Unidirectional Surface Research Articles

Unidirectional Surface Waves in Bi-Anisotropic Media

We show theoretically that unidirectional surface waves can propagate on the surface of homogeneous bi-anisotropic layers with an anti-symmetric chirality tensor. These materials mimic the electromagnetic behavior of an anisotropic medium with gyrotropic permittivity and permeability tensors that operate on pseudo-electromagnetic fields. The unidirectional waves have transverse pseudo-electric or magnetic polarizations and pass through an obstacle without backscattering if the obstacle does not cause polarization change. The bi-anisotropic medium can be realized as a metamaterial comprising omega particles tailored to achieve the constitutive parameters desired.

Coupled Topological Surface Modes in Gyrotropic Structures: Green's Function Analysis

At a transition in a wave-guiding structure, part of the incident energy is transmitted and part of the energy is reflected. When the waveguide has non-trivial topological properties, however, the transition may occur with no backscattering, and with unusual modal coupling/transformations. Within this context, we discuss the response of a nonreciprocal topological structure composed of two nearby interfaces between oppositely-biased gyrotropic media and an isotropic medium, which support unidirectional surface modes (topological modes). We provide an exact Green's function analysis of this structure, and we discuss how the topological surface modes are modified when the two interfaces are brought closer and eventually merged. We show that the resulting mode conversion is independent of the transition geometry.

Influence of environmental friendly multiphase lubricants on the friction and transfer layer formation during sliding against textured surfaces

Both surface texture and multiphase lubricants (additives in base oil) influence the friction and wear performance during sliding. However, the synergy of their interaction is still unknown. In the present investigation, an effort is made to understand the influence of environmental friendly multiphase lubricants on the friction and transfer layer formation during sliding against various surface textures. In this study, unidirectional surface textures were created and sliding tests were conducted against various grinding angles of the unidirectional texture. The sliding tests were conducted in multiphase lubricants consisting of canola oil (biolubricant) and nano-additives (graphene) at various concentrations. The results showed that the coefficient of friction (COF) and transfer layer formation was influenced by the grinding angle. However, it was found that this influence can be minimized by using an optimum concentration of graphene in canola oil. A minimum COF of 0.05 ± 0.01 could be achieved for various grinding angles by utilizing a specific concentration of graphene. It was found that a high concentration of Gr (3 wt%) was required when sliding at 0° and a low concentration of Gr (1 wt%) was required when sliding at 45° to achieve the minimum COF. This was attributed to the variations in asperity and Gr additive interaction when sliding at different grinding angles. The present study details the observed friction and transfer layer formation results and analyzes the mechanism of multiphase lubrication during sliding at various grinding angles. An asperity interaction model for sliding against various grinding angles and in a multiphase lubricant with varying additive concentrations has also been presented.

Optical torque on a two-level system near a strongly nonreciprocal medium

We investigate the quantum optical torque on an atom interacting with an inhomogeneous electromagnetic environment described by the most general linear constitutive relations. The atom is modeled as a two-level system prepared in an arbitrary initial energy state. Using the Heisenberg equation of motion (HEM) and under the Markov approximation, we show that the optical torque has a resonant and non-resonant part, associated respectively with a spontaneous-emission process and Casimir-type interactions with the quantum vacuum, which can both be written explicitly in terms of the system Green function. Our formulation is valid for any inhomogeneous, dissipative, dispersive, nonreciprocal, and bianisotropic structure. We apply this general theory to a scenario in which the atom interacts with a material characterized by strong nonreciprocity and modal unidirectionality. In this case, the main decay channel of the atom energy is represented by the unidirectional surface waves launched at the nonreciprocal material-vacuum interface. To provide relevant physical insight into the role of these unidirectional surface waves in the emergence of non-trivial optical torque, we derive closed-form expressions for the induced torque under the quasi-static approximation. Finally, we investigate the equilibrium states of the atom polarization, along which the atom spontaneously tends to align due to the action of the torque. Our theoretical predictions may be experimentally tested with cold Rydberg atoms and superconducting qubits near a nonreciprocal material. We believe that our general theory may find broad application in the context of nano-mechanical and bio-mechanical systems.

Resonant Transmission through Topological Metamaterial Grating

AbstractRecently, artificial photonic structures that exhibit nontrivial topological properties have attracted growing attention due to their capability of achieving one‐way backscatter immune transport of light. While photonic crystals are predominantly employed for achieving nontrivial topologies, effective medium approach based on metamaterials has been recently proposed for realizing topologically protected unidirectional surface states. In this article, a microscopic model to investigate the transmission of topological metamaterial grating is constructed based on the scattering processes involving unidirectional surface states. The numerically simulated transmission efficiency of the grating can be precisely reproduced by the model. The model demonstrates that the sharp transmission resonance of the grating results from the constructive interference of the topologically protected one‐way surface states. The present work provides an intuitive picture for understanding the scattering processes and resonance behaviors of the topologically protected one‐way surface states. Benefitting from the sharp spectral features of the supported resonances, the proposed grating structure may be potentially used for sensing applications.

Liquids Unidirectional Transport on Dual-Scale Arrays.

Liquids unidirectional transport has cutting-edge applications ranging from fog collection, oil-water separation, to microfluidic devices. Despite extensive progresses, existing man-made surfaces with asymmetric wettability or micro/nanoscales structures are still limited by complex fabrication techniques or obscure essential transport mechanisms to achieve unidirectional transport with both high speeds and large volumes. Here, we demonstrate the three-dimensional printed micro/macro dual-scale arrays for rapid, spontaneous, and continuous unidirectional transport. We reveal the essential directional transport mechanism via a Laplace pressure driven theory. The relationship between liquid unidirectional transport and surface morphology parameter is systematically explored. Threshold values to achieve unidirectional transport are determined. Significantly, dual-scale arrays even facilitate liquid's uphill running, microfluidics patterning, and liquid shunting in target directions without external energy input. Free combination of dual-scale island arrays modules, just like LEGO bricks, achieves fast liquid transport on demand. This dual-scale island array can be used to build smart laboratory-on-a-chip devices, printable microfluidic integration systems, and advanced biochemistry microreactors.

Riverbed Hydrologic Exchange Dynamics in a Large Regulated River Reach

AbstractHydrologic exchange flux (HEF) is an important hydrologic component in river corridors that includes both bidirectional (hyporheic) and unidirectional (gaining/losing) surface water‐groundwater exchanges. Quantifying HEF rates in a large regulated river is difficult due to the large spatial domains, complexity of geomorphologic features and subsurface properties, and the great stage variations created by dam operations at multiple time scales. In this study, we developed a method that combined numerical modeling and field measurements for estimating HEF rates across the riverbed in a 7 km long reach of the highly regulated Columbia River. A high‐resolution computational fluid dynamics (CFD) modeling framework was developed and validated by field measurements and other modeling results to characterize the HEF dynamics across the riverbed. We found that about 85% of the time from 2008 to 2014 the river was losing water with an annual average net HEF rates across the riverbed (Qz) of −2.3 m3 s−1 (negative indicating downwelling). June was the only month that the river gained water, with monthly averaged Qz of 0.8 m3 s−1. We also found that the daily dam operations increased the hourly gross gaining and losing rate over an average year of 8% and 2%, respectively. By investigating the HEF feedbacks at various time scales, we suggest that the dam operations could reduce the HEF at seasonal time scale by decreasing the seasonal flow variations, while also enhance the HEF at subdaily time scale by generating high‐frequency discharge variations. These changes could generate significant impacts on biogeochemical processes in the hyporheic zone.

Strategic Design and Fabrication of Nerve Guidance Conduits for Peripheral Nerve Regeneration.

Nerve guidance conduits (NGCs) have been drawing considerable attention as an aid to promote regeneration of injured axons across damaged peripheral nerves. Ideally, NGCs should include physical and topographic axon guidance cues embedded as part of their composition. Over the past decades, much progress has been made in the development of NGCs that promote directional axonal regrowth so as to repair severed nerves. This paper briefly reviews the recent designs and fabrication techniques of NGCs for peripheral nerve regeneration. Studies associated with versatile design and preparation of NGCs fabricated with either conventional or rapid prototyping (RP) techniques have been examined and reviewed. The effect of topographic features of the filler material as well as porous structure of NGCs on axonal regeneration has also been examined from the previous studies. While such strategies as macroscale channels, lumen size, groove geometry, use of hydrogel/matrix, and unidirectional freeze-dried surface are seen to promote nerve regeneration, shortcomings such as axonal dispersion and wrong target reinnervation still remain unsolved. On this basis, future research directions are identified and discussed.

Bifunctional gap-plasmon metasurfaces for visible light: polarization-controlled unidirectional surface plasmon excitation and beam steering at normal incidence

Integration of multiple diversified functionalities into a single, planar and ultra-compact device has become an emerging research area with fascinating possibilities for realization of very dense integration and miniaturization in photonics that requires addressing formidable challenges, particularly for operation in the visible range. Here we design, fabricate and experimentally demonstrate bifunctional gap-plasmon metasurfaces for visible light, allowing for simultaneous polarization-controlled unidirectional surface plasmon polariton (SPP) excitation and beam steering at normal incidence. The designed bifunctional metasurfaces, consisting of anisotropic gap-plasmon resonator arrays, produce two different linear phase gradients along the same direction for respective linear polarizations of incident light, resulting in distinctly different functionalities realized by the same metasurface. The proof-of-concept fabricated metasurfaces exhibit efficient (>25% on average) unidirectional (extinction ratio >20 dB) SPP excitation within the wavelength range of 600–650 nm when illuminated with normally incident light polarized in the direction of the phase gradient. At the same time, broadband (580–700 nm) beam steering (30.6°–37.9°) is realized when normally incident light is polarized perpendicularly to the phase gradient direction. The bifunctional metasurfaces developed in this study can enable advanced research and applications related to other distinct functionalities for photonics integration.

Experimental wind tunnel study of a smart sensing skin for condition evaluation of a wind turbine blade

Condition evaluation of wind turbine blades is difficult due to their large size, complex geometry and lack of economic and scalable sensing technologies capable of detecting, localizing, and quantifying faults over a blade’s global area. A solution is to deploy inexpensive large area electronics over strategic areas of the monitored component, analogous to sensing skin. The authors have previously proposed a large area electronic consisting of a soft elastomeric capacitor (SEC). The SEC is highly scalable due to its low cost and ease of fabrication, and can, therefore, be used for monitoring large-scale components. A single SEC is a strain sensor that measures the additive strain over a surface. Recently, its application in a hybrid dense sensor network (HDSN) configuration has been studied, where a network of SECs is augmented with a few off-the-shelf strain gauges to measure boundary conditions and decompose the additive strain to obtain unidirectional surface strain maps. These maps can be analyzed to detect, localize, and quantify faults. In this work, we study the performance of the proposed sensing skin at conducting condition evaluation of a wind turbine blade model in an operational environment. Damage in the form of changing boundary conditions and cuts in the monitored substrate are induced into the blade. An HDSN is deployed onto the interior surface of the substrate, and the blade excited in a wind tunnel. Results demonstrate the capability of the HDSN and associated algorithms to detect, localize, and quantify damage. These results show promise for the future deployment of fully integrated sensing skins deployed inside wind turbine blades for condition evaluation.

Perturbation analysis of subphase gas and meniscus curvature effects for longitudinal flows over superhydrophobic surfaces

Integral expressions for the first-order correction to the effective slip length for longitudinal flows over a unidirectional superhydrophobic surface with rectangular grooves are determined under the assumptions that the meniscus curvature is small and the viscosity contrast between the groove-trapped subphase gas and the working fluid is significant. Both pressure-driven channel flows and semi-infinite shear flows are considered. Reciprocity ideas, based on use of Green’s second identity, provide the integral expressions with integrands dependent on known flat-meniscus solutions found by Philip (Z. Angew. Math. Phys., vol. 23, 1972, pp. 353–372). The results extend earlier work by Sbragaglia & Prosperetti (Phys. Fluids, vol. 19, 2007, 043603) on how weak meniscus curvature affects hydrodynamic slip. In particular, we derive a new integral expression for the first-order slip length correction due to weak meniscus curvature.

Fluid Flow and Mixing Induced by AC Continuous Electrowetting of Liquid Metal Droplet

In this work, we proposed a novel design of a microfluidic mixer utilizing the amplified Marangoni chaotic advection induced by alternating current (AC) continuous electrowetting of a metal droplet situated in electrolyte solution, due to the linear and quadratic voltage-dependence of flow velocity at small or large voltages, respectively. Unlike previous researchers exploiting the unidirectional surface stress with direct current (DC) bias at droplet/medium interface for pumping of electrolytes where the resulting flow rate is linearly proportional to the field intensity, dominance of another kind of dipolar flow pattern caused by local Marangoni stress at the drop surface in a sufficiently intense AC electric field is demonstrated by both theoretical analysis and experimental observation, which exhibits a quadratic growth trend as a function of the applied voltage. The dipolar shear stress merely appears at larger voltages and greatly enhances the mixing performance by inducing chaotic advection between the neighboring laminar flow. The mixer design developed herein, on the basis of amplified Marangoni chaotic advection around a liquid metal droplet at larger AC voltages, has great potential for chemical reaction and microelectromechanical systems (MEMS) actuator applications because of generating high-throughput and excellent mixing performance at the same time.

Robust electrical uni-directional de-icing surface with liquid metal (Ga90In10) and ZnO nano-petal composite coatings

A novel superhydrophobic electro-conductive coating with liquid metal (Ga90In10) and ZnO nano-petals is fabricated by integrating the methods of spraying, crystal growth and low surface energy modification. The results indicate that the paper-based surface could be superhydrophobic in cold environment and successfully realize uni-directional ice driving with the help of thermal gradient. We anticipate that the combination of excellent superhydrophobic performance of the composite structure with inherent conductive advantage of liquid metal will find a vast application prospect.

Corrugation-assisted metal-coated angled fiber facet for wavelength-dependent off-axis directional beaming.

We propose a fiber-optic-plasmonic hybrid device that is based on a corrugation-assisted metal-coated angled fiber facet (CA-MCAFF) for wavelength-dependent off-axis directional beaming (WODB). The device breaks into two key structures: One is the MCAFF structure, which is a modified Kretschmann configuration implemented onto a fiber platform, thereby being able to generate a unidirectional surface plasmon with dramatically enhanced properties in terms of non-confined diffracted radiation loss and operational bandwidth. The other is the periodic corrugation structure put on the MCAFF, thereby enabling WODB functionality out of the whole structures. The corrugated metal surface out-couples the surface plasmon mode to free-space optical radiation into a direction that varies with the wavelength of the optical radiation with excellent linearity. We perform extensive numerical investigations based on the finite-element-method and analyze the out-coupling efficiency (OCEout) and spectral bandwidth (SBout) of the proposed device for various designs and conditions. We determine the seven structural parameters of the device via taking sequential optimization steps. We deduce two optimal conditions particularly for the fiber-facet angle, in terms of the averaged OCEout or the SBout in the whole visible wavelength range (400 - 700 nm), which eventually leads to OCEout = 30.4% and SBout = 230 nm or to OCEout = 24.5% and SBout = 245 nm, respectively. These results suggest substantial enhancements in both OCEout and SBout, in comparison with the performance properties of a typical nano-slit-based device having a similar type of WODB functionality. The proposed CA-MCAFF is a simple, compact and efficient WODB device that is fully compatible with the state-of-the-art optical fiber technology.

Tunable unidirectional surface plasmon polariton launcher utilizing a graphene-based single asymmetric nanoantenna

We design and numerically investigate a graphene-based asymmetric nanoantenna microstructure that can be used to realize electrically controllable, unidirectionally propagating broadband surface plasmon polaritons. The device geometry facilitates the simultaneous excitation of two localized surface plasmons resonances in the whole structure, and consequently, the asymmetric nanoantenna can be considered as being composed of two oscillating magnetic dipoles, wherein the interference of the radiated electromagnetic waves leads to a unidirectional propagation effect. Our results indicate that our proposed active device is promising for realizing compactable, tunable, terahertz plasmonic light sources.

Bioinspired Surfaces: A Novel Bioinspired Continuous Unidirectional Liquid Spreading Surface Structure from the Peristome Surface of Nepenthes alata (Small 4/2017)

A novel unidirectional liquid spreading surface inspired by the N. alata peristome is proposed and fabricated by H. Chen and co-workers in article number 1601676, via two-step UV photolithography. Its superior unidirectional liquid spreading capability is demonstrated, and the underlying mechanism of unidirectional spreading is investigated. This bio-inspired surface will advance a variety of applications such as microfluidic devices and self-lubrication in mechanical engineering.

Uni-directional liquid spreading control on a bio-inspired surface from the peristome of Nepenthes alata

Uni-directional liquid spreading surface structures inspired by the Nepenthes alata peristome and the control of the surface wettability and structural features were investigated.

Topologically Protected Unidirectional Surface States in Biased Ferrites: Duality and Application to Directional Couplers

Topologically Protected Unidirectional Surface States in Biased Ferrites: Duality and Application to Directional Couplers

Plasmonic polarization nano-splitter based on asymmetric optical slot antenna pairs.

We propose and experimentally demonstrate a plasmonic polarization nano-splitter composed of asymmetric optical slot antenna pairs. Broadband polarization-controlled unidirectional surface plasmon polariton (SPP) launching and splitting are achieved experimentally using an asymmetric optical slot antenna pair array. Both transverse-electric and transverse-magnetic-polarized incident light is coupled to SPPs on the metal surface, but with opposite directions. The measured extinction ratio for the two opposite propagating directions is larger than 5dB within a bandwidth of 160nm and reaches up to ∼12 dB at an incident wavelength of 790nm. This plasmonic polarization nano-splitter, together with the polarization-controlled unidirectional SPP coupler, may have promising applications in the nano-optics and integrated optical circuits.

A Novel Bioinspired Continuous Unidirectional Liquid Spreading Surface Structure from the Peristome Surface of Nepenthes alata.

A novel unidirectional liquid spreading surface with an inclined arc pitted groove, inspired by the continuous unidirectional liquid spreading mechanism on the peristome surface of N. alata, is explored and fabricated by two-step UV lithography. Its superior unidirectional liquid spreading capability to that of other surface patterns is demonstrated, and its unidirectional liquid spreading mechanism is investigated.