Recent strides in micro‐ and nanofabrication technologies have enabled researchers to design and develop micro‐ and nanoscale robotic systems with enhanced power, functionality, and versatility. Because of their capability of remote actuation and navigation, synthetic micro‐ and nanomotors powered by oscillating magnetic fields have recently gained considerable attention. In this article, a new type of magnetic surface walker that can achieve speeds of up to 18.6 µm s−1 (≈4 body length s−1) in an oscillating magnetic field operated at 25 Hz and ≈2.7 mT is reported. Two magnetic Janus microspheres spontaneously form a microdimer via magnetic dipolar interactions, and this microdimer rolls its two “feet” back and forth in an alternating fashion. In addition to propulsion, the oscillating magnetic field can also precisely steer these surface walkers through complicated structures, and an extensive discussion of their performance in various experimental conditions is provided. The reported propulsion mechanism opens new possibilities for the design of remotely actuated microrobots for a wide range of applications.

Micromotors powered by megahertz ultrasound, first reported about 5 years ago, have lately been considered a promising platform for a wide range of microscale applications, yet we are only at the early stage of understanding their operating mechanisms. Through carefully designed experiments, and by comparing the results to acoustic theories, we present here an in-depth study of the behaviors of particles activated by ultrasound, especially their in-plane orbiting and spinning dynamics. Experiments suggest that metallic microrods orbit in tight circles near the resonance ultrasound frequency, likely driven by localized acoustic streaming due to slightly bent particle shapes. On the other hand, particle spins around their long axes on nodal lines, where phase-mismatched orthogonal sound waves possibly produce a viscous torque. Intriguingly, such a torque spins metal-dielectric Janus microspheres back and forth in an unusual "rocking chair" fashion. Overall, our observations and analysis provide fresh and much needed insights on the interesting particle dynamics in resonating ultrasound and could help with developing more powerful and controllable micromachines with biocompatible energy sources.

One of the recent frontiers of nanotechnology research involves machines that operate at nano- and microscales, also known as nano/micromotors. Their potential applications in biomedicine, environmental sciences and engineering, military and defense industries, self-assembly, and many other areas have fueled an intense interest in this topic over the last 15 years. Despite deepened understanding of their propulsion mechanisms, we are still in the early days of exploring the dynamics of micromotors in complex and more realistic environments. Confinements, as a typical example of complex environments, are extremely relevant to the applications of micromotors, which are expected to travel in mucus gels, blood vessels, reproductive and digestive tracts, microfluidic chips, and capillary tubes. In this review, we summarize and critically examine recent studies (mostly experimental ones) of micromotor dynamics in confinements in 3D (spheres and porous network, channels, grooves, steps, and obstacles), 2D (liquid–liquid, liquid–solid, and liquid–air interfaces), and 1D (chains). In addition, studies of micromotors moving in the bulk solution and the usefulness of acoustic levitation is discussed. At the end of this article, we summarize how confinements can affect micromotors and offer our insights on future research directions. This review article is relevant to readers who are interested in the interactions of materials with interfaces and structures at the microscale and helpful for the design of smart and multifunctional materials for various applications.