Longqiu Li

Magnetically Propelled Fish-Like Nanoswimmers

The swimming locomotion of fish involves a complex interplay between a deformable body and induced flow in the surrounding fluid. While innovative robotic devices, inspired by physicomechanical designs evolved in fish, have been created for underwater propulsion of large swimmers, scaling such powerful locomotion into micro-/nanoscale propulsion remains challenging. Here, a magnetically propelled fish-like artificial nanoswimmer is demonstrated that emulates the body and caudal fin propulsion swimming mechanism displayed by fish. To mimic the deformable fish body for periodic shape changes, template-electrosynthesized multisegment nanowire swimmers are used to construct the artificial nanofishes (diameter 200 nm; length 4.8 μm). The resulting nanofish consists a gold segment as the head, two nickel segments as the body, and one gold segment as the caudal fin, with three flexible porous silver hinges linking each segment. Under an oscillating magnetic field, the propulsive nickel elements bend the body and caudal fin periodically to generate travelling-wave motions with speeds exceeding 30 μm s-1 . The propulsion dynamics is studied theoretically using the immersed boundary method. Such body-deformable nanofishes exhibit a high swimming efficiency and can serve as promising biomimetic nanorobotic devices for nanoscale biomedical applications.

A unified model of drag force for bubble-propelled catalytic micro/nano-motors with different geometries in low Reynolds number flows

Motion of catalytic micro/nano-motors with various geometries is mainly determined by the drag force and the propulsion force acting on the motors as they move in low Reynolds number flows. A unified solution of drag force along with drag coefficient for all circular cross-sectional types of micro/nano-motors is derived. The effect of the geometric parameters of a micro/nano-motor, such as the semi-cone angle θ, the ratio ξ of length to larger radius, on the drag coefficient is identified. Results provided in this work are useful for optimizing the design and fabrication of catalytic micro/nano-motors, which can be potentially used in biomedical and environmental engineering.

Autonomous Collision-Free Navigation of Microvehicles in Complex and Dynamically Changing Environments

Self-propelled micro- and nanoscale robots represent a rapidly emerging and fascinating robotics research area. However, designing autonomous and adaptive control systems for operating micro/nanorobotics in complex and dynamically changing environments, which is a highly demanding feature, is still an unmet challenge. Here we describe a smart microvehicle for precise autonomous navigation in complicated environments and traffic scenarios. The fully autonomous navigation system of the smart microvehicle is composed of a microscope-coupled CCD camera, an artificial intelligence planner, and a magnetic field generator. The microscope-coupled CCD camera provides real-time localization of the chemically powered Janus microsphere vehicle and environmental detection for path planning to generate optimal collision-free routes, while the moving direction of the microrobot toward a reference position is determined by the external electromagnetic torque. Real-time object detection offers adaptive path planning in response to dynamically changing environments. We demonstrate that the autonomous navigation system can guide the vehicle movement in complex patterns, in the presence of dynamically changing obstacles, and in complex biological environments. Such a navigation system for micro/nanoscale vehicles, relying on vision-based close-loop control and path planning, is highly promising for their autonomous operation in complex dynamic settings and unpredictable scenarios expected in a variety of realistic nanoscale scenarios.

An electrical contact resistance model including roughness effect for a rough MEMS switch

Understanding the contact behavior of a microelectromechanical system (MEMS) switch is of great importance to reach a high-reliability level for micro-switch applications. The deformation occurring at the many a-spots produced by the mechanical contact of asperities can be elastic, elastic–plastic, fully plastic or any combination of the three types of deformation and has a strong effect on the contact resistance. This work presents an electrical contact resistance model for a rough MEMS switch. A dimensionless parameter k, which is a function of the electron mean free path and the roughness parameters of contacting surfaces, is introduced to identify the contact resistance. The results show that the dimensionless contact resistance is a strong function of the dimensionless contact load, the plasticity index ψ and the parameter k. The theoretical results are in good agreement with some of the experimental results.

Selective optical trapping based on strong plasmonic coupling between gold nanorods and slab

A resonance plasmon mode is formed between a gold nanorod and an infinite slab in infrared range, with local electric field enhancement factor over 40. A strong optical attractive force is exerted on the rod by the slab at resonance frequency. Based on Maxwell stress tensor method and numerical simulations, the optical force was calculated to be over 2.0 nN/(mW/μm2). For a fixed incident wavelength, the enhanced optical force is obtained only for the rods with particular length when the diameter is fixed. This strong optical force could be used as a possible selective optical trapping technique in the future.

Indefinite Plasmonic Beam Engineering by In-plane Holography

Recent advances in controlling the optical phase at the sub-wavelength scale by meta-structures offer unprecedented possibilities in the beam engineering, holograms, and even invisible cloaks. In despite of developments of plasmonic beam engineering for definite beams, here, we proposed a new holographic strategy by in-plane diffraction process to access indefinite plasmonic beams, where a counterintuitive oscillating beam was achieved at a free metal surface that is against the common recognition of light traveling. Beyond the conventional hologram, our approach emphasizes on the phase correlation on the target, and casts an in-depth insight into the beam formation as a kind of long depth-of-field object. Moreover, in contrast to previous plasmonic holography with space light as references, our approach is totally fulfilled in a planar dimension that offers a thoroughly compact manipulation of the plasmonic near-field and suggests new possibilities in nanophotonic designs.

Direct observation of guided-mode interference in polymer-loaded plasmonic waveguide

We report a direct observation of guided-mode interference in polymer-loaded plasmonic waveguides by the technique of leakage radiation microscopy (LRM). Spatial beating patterns of the interferences were clearly characterized with respect to different structural parameters, and the interference properties were analyzed in detail. Besides, the capability of LRM for characterizing the multiple modes was also discussed extensively. Our finding not only offers an efficient technique in analyzing the guided modes and their interference, but also provides a definite guideline in evaluating the validity of LRM and deepens further studies on the dielectric-loaded hybrid waveguide system.

The Onset of Plastic Yielding in a Spherical Shell Compressed by a Rigid Flat

The onset of plastic yielding in a spherical shell loaded by a rigid flat is analyzed using finite element analysis. The effect of spherical shell geometry and material properties on the critical normal load, critical interference, and critical contact area, at the onset of plastic yielding, is investigated and the location where plastic yielding first occurs is determined. A universal dimensionless shell parameter, which controls the behavior of the spherical shell, is identified. An empirical relation is found for the load-interference behavior of the spherical shell prior to its plastic yielding. A limiting value of the dimensionless shell parameter is identified above which the shell behaves like a solid sphere. DOI: 10.1115/1.4001994

Locomotion of chemically powered autonomous nanowire motors

Physical insights on the hydrodynamics and locomotion of self-propelled nanowire motor under nonequilibrium steady state are investigated using finite element method in accordance with hybrid molecular dynamics/multiparticle collision dynamics and rigid body dynamics. Nanowire motor is discretized into finite segments, and forces of solvent molecule acting on the motor are assumed to be the sum of forces acting on all segments of the motor. We show that the locomotion of nanowire motor is mainly determined by the imbalance forces acting on the catalytic and noncatalytic segments. The average velocity along the axis increases significantly as a function of time prior to reaching equilibrium. The length of nanowire motor shows negligible effect on the velocity of the motor. Preliminary experimental results are provided to validate the current model.

Discussion of electrical discharge machining in gas

Summary form only given. For the microcosmical understanding of electrical discharge machining (EDM) in gas (dry EDM), the influences of such parameters as polarity and gas pressure on EDM in gas were discussed based on theory of gas discharge and experiments; meanwhile, the influencing factors of material remove rate (MRR) in dry EDM and reasons for the low relative wear ratio (RWR) are analyzed mainly according to the gas medium characteristics. Positive polarity was recommended in dry EDM because electrodes play main roles in collision and ionization; the notable feature of dry EDM is very low RWR and the reasons for low RWR are analyzed as follows: (1) the passage developing to anode is stronger than the passage toward cathode, which can be used to explain that the energy absorbed by anode is larger than the energy absorbed by cathode, and that is main reason for lower RWR. (2) Tool electrode was protected by the debris adheres on the tool. Additionally, discharge passage extends rapidly in gas medium although it is limited by liquid in conventional EDM, which leads to weaker workpiece cool effect and low MRR in dry EDM; Finally, to ensure machining process stable at the sparkle discharge state, a certain gas pressure is necessary to strengthen deionization in dry EDM and to keep discharge points disperse in gap.