Thanks to their capabilities of converting various energy into motions, micro/nanomachines
are believed to bring about revolutionary changes in many fields, such as biomedicine,
micro/nanoengineering, and environmental monitoring and remediation. Even though remarkable
progress has been made in the last few decades, there are still some great challenges,
such as toxic fuels, low controllability, poor intelligence, single functions, etc.,
that need to be overcome so that micro/nanomachines can work safely and effectively
in living bodies, natural water systems, and micro/nanofactories. In addition, swarmings
and assemblies of micro/nanomachines which have collective behaviors may cooperatively
perform complex biological or engineering tasks that cannot be completed by single
ones. In this present Special Issue, we have published 10 papers, covering topics
ranging from the design strategies, motion control, and applications of single micro/nanomachines
[1,2,3,4,5,6,7], to the collective behaviors of micro/nanomachines [1,8,9,10].
Among the 10 published papers in this Special Issue, there are three reviews and four
research papers that concern single micro/nanomachines. Ning et al. introduced the
design from aspects of materials, geometries and fuels, the motion control strategies,
and the potential applications of micromotors, and they outlined some future research
directions [1]. Considering the unique advantages of tubular micro/nanomotors in driving
force and surface functionalization, Zha et al. overviewed in detail the propulsion
mechanisms, fabrication techniques, and applications, and pointed out some challenging
problems of the existing tubular micro/nanomotors, as well as possible solutions to
be explored in the near future [2]. Chen et al. summarized the recent advances in
the design, manufacture, structural features, motion performance, and motion manipulation
of light-powered micro/nanomotors and came up with some of their challenges and opportunities
[3]. Jiao et al. reported a magnetic and fluorescent hybrid Janus micromotor by embedding
magnetic nanoparticles and fluorescent dyes into the microparticles in a one-step
process [4]. Sun et al. investigated the motion behaviors of self-powered liquid metal
droplet machines under an external electric field, and put forward two non-dimensional
parameters (Ä and Ö) to evaluate the ratio of the forces that resulted from the electric
field to the fluidic viscous force, and the ratio of the friction force to the fluidic
viscous force [5]. Li et al. proposed a self-propelled Janus foam motor, which can
effectively integrate intriguing behaviors of the self-propulsion, efficient oil capture,
and spontaneous self-assembly [6]. Feng et al. demonstrated three-dimensional (3D)
control of the microrobot within a microfluidic chip using balanced magnetic and buoyancy
forces, and the microrobot could grip particles (200 μm) and deliver it in a 3D space
[7].
Motivated by the intriguing collective behaviors and the swarm intelligence of lives
in nature, researchers are also dedicated to the understanding, construction, and
manipulation of micro/nanomachine swarms and assemblies in this Special Issue. Shi
et al. theoretically studied the pair dynamics of two self-propelled sphere dimers
in the chemically active medium, which may shed light on the understanding of the
collective dynamics of synthetic micro/nanomachines, as pair dynamics are the basic
elements of the larger scale systems [8]. Ning et al. briefly introduced swarming,
collective, and adaptive behaviors of interactive micromotors in the view of dynamic
interactions between them [1]. Liu et al. summarized and compared the assembly and
swarming of synthetic micro/nanomachines by the fuel induced methods (enzyme, hydrogen
peroxide, hydrazine, etc.) and fuel-free induced approaches (electric, ultrasound,
light, and magnetic) [9]. Zhang et al. streamlined the recent developments in light-controlled
swarming and the assembly of colloidal particles based on the interactions that have
arisen from optical forces, photochemical reactions, photothermal effects, and photoisomerizations,
and also discussed the potential applications, challenges, and future prospects [10].
We wish to express our gratitude to all of the authors who submitted their papers
to this Special Issue. We are also very grateful to all of the reviewers who helped
us in an attentive and timely manner to improve the quality of this Special Issue.