Small power: Autonomous nano- and micromotors propelled by self-generated gradients

In this article we review the development, current status and future prospects of nano-and microscale motors propelled by locally generated fields and chemical gradients. These motors move autonomously in fluids by converting different sources of energy into mechanical work. Most commonly they are particles that are similar in their largest dimensions to bacteria (a few microns) or eukaryotic cells (10—20 ␮m). Their shapes and compositions are designed to break symmetry in some way to create a local gradient (chemical, acoustic, thermal, etc.). A few important principles are introduced for readers to understand the physics of powered movement on small length scales. Interesting collective and emergent behaviors, as well as current and developing applications of these motors are also reviewed. Nano-and micromotors that are propelled by other mechanisms such as bubble recoil and magnetic induction are also briefly discussed.