Active Synthetic Microrotors: Design Strategies and Applications.

Microrotors are microscopic objects that convert energy stored in the environment into spontaneous rotation, in the form of spinning along an axis, rolling on a surface, or orbiting in circles. Because of its distinct dynamics and the vertical flows around it, a microrotor is potentially useful for applications, including drug delivery, minimally invasive surgery, fluid mixing, and sensing. It is also useful as a model system to probe the collective behaviors among rotating micro-objects. In this review article, we comprehensively review the recent experimental progress in designing, synthesizing, and using microrotors. For applications, particular emphasis is placed on microfluidic mixing, biomedicine, and collective behaviors. In the end, we comment on how microrotors can be made more biocompatible and more controllable and rotate in more ways and the challenges therein. A key feature of this review article is to introduce three ways in which to classify a microrotor: the nature of its rotational behavior (spinners, rollers, or orbiters), the cause of its rotation (whether chiral symmetry is broken by shapes, chemical compositions, or the way energy is applied), and its power source (whether powered by chemical reactions, electric or magnetic fields, light, or ultrasound). This review article will help materials scientists and chemists in designing micromachines and microrotors, help engineers in finding appropriate microrotors for a specific application, and help physicists in finding appropriate model systems.

Generic Rules for Distinguishing Autophoretic Colloidal Motors.

Distinguishing the operating mechanisms of nano- and micromotors powered by chemical gradients, i.e. "autophoresis", holds the key for fundamental and applied reasons. In this article, we propose and experimentally confirm that the speeds of a self-diffusiophoretic colloidal motor scale inversely to its population density but not for self-electrophoretic motors, because the former is an ion source and thus increases the solution ionic strength over time while the latter does not. They also form clusters in visually distinguishable and quantifiable ways. This pair of rules is simple, powerful, and insensitive to the specific material composition, shape or size of a colloidal motor, and does not require any measurement beyond typical microscopy. These rules are not only useful in clarifying the operating mechanisms of typical autophoretic micromotors, but also in predicting the dynamics of unconventional ones that are yet to be experimentally realized, even those involving enzymes.