Tunable Manipulation Research Articles

Controlling phase separations and reactions in trapped microfluidic droplets

Microfluidics and droplet-based assays are the basis for numerous high-throughput experiments, including bio-inspired microreactors and selection platforms for directed evolution. While elaborate techniques are available for the production of picoliter-sized droplets, there is an increasing demand for subsequent manipulation and control of the droplet interior. Here, we report on a straightforward method to rapidly adjust the size of single to several hundred double-emulsion droplets in a microfluidic sieve by varying the carrier fluid’s salt concentration. We show that the concomitant concentration changes in the droplet interior can drive a reversible demixing transition in a biomimetic binary fluid. As another application, we show that growing and shrinking of trapped droplets can be utilized to achieve a reversible dissociation of double-stranded DNA into single strands, i.e. cycles of reversible DNA hybridization, similar to PCR cycles, can be achieved by reversibly changing the droplet size at constant temperature. Altogether, our approach shows how a simple and temporally tunable manipulation of the size and the chemistry in prefabricated droplets can be achieved by an external control parameter.

Dual-curvilinear beam enabled tunable manipulation of high- and low-refractive-index particles

We present an innovative approach for the simultaneous agile manipulation of high-refractive-index (HRI) and low-refractive-index (LRI) particles. Our method involves introducing a dual-curvilinear optical vortex beam (DC-OVB) generated by superimposing a pair of curved beams: HRI and LRI particles are controlled by the bright curve and the dark channel between the two curves, respectively. The proposed DC-OVB provides customizable motion paths and velocities for both LRI and HRI particles. Each curve of the DC-OVB can support a distinct orbital flow density (OFD), enabling the application of torques to HRI and LRI particles, guiding them to orbit along specified trajectories and prompting them to execute various curvilinear motions simultaneously, including curvilinear movement, revolution, and rotation.

Active Tunable Elastic Metasurface for Abnormal Flexural Wave Transmission

An active elastic metasurface has more flexibility than a passively modulated elastic metasurface, owing to the manipulation of the phase gradient that can be realized without changing the geometrical configuration. In this study, a negative proportional feedback control system was employed to provide positive active control stiffness for adaptive unit cells, with the aim of achieving the active modulation of the phase gradient. The relationship between the control gain and the phase velocity of the flexural wave was derived, and the transfer coefficients and phase shifts of the flexural wave through the adaptive unit cells were resolved using the transfer matrix method. Finite element simulations for wave propagations in the adaptive unit cells were conducted, and they verified the analytic solutions. Based on this theoretical and numerical work, we designed active elastic metasurfaces with adaptive unit cells with sub-wavelength thicknesses according to the generalized Snell’s law. These metasurfaces show flexibility in achieving abnormal functions for transmitted waves, including negative refraction and wave focusing, and transforming guided waves at different operating frequencies by manipulating the control gain. Therefore, the proposed active metasurface has great potential in the fields of the tunable manipulation of elastic waves and the design of smart devices.

Wave Manipulation in Intelligent Metamaterials: Recent Progress and Prospects

AbstractMetamaterials (MMs), which include phononic crystals (PCs) as a particular type, exhibit anomalous wave propagation properties through artificial design of topologies or lattice forms of unit‐cells. Recent advancements in MMs signify an ascendant research trend, providing promising design ideas and means for unprecedented wave propagation properties. The imperative for on‐demand, real‐time active control of wave propagation underscores the significance of tunable manipulation of acoustic/elastic waves, promoting the design and development of tunable MMs. Furthermore, the versatility of intelligent materials and their ongoing development and innovation contribute significantly to the emergence of diverse intelligent MMs. This comprehensive survey provides an overview of recent advancements and current research trends in the interdisciplinary field of intelligent MMs with electro‐/magneto‐mechanical couplings. The primary objective of the review is to emphasize significant progress in agile manipulation of acoustic/elastic waves in electro‐/magneto‐mechanical coupled MMs, followed by an in‐depth exploration of intelligent metasurfaces, topological MMs, non‐Hermitian parity‐time symmetric wave systems, odd elastic MMs, and spatiotemporally modulated MMs. Special emphasis is given to multi‐field coupling effects. The review concludes with a summary and outlines potential prospects, offering a timely and informative guide for future studies on actively tunable PCs and MMs in practical engineering applications.

Tunable multiband circular dichroism and asymmetric transmission enabled by chiral quasi-BICs in dielectric metasurfaces covered with graphene

High-Q chiral resonances have attracted great interest recently due to their practical photonic applications. For generating such resonances, chiral quasi-bound states in the continuum (quasi-BICs) have recently been proposed and experimentally demonstrated by many researchers, while their dynamic tunability has still not been discussed. In this work, we show multiband circular dichroism and asymmetric transmission based on high-Q chiral quasi-BICs in dielectric metasurfaces, which are dynamically tunable in the mid-infrared range by using graphene monolayers covering the metasurfaces. By changing the Fermi energy, the surface conductivity of graphene is tunable in a wide range, resulting in tunable chiroptical responses of the metasurfaces. Since the metasurfaces exhibit high-Q resonances with narrow linewidths, tuning the metasurfaces results in large modulation depths of circular dichroism and asymmetric transmission, which is of great importance for photonic applications, including tunable polarization manipulation and polarization-dependent detection.

Investigation of a new magnetorheological elastomer metamaterial plate with continuous programmable properties for vibration manipulation

Metamaterial plates have attracted considerable attention due to their unprecedented elastic wave manipulation abilities. Currently, the band gap and other properties of traditional metamaterial plates are fixed, and the material parameters of most metamaterials cannot be dynamically and continuously adjusted. To address this issue, a two-dimensional (2D) magnetorheological elastomer (MRE) programmable metamaterial plate (MREPMP) with local resonance was proposed. The proposed MREPMP was composed of a thin steel plate and a periodic array of resonators, consisting of MREs and electromagnetic mass blocks. It not only has a compact structural design, but also enables real-time vibration control and programming wave manipulation by altering the amplitude of the current. The dispersion relation of the proposed MREPMP was theoretically calculated based on the plane wave expansion (PWE) method and the simulated transmissibility was numerically calculated using the finite element method in COMSOL. Subsequently, a prototype of the MREPMP was fabricated, and vibration tests were conducted to assess its programmable vibration manipulation functionalities. Two different types of defect paths were used as excitations and the corresponding real-time vibration manipulation performance was analyzed. The simulation results agree with the experimental results, showing that the MREPMP can dynamically tune the band gap and activate different waveguides through continuous online programming. This paper opens up a new idea for tunable manipulation of wave propagation.

A structural blueprint for interleukin-21 signal modulation

Interleukin-21 (IL-21) plays a critical role in generating immunological memory by promoting the germinal center reaction, yet clinical use of IL-21 remains challenging because of its pleiotropy and association with autoimmune disease. To better understand the structural basis of IL-21 signaling, we determine the structure of the IL-21-IL-21R-γc ternary signaling complex by X-ray crystallography and a structure of a dimer of trimeric complexes using cryo-electron microscopy. Guided by the structure, we design analogs of IL-21 by introducing substitutions to the IL-21-γc interface. These IL-21 analogs act as partial agonists that modulate downstream activation of pS6, pSTAT3, and pSTAT1. These analogs exhibit differential activity on T and B cell subsets and modulate antibody production in human tonsil organoids. These results clarify the structural basis of IL-21 signaling and offer a potential strategy for tunable manipulation of humoral immunity.

Plasmonic vortices for tunable manipulation of target particles, using arrays of elliptical holes in a gold layer

Here, we numerically prove that light with linear polarization can be coupled to surface plasmon polaritons at an elliptical hole perforated in a gold layer to generate plasmonic vortex (PV). Benefiting from the smooth variation of the minor to major ellipse axes, a gradual variation in the phase profile of the generated PV is achieved. Regarding this, three types of independent arrays of elliptical holes are presented, which can produce uniform and high quality PVs with different topological charges at the center of the arrays. The first array can produce PV with topological charges of + 1 and − 1, depending on the polarization orientation of the incident light. In the second one, the topological charge of the PV can be switched between 0 and + 2, by switching the polarization direction of the incident light. In the third array, a robust PV with topological charge of + 1 is generated independent of possible tolerances in the polarization orientation. In order to use the generated PVs for plasmonic tweezing application, there are side fringes around the central vortex of the arrays that should be eliminated. To produce a single vortex, we propose metal-insulator-metal (MIM) structures, screening excessive fringes and allowing the central PVs to leak out. It is also demonstrated by simulation that target particles, such as gold and polystyrene spheres of subwavelength dimensions, can be efficiently manipulated by our MIM designs, suitable for different applications including local mixing, and applying switchable torque or force to target particles to explore their complete elastic characteristics.

Highly morphing and reconfigurable fluid–solid interactive metamaterials for tunable ultrasonic guided wave control

Fluid–structural interactions enable the alternation of local resonance behaviors of elastic metamaterial unit cells. Magnetically active ferrofluids facilitate reconfiguration couplings for breaking and tunneling ultrasonic wave energy transmission. This Letter presents a magnetic fluid–solid interactive metamaterial to achieve the tunable manipulation of multimodal, dispersive ultrasonic guided waves. It is revealed that the phenomenon of the fluid–structure interaction plays an indispensable role in the achievement of bandgap formation and translation. The tunable mechanism stems from the variation of the fluid–solid coupling reconfiguration arising from liquid morphing via electromagnetic stimuli. The tunable wave control performance was explicitly validated through both numerical simulations and experimental verifications. Such an active metamaterial system may possess application potential for future highly flexible and tunable wave control, e.g., selective-tunnel waveguiding and adaptive mechanical frequency filtering.

Theoretical analysis of guided waves propagation in periodic piezoelectric plates with shunting circuits

The tunable manipulation of guided waves in plates brings out great potential applications in engineering practices, and the electromechanical coupling effects of piezoelectric material with shunting circuits have exhibited powerful tunability and flexibility for guided wave propagation. In this paper, a theoretical model is established to analyze the guided wave propagation in one-dimensional periodic piezoelectric plate constructed from a periodic array of anisotropic piezoelectric materials under periodic electrical boundary conditions. The extended Stroh formalism incorporating with the plane wave expansion method is developed to transform the wave motion equations of the periodic piezoelectric plate into a linear eigenvalue system, and a more concise and elegant solution of generalized displacement and generalized stress can be derived. There are various dispersion relations in terms of the altering electrical boundary conditions to be acquired, if the thin electrodes with shunting circuits are attached periodically to both surfaces of the piezoelectric plate. Analytical results show that the coupling of the local electric resonant mode and propagating elastic wave modes can induce hybridization bandgaps, and the bandgaps of Lamb waves and SH waves in the piezoelectric plate can be tuned by designing appropriate material polarization orientations and shunting circuits. In addition, the Bragg bandgaps can also be influenced by the external circuits. Results indicate that the proposed theoretical model can effectively analyze the performances of guided waves in periodic piezoelectric plate and provide useful theoretical guidance for designing smart wave control devices.

Antiferromagnetism: An efficient and controllable spin source

Antiferromagnetic (AFM) spintronics is an emerging field, with significant advances in particular in the study of the tunable spin generation, transport, manipulation, and detection based on the control and probe of AFM moments. The Néel-vector-dependent spin current generation in AFM materials makes them an efficient and controllable spin source, paving the way for future spintronic devices with ultralow power consumption and high operability. Herein, we aim to provide a comprehensive review of recent progress in charge-spin conversion mediated by AFMs. First, we present a brief summary of several typical characterization techniques of charge-spin conversion. Then, we focus on the recent efforts for improving the charge-spin conversion efficiency and performances of spin torques in the antiferromagnet/ferromagnet bilayer. In the third part, we review the controllable spin current and multidirectional spin polarization generated by AFMs and the resultant field-free magnetization switching. Finally, we discuss the prospects of the AFM spin sources, which will inspire more in-depth studies and advance practical applications.

Tunable Terahertz Wavefront Modulation Based on Phase Change Materials Embedded in Metasurface.

In the past decades, metasurfaces have shown their extraordinary abilities on manipulating the wavefront of electromagnetic wave. Based on the ability, various kinds of metasurfaces are designed to realize new functional metadevices based on wavefront manipulations, such as anomalous beam steering, focus metalens, vortex beams generator, and holographic imaging. However, most of the previously proposed designs based on metasurfaces are fixed once design, which is limited for applications where light modulation needs to be tunable. In this paper, we proposed a design for THz tunable wavefront manipulation achieved by the combination of plasmonic metasurface and phase change materials (PCMs) in THz region. Here, we designed a metal-insulator-metal (MIM) metasurface with the typical C-shape split ring resonator (CSRR), whose polarization conversion efficiency is nearly 90% for circular polarized light (CPL) in the range of 0.95~1.15 THz when PCM is in the amorphous state, but the conversion efficiency turns to less than 10% in the same frequency range when PCM switches into the crystalline state. Then, benefiting from the high polarization conversion contrast of unit cell, we can achieve tunable wavefront manipulation by utilizing the Pancharatnam–Berry (PB) phase between the amorphous and crystalline states. As a proof-of-concept, the reflective tunable anomalous beam deflector and focusing metalens are designed and characterized, and the results further verify their capability for tunable wavefront manipulation in THz range. It is believed that the design in our work may pave the way toward the tunable wavefront manipulation of THz waves and is potential for dynamic tunable THz devices.

Dual tunable terahertz polarization conversion enabled by Double-Layer Graphene Metasurface

Tunable manipulation on the polarization states of electromagnetic waves is the greatly significant and largely desired fields including information encryption of electromagnetic waves, photodetection, and optical microscope. However, such tunability is usually limited by narrow-working bandwidth. In this paper, we propose a novel design of a dynamic tunable multi-function Double-Layer Graphene Metasurface (DLGM) in THz frequency range, assisted by the graphene plasmon resonance (GPR), which enables to realize dynamic tunable polarization states in the same frequency, and achieve polarization conversion in a broadband frequency range. Moreover, by tuning the Fermi level at different times in each layer of graphene, our design can be applied in time-domain encryption of electromagnetic waves since the frequency range of polarization conversion (circular polarization) as well as the polarization states at specific frequency can be controlled at different times corresponding to the Fermi level. Finally, the mechanism of the phase difference induced by GPR is revealed in our proposed design and our numerical calculation shows such polarization conversion can cover broader THz region compared with previous results.

Tunable Manipulation of Enantiomers by Vector Exceptional Points

Tunable Manipulation of Enantiomers by Vector Exceptional Points

Terahertz Metamaterial Modulator Based on Phase Change Material VO2

In this paper, a new type of terahertz (THz) metamaterial (MM) modulator has been presented with bifunctional properties based on vanadium dioxide (VO2). The design consists of a VO2 resonator, polyimide substrate, frequency selective surface (FSS) layer, and VO2 film. Based on the metal-insulator transition (MIT) of VO2, this structure integrated with VO2 material can achieve the dynamic modulation on both transmission and reflection waves at 2.5 THz by varying the electrical conductivity value of VO2. Meanwhile, it also exhibits adjustable absorption performance across the whole band from 0.5–7 THz. At the lower conductivity (σ = 25 S/m), this structure can act as a bandpass FSS, and, at the high conductivity (σ = 2 × 105 S/m), it behaves like a wideband absorber covering 2.52–6.06 THz with absorption A > 0.9, which realizes asymmetric transmission. The surface electric field distributions are illustrated to provide some insight into the physical mechanism of dynamic modulation. From the simulated results, it can be observed that this design has the capability of controlling tunable manipulation on both transmission/reflection responses at a wide frequency band. This proposed design may pave a novel pathway towards thermal imaging, terahertz detection, active modulators, etc.

A new strategy towards spectral selectivity: Selective leaching alloy to achieve selective plasmonic solar absorption and infrared suppression

Cost-effective and spectrally selective solar absorbers that possess high solar absorptance, low thermal emittance, and superior thermal stability are essential for photothermal conversion applications, e.g., industrial heating, solar desalination, photothermal catalysis, and concentrating solar power systems. Here, a new strategy using a selective leaching reaction is demonstrated for transfiguring the broad-spectrum and highly reflective aluminum alloys into plasmonic-nanostructure selective solar absorbers (PNSSAs). Enabled by surface plasmon resonance, this strategy via assembling copper nanostructured thin film on an alloy mirror yields tunable manipulation of the spectral selectivity, high and omnidirectional solar absorptance (0.94 from 0 to 60°), low thermal emittance (0.03 at 100 °C), and excellent thermomechanical stability. Featured with merits of competitive performance of spectral selectivity, the feasibility of solution-processed fabrication, and cost-effectiveness of raw materials and chemicals, selective-leaching-alloy to achieve PNSSAs is a promising and universal approach for achieving high photothermal efficiency (85%) of solar thermal energy harvesting. The compatibility of this strategy with other metal alloys, such as steel and superalloys, extends its applications to fabricating mid- and high-temperature selective solar absorbers.

Elastic wave velocities in finitely pre-stretched soft fibers

Elastic wave velocity in a soft fiber that varies depending on material constitution and axial stress level is an essential measure of mechanical signals in many technical applications. In this work, based on the small-on-large theory, we establish a model of linear elastic wave propagation in a finitely pre-stretched soft fiber. The formulas of longitudinal (L-) and transverse (T-) wave velocities are provided and validated by numerical simulations as well as by experimental data on spider silk. The influences of material constitution, compressibility, and pre-stress on the wave propagation are investigated. We found that with increasing pre-stress, the variation of L-wave velocity highly relies on the concavity of the stress–strain curve. In contrast, an increase of T-wave velocity exhibits regardless of any constitutive model. For both L- and T-waves, the variation of the velocities is more significant in a compressible fiber than that in a nearly-incompressible one. Moreover, for minuscule pre-stress, we propose a modified formula for T-wave velocity based on the Rayleigh beam theory, which reveals the competition mechanism between “string vibration” and “beam vibration.” This may provide a reliable theoretical basis for precise mechanical characterization of soft fibers and open a route for lightweight, tunable wave manipulation devices.

Acoustic Metagrating Circulators: Nonreciprocal, Robust, and Tunable Manipulation with Unitary Efficiency

Nonreciprocal signal operation is highly desired for various acoustic applications, where protection from unwanted backscattering can be realized so that transmitting and receiving signals are processed in a full-duplex mode. Here we present the realization of a class of nonreciprocal circulators based on simply structured acoustic metagratings, which consist only of a few solid cylinders and a steady fluid flow with low velocity. These innovative metagratings are intelligently designed via a diffraction analysis of the linearized potential flow equation and a genetic-algorithm-based optimization process. Unitary reflection efficiency between desired ports of the circulators are demonstrated through full-wave numerical simulations, confirming nonreciprocal and robust circulation of the acoustic signal over a broad range of flow velocity magnitude and profile. Our design provides a feasible degree of tunability, including switching from reciprocal to nonreciprocal operation and reversing the handedness of the circulator, presenting a convenient but efficient approach for the realization of nonreciprocal acoustic devices from wavelength-thick metagratings. It may find applications in various scenarios including underwater communication, energy harvesting, and acoustic sensing.

Dynamically controlled dielectrophoresis using resonant tuning.

Electrically polarizable micro- and nanoparticles and droplets can be trapped using the gradient electric field of electrodes. But the spatial profile of the resultant dielectrophoretic force is fixed once the electrode structure is defined. To change the force profile, entire complex lab-on-a-chip systems must be re-fabricated with modified electrode structures. To overcome this problem, we propose an approach for the dynamic control of the spatial profile of the dielectrophoretic force by interfacing the trap electrodes with a resistor and an inductor to form a resonant resistor-inductor-capacitor (RLC) circuit. Using a dielectrophoretically trapped water droplet suspended in silicone oil, we show that the resonator amplitude, detuning, and linewidth can be continuously varied by changing the supply voltage, supply frequency, and the circuit resistance to obtain the desired trap depth, range, and stiffness. We show that by proper tuning of the resonator, the trap range can be extended without increasing the supply voltage, thus preventing sensitive samples from exposure to high electric fields at the stable trapping position. Such unprecedented dynamic control of dielectrophoretic forces opens avenues for the tunable active manipulation of sensitive biological and biochemical specimen in droplet microfluidic devices used for single-cell and biochemical reactionanalysis.

Electronically controlled liquid crystal terahertz polarization manipulation under static magnetic field pre-anchoring

In this work, a new strategy was proposed for active control of mm-thick liquid crystals (LC) cell to realize the polarization manipulation in terahertz (THz) regime, which through the electric field control and static magnetic field pre-anchoring. The LC cell was fabricated by a nematic 5CB LC and two silica substrates that were coated with the graphite layer as the transparent electrode. Under the pre-anchoring of the static magnetic field, the optical axis of LC can be precisely controlled by the variable electric field. By using a THz-TDS with a wire grid polarizer, the output THz polarization from the LC cell can be deduced from the amplitude and phase shift of ±45° components. Here, we systematically analyzed three different outfield configurations. Only if the ±45° components that output from the polarizer have phase shifts, can the polarization state conversion be realized. The results show that the linear to circular or the cross-polarization conversion were realized under the specific electric field. This work provides a new approach for the thick-LC layer anchoring and orientation control, and also the tunable polarization manipulation of THz LC devices.