of motion of linearly propelling nanorobots, demonstrating directional reversal when switching between one method of actuation and another but have demonstrated no rotational modes of motion. − Reports of methods where one of the actuation methods utilizes a chemical fuel, − typically hydrogen peroxide, suffer from the rapid depletion of the chemical fuel during operation, which influences the mode of motion in an uncontrolled way.…”

Open Questions of Chemically Powered Nano- and Micromotors

“…Three competing mechanisms explain the self-propulsion of a Pt Janus microsphere in H 2 O 2 . Reprinted or adapted with permission under a Creative Commons CC-BY 4.0 from ref . Copyright 2023 Annual Reviews.…”

Micro/Nanorobotic Swarms: From Fundamentals to Functionalities

“…To date, different types of external fields have been applied to actuate micro/nanoagents (Figure ), including magnetic, − electric, − optical, − and acoustic fields. − In addition to these fields, chemical reaction − and topographical patterns − have also been investigated to power and guide the agents, respectively. Existing reviews have focused on individual micro/nanoagents, including their evolution, fabrication and functionalization, − modeling and motion control, − biomedical applications, − and environmental applications. − Compared with a single agent, micro/nanorobotic swarms have higher robustness (e.g., higher tolerance to the failure of a few agents), higher motion dexterity and flexibility, and better scalability. They are poised to transform biology and medicine because of their large surface-to-volume ratio for drug loading, capability of maintaining integrity in resistance to external disturbances, and compatibility with many imaging modalities.…”