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.
Keywords: active microrotors; biomedical application; breaking chiral symmetry; catalytic; collective behavior; external fields; micromotors; orbiting; rolling; spinning.