Efficient Propulsion and Hovering of Bubble-Driven Hollow Micromotors underneath an Air-Liquid Interface

Sub-Nanogram Resolution Measurement of Inertial Mass and Density Using Magnetic-Field-Guided Bubble Microthruster

Artificial micro/nanomotors using active particles hold vast potential in applications such as drug delivery and microfabrication. However, upgrading them to micro/nanorobots capable of performing precise tasks with sophisticated functions remains challenging. Bubble microthruster (BMT) is introduced, a variation of the bubble-driven microrobot, which focuses the energy from a collapsing microbubble to create an inertial impact on nearby target microparticles. Utilizing ultra-high-speed imaging, the microparticle mass and density is determined with sub-nanogram resolution based on the relaxation time characterizing the microparticle's transient response. Master curves of the BMT method are shown to be dependent on the viscosity of the solution. The BMT, controlled by a gamepad with magnetic-field guidance, precisely manipulates target microparticles, including bioparticles. Validation involves measuring the polystyrene microparticle mass and hollow glass microsphere density, and assessing the mouse embryo mass densities. The BMT technique presents a promising chip-free, real-time, highly maneuverable strategy that integrates bubble microrobot-based manipulation with precise bioparticle mass and density detection, which can facilitate microscale bioparticle characterizations such as embryo growth monitoring.

Role of Bubble Evolution in the Bubble-Propelled Janus Micromotors

Bubble-propelled Janus micromotors have attracted extensive attention in recent years and have been regarded as powerful tools in the environmental and medical fields due to their excellent movement ability. The movement ability can mainly be attributed to the periodic growth, detachment, and/or collapse of the bubble. However, subjected to the experimental conditions, the mechanism of bubble evolution on the motion of the micromotor could not be elucidated clearly. In this work, a finite element method was employed for exploring the role of bubble evolution in bubble-propelled Janus micromotors, which emphasized the growth and collapse of bubbles. After the proposed model was verified by the scallop theorem, the influence of the growth and rapid collapse of bubbles on micromotors was investigated. Results show that the growth and collapse of a bubble can drive the micromotor to produce a displacement, but the displacement caused by a bubble collapse is significantly greater than that caused by bubble growth. The reasons for this phenomenon are analyzed and explained. In addition to the influence of bubble size, the collapse time of the bubble is also investigated.

Intrinsic Properties Enabled Metal Organic Framework Micromotors for Highly Efficient Self-Propulsion and Enhanced Antibacterial Therapy

Miniaturization of synthetic micro/nanomotors (MNMs) brings great application prospects but limits their functionalization ability. Here, we report self-fueled metal organic framework (MOF) micromotors that are endowed with capabilities of self-propulsion and antibacterial therapy by their material's intrinsic properties. The spontaneous degradation of the MOF micromotors in water would release their own constituting components of ions which act as fuels to propel themselves by ionic diffusionphoresis with a high energy conversion efficiency. Meanwhile, the metal cations released from the MOF micromotors can also serve as antibacterial reagents to kill Escherichia coli (E. coli) with motion enhanced efficacy, which could significantly accelerate the wound closure in a bacterially infected wound model in vivo. Our work provides a general guidance for constructing functional MNMs by taking advantage of the motors' own materials to achieve self-propulsion and on-demand task assignments, which would promote future development of highly integrated micro/nanorobotic systems at micro/nanoscale.

Nonequilibrium atomistic molecular dynamics simulation of tubular nanomotor propelled by bubble propulsion

We develop a molecular nanoscaled model for tubular motors propelled by bubble propulsion. The motor is modeled by a carbon nanotube, and the bubble is represented by a particle interacting with water by a time-dependent potential. Effects of liquid viscosity, fuel concentration, geometry, and size of the tube on the performance of the motor are effectively encoded into two parameters: time scales of the bubble expansion and bubble formation. Our results are qualitatively consistent with experimental data of much larger motors. Simulations suggest that (i) the displacement of the tube is optimized if two time scales are as short as possible, (ii) the compromise between the performance and fuel consumption is achieved if the bubble formation time is shorter than the velocity correlation time of the tube, (iii) the motor efficiency is higher with slow expansion, short formation of the bubble than fast growth but long formation time, and (iv) the tube is propelled by strong forces on the order of mN, reaching high speeds up to ∼60 m/s. Our simulation may be useful for refining and encouraging future experimental work on nanomotors having the size of a few nanometers. The tiny size and high speed motors could have great potential applications in real life.

A Review of Micromotors in Confinements: Pores, Channels, Grooves, Steps, Interfaces, Chains, and Swimming in the Bulk

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.

Disintegrating polymer multilayers to jump-start colloidal micromotors

Colloidal systems with autonomous mobility are attractive alternatives to static particles for diverse applications. We present a complementary approach using pH-triggered disintegrating polymer multilayers for self-propulsion of swimmers. It is illustrated both experimentally and theoretically that homogenously coated swimmers exhibit higher velocity in comparison to their Janus-shaped counterparts. These swimmers show directional and random motion in microfluidic channels with a steep and shallow pH gradient, respectively. Further, a higher number of deposited polymer multilayers, steeper pH gradients and lower mass of the swimmers result in higher self-propulsion velocities. This new self-propulsion mechanism opens up unique opportunities to design, for instance, fast and yet biocompatible swimmers using the diverse tools of polymer chemistry to custom-synthesise the polymeric building blocks to assemble multilayers.