Air-material Interface Research Articles

Experimental Investigations on the Ferromagnetic Resonance and Absorbing Properties of a Ferrofluid in the Microwave Range

Measurements of complex magnetic permeability, μ(f,H) = μ′(f,H) − iμ″(f,H) and dielectric permittivity ε(f,H) = ε′(f,H) − iε″(f,H), in the frequency range, f of (0.4–7) GHz, and polarizing field, H of (0–135) kA/m, were performed, for a kerosene-based ferrofluid with magnetite nanoparticles. Based on these measurements, the phenomenon of ferromagnetic resonance was highlighted and some microwave propagation parameters of the ferrofluid were determined: the attenuation constant, α(f,H), and the reflection coefficient, R(f,H), at the air-material interface, at the normal incidence. Knowing these parameters we proposed a theoretical model establishing for the first time an equation that allows the calculation of the overall reflection coefficient, Rw(f,H), at the normal incidence of the wave, for a ferrofluid of thickness d, deposited on a totally reflective support, following multiple internal reflections of the electromagnetic wave in the material. The results show that by increasing both, H, and d, the parameter, Rw(f,H) presents a minimum that decreases from 0.90 (for d = 2 mm) to 0.64 (for d = 10 mm), at frequency f = 5 GHz, which indicates an increase in the absorption of the electromagnetic wave by the ferrofluid. The obtained results are useful in the study of some materials that could be used as electromagnetic absorbers in the microwave range, by the determination of the overall reflection coefficient, Rw(f,H), controlled both by the thickness, d, of the absorber and by the external applied field, H.

Role of metal inserts for ‘targeted’, ‘uniform’ or ‘controlled’ microwave heating objectives involving generalized sets of dielectrics (Group 1 – 4)

Microwave heating occurs volumetrically leading to faster processing than conventional heating processes. Over the last few decades, the potential of microwave heating has been investigated for various thermal processing applications such as food processing, material synthesis, fuel production and waste treatment/remediation. Current work investigates the role of the metal inserts towards targeted, uniform and/or controlled microwave heating for the four groups of dielectrics (Group 1 – 4) involving lateral and isotropic microwave incidences. The microwave induced heat generation and evolving temperature distributions within the materials in the absence and presence of the metal insert are numerically determined via the mathematical models comprising of Helmholtz equation, energy balance equation and an integro-differential boundary condition representing the scattering of the microwave at the air-material interface. Galerkin finite element method with biquadratic elements along with Crank-Nicolson time integration scheme has been employed to solve the governing equations. It has been found that the metal insert can suitably alter the heating characteristics leading to either faster heating, better heating uniformity or more intense targeted heating of the materials. Uniform microwave heating is generally difficult to attain except for very small samples. This work shows that the metal inserts can significantly reduce the heating inhomogeneity of the moderately large samples involving Group 1 to Group 3 materials. In addition, the targeted heating effect of the materials (Groups 1 to 4) can also be intensified by the use of the metal insert, as has been illustrated for various test cases. Especially, the metal insert can lead to targeted heating at the incident surface irrespective of the material or sample dimension, and that is difficult to attain except for the large samples. The metal inserts are also found to be efficient to control the rapid microwave heating experienced by the small samples of lossy materials (Group 1 and 3 materials) or accelerate the slower heating of the large samples resulting in significant energy savings, especially for Group 1 to Group 3 materials.

Combining radar and ultrasound imaging for surface echo compensation and augmented visibility of interior structures in NDT applications

Millimeter-wave radar imaging is a promising technique in non-destructive testing. When a small material defect is located closely to the test object’s surface, the strong reflection caused by the air-material interface can overlap with the defect’s weak reflection and mask it. If the image resolution is not sufficient, the defect cannot be resolved and will not be recognizable in the reconstruction image. To solve this problem, surface echo compensation techniques have been invented. However, these techniques are usually restricted to planar surfaces, which for most applications is not the case. This article presents a technique for surface echo compensation of arbitrarily shaped objects. We propose to combine radar and air-coupled ultrasound. Since aircoupled ultrasound is not able to penetrate solids, an air-coupled ultrasound imaging system is only able to detect the surface of a solid object. The idea of our proposed concept is to employ the ultrasound signal to compensate for the surface echo in the radar signal. The proposed method is based on the idea of using signals of equal wavelengths for both wave types and an amplitude calibration. Then, the resulting signals for radar and ultrasound are formally equal, which makes it possible to directly combine them numerically. That way, a modified radar image can be obtained which only contains the reflections from the inside and highlights interior defects.

Magnetically tuning microwave propagation parameters in ferrofluids.

The paper reports on the frequency (f) and static magnetic field (H) dependencies of the microwave propagation parameters, in the ranges 0.1-6 GHz and 0-90.7 kA/m, of a kerosene-based ferrofluid with magnetite particles, filtered in magnetic field gradient. In the investigated range, the sample exhibits ferromagnetic resonance phenomenon and Maxwell-Wagner dielectric relaxation. Unlike the usual way of studying the propagation of microwaves through different media, in this paper we have defined an overall reflection coefficient, Rw(f, H), of a material with thickness, w, deposited on a total reflective support, which takes into account both the attenuation of wave within the material and the reflection at the air-material interface. Based on the measured relative magnetic permeability, [Formula: see text], and relative dielectric permittivity, [Formula: see text], a comprehensive and meaningful set of microwave propagation parameters are determined. Apart from Rw(f, H), this set of parameters of ferrofluid includes the attenuation constant of the electromagnetic wave, [Formula: see text](f, H), the phase constant [Formula: see text](f, H), the real, n'(f, H), and imaginary, n"(f, H), components of the refractive index, the reflection coefficient at the interface air-material, R(f, H), and the quarter wavelength in material, [Formula: see text](f, H). Based on the theoretical considerations and characteristics of ferrofluid, simplified and practical formulas of the propagation parameters are given and also possible applications of the results are suggested (such as electromagnetic absorber, phase shifter, microwave lenses and vibration sensor). This connection between theory and experimental results offers an example for the preliminary design of microwave applications of ferrofluids and, by extension, for any material consisting of magnetic nanoparticles dispersed in a dielectric matrix.

Time-domain simulations of sound propagation in a flow duct with extended-reacting liners

A general methodology to simulate acoustic propagation in ducts with extended-reacting liners in the time domain is presented, including a generic perforated sheet on the air-material interface. The Linearized Euler Equations (LEE) with a mean flow profile are solved in the duct and the linearized equations on an equivalent fluid are solved in the liner material. The auxiliary differential equation method (ADE) is used to prevent the computation of convolution integrals, and leads to a formulation compatible with high-order numerical schemes. The methodology is illustrated for the case of liners consisting of rigid-frame porous materials and a prototype of locally-resonant acoustic metamaterials. A one-dimensional (1D) test case is first used to validate the algorithm and assess the numerical error. The numerical order of the algorithm is the expected one, independently of the interface, as long as the number of poles retained in the partial fraction expansions involved in the formulation is high enough. The algorithm is then applied to a realistic two-dimensional (2D) configuration in a duct with flow, and is used to illustrate the restrained validity of the locally-reacting approximation. Finally, the impact of the flow Mach number on the acoustic performance of porous and metamaterial extended-reacting liners is briefly assessed.

3D Simulation of acoustic waves propagating through porous domain using PUFEM tetrahedral elements

Partition of Unity Finite Element Method is one of the new deterministic prediction techniques developed for tackling medium and high frequency wave problems, because it has the overwhelming advantage over traditional Finite Element Method due to its high efficiency and low computational cost properties. In this paper, PUFEM is further developed to simulate the 3D sound pressure field where porous material is included and modeled as an equivalent homogeneous fluid. Lagrange multipliers are applied at the air and porous material interface. The performance and efficiency of the established numerical model using 3D PUFEM elements are analyzed, it is shown that PUFEM technique can effectively reduce the computational burden while still maintaining the sufficient accuracy for the simulation results of 3D acoustic problems.

A Galerkin finite element based analysis on the microwave heating characteristics of lossy samples in the presence of low and high lossy containers

The coupled electromagnetic wave equations and energy balance equations are solved via Galerkin finite element method with Crank-Nicholson time integration scheme to numerically simulate the microwave heating of model lossy samples enclosed in various low (alumina) and high (SiC) lossy containers. The primary objective of this work is to identify the alteration of heating or power absorption characteristics of the lossy sample in the presence of containers and analyze the role of container thickness. Microwave propagation is modeled via Helmholtz equations, which are the time-harmonic representation of Maxwell equations. The scattering at the air-material interface is efficiently modeled via an integro-differential radiation boundary condition. Power absorption and temperature distribution are evaluated for a range of container thicknesses and sample dimensions representing thin sample with uniform power absorption, intermediate sample with resonating power absorption and thick sample with exponentially attenuated power absorption. For each case, simulations are also carried out in the absence of the containers. It is found that the power absorption and temperature distribution are strongly influenced by both the low (alumina) and high (SiC) lossy containers. Overall, power absorption and/or heating quality can be greatly enhanced in the presence of alumina containers. SiC containers suppress power absorption within the sample. However, SiC containers lead to more controlled and uniform heating. Thickness of the container is found to be the critical factor for enhanced power absorption or heating quality of the sample.

CFD modeling of convective scalar transport in a macroporous material for drying applications

Convective drying is one of the most important forms of moisture removal in macroporous materials exposed to environmental loads. In this study, the influence of airflow on the convective removal of water vapor (passive scalar) from porous asphalt (PA), a macroporous material, is investigated for different wind speeds and levels of liquid water height within the material, using computational fluid dynamics (CFD). The PA domain for the CFD simulations is reconstructed from three-dimensional X-ray micro-computed tomography images. Steady Reynolds-averaged Navier-Stokes (RANS) k-ε model with low Reynolds number modeling for the near-wall region is used for the turbulence modeling. Simulations are carried out for three wind speeds, 0.1, 1.5 and 10 m/s, and three depths of the scalar (vapor) source, 0.13, 15 and 29 mm, within the specimen. Only the highest wind speed has a clear influence on the total vapor flux measured at the interface. Additionally, at the highest wind speed, air entrainment in pores near the air-material interface plays an important role in vapor removal. At lower wind speeds, diffusion within the material is the dominant vapor transport mechanism. At a wind speed of 10 m/s, strong vortices are observed which inhibit vapor removal in cavity-type pores by recirculating vapor within the pore. However, at the same wind speed, vapor transport in a non-cavity-type pore system is found to be very strong due to strong air convection.

Convective drying of fruit: A deeper look at the air-material interface by conjugate modeling

A better physical understanding but also prediction of convective drying processes of fruit is essential for further process optimization. This study uses validated conjugate modeling to gain insight in how fruit drying kinetics are related to the convective heat and mass exchange with the surrounding turbulent airflow via the fruit surface. Conjugate modeling implies that the heat and mass transport in both air and fruit domains are solved simultaneously. We explore the impact of several model assumptions and different convective drying conditions. The conjugate model is inherently more accurate than the use of constant convective transfer coefficients (CTCs), so the non-conjugate approach. However the gain in accuracy was found to be limited in terms of overall fruit drying kinetics, such as total mass loss. Nevertheless, conjugate modeling allowed to identify spatial and temporal variability in CTCs, which locally affected drying rates and internal moisture content distribution. Thereby, we identified the occurrence of negative convective transfer coefficients, which led to rehydration at specific locations on the fruit surface, due to the surrounding high-humidity microclimate. The ability to identify the direct relation between non-uniformities in the airflow to those in the tissue is a unique trait of the conjugate approach. Furthermore, it was shown that isothermal modeling should not be used, even for near-isothermal conditions such as low-temperature drying, and that including thermal radiation exchange with the environment clearly affected the drying rates. Regarding the drying conditions, the impact of the air speed and approach flow temperature was found to be smaller compared to altering the approach flow humidity. When direct solar radiation was present, the presence of airflow provided significant cooling of the fruit, which is beneficial for preserving heat-sensitive nutritional compounds in the fruit, and also enhanced the drying rate. This study will aid drying technologists to define the required complexity of their model.

Universal threshold for femtosecond laser ablation with oblique illumination

We quantify the dependence of the single-shot ablation threshold on the angle of incidence and polarization of a femtosecond laser beam, for three dissimilar solid-state materials: a metal, a dielectric, and a semiconductor. Using the constant, linear value of the index of refraction, we calculate the laser fluence transmitted through the air-material interface at the point of ablation threshold. We show that, in spite of the highly nonlinear ionization dynamics involved in the ablation process, the so defined transmitted threshold fluence is universally independent of the angle of incidence and polarization of the laser beam for all three material types. We suggest that angular dependence of ablation threshold can be utilized for profiling fluence distributions in ultra-intense femtosecond laser beams.

Turbulent airflow above a full-scale macroporous material: Boundary layer characterization and conditional statistical analysis

Convective drying of macroporous materials is governed by the complex interaction between airflow above and into the material, the roughness of the air–material interface and the characteristics of the material pore system. In this study, we experimentally investigate this interplay in detail and at full-scale, using porous asphalt (PA) as a model material. The characteristics of the turbulent flow in the immediate vicinity of the material surface are studied with full-scale wind tunnel experiments at three flow speeds over two types of PA with different surface porosities and surface pore sizes, and are compared to similar measurements over a smooth and impermeable reference material. It is shown that, above a certain wall-normal distance, turbulence profiles can be scaled to make them independent of the flow speed. However, at low speed, the scaling breaks down due to a combination of organized turbulent structures of high intensity and a low turbulence background. No generally valid scaling applicable at all tested air speeds is found close to the surface, where drying occurs. Hence, realistic drying experiments must be performed at full scale and for the entire range of velocities of interest.

Modeling ozone removal to indoor materials, including the effects of porosity, pore diameter, and thickness.

We develop an ozone transport and reaction model to determine reaction probabilities and assess the importance of physical properties such as porosity, pore diameter, and material thickness on reactive uptake of ozone to five materials. The one-dimensional model accounts for molecular diffusion from bulk air to the air-material interface, reaction at the interface, and diffusive transport and reaction through material pore volumes. Material-ozone reaction probabilities that account for internal transport and internal pore area, γ(ipa), are determined by a minimization of residuals between predicted and experimentally derived ozone concentrations. Values of γ(ipa) are generally less than effective reaction probabilities (γ(eff)) determined previously, likely because of the inclusion of diffusion into substrates and reaction with internal surface area (rather than the use of the horizontally projected external material areas). Estimates of γ(ipa) average 1 × 10(-7), 2 × 10(-7), 4 × 10(-5), 2 × 10(-5), and 4 × 10(-7) for two types of cellulose paper, pervious pavement, Portland cement concrete, and an activated carbon cloth, respectively. The transport and reaction model developed here accounts for observed differences in ozone removal to varying thicknesses of the cellulose paper, and estimates a near constant γ(ipa) as material thickness increases from 0.02 to 0.16 cm.

Condition for Perfect Resonant Antireflection

ABSTRACTReflection occurs at an air-material interface. The development of antireflection schemes, which aims to cancel such reflection, is important for a wide variety of applications including solar cells and photodetectors. Recently, it has been demonstrated that a periodic array of resonant subwavelength objects placed at an air-material interface can significantly reduce reflection that otherwise would have occurred at such an interface. Here, we introduce the theoretical condition for complete reflection cancellation in this resonant antireflection scheme. Using both general theoretical arguments and analytical temporal coupled-mode theory formalisms, we show that in order to achieve perfect resonant antireflection, the periodicity of the array needs to be smaller than the free-space wavelength of the incident light for normal incidence, and also the resonances in the subwavelength objects need to radiate into air and the dielectric material in a balanced fashion. Our theory is validated using first-principles full-field electromagnetic simulations of structures operating in the infrared wavelength ranges. For solar cell or photodetector applications, resonant antireflection has the potential of providing a low-cost technique for antireflection that does not require nanofabrication into the absorber materials, which may introduce detrimental effects such as additional surface recombination. Our work here provides theoretical guidance for the practical design of such resonant antireflection schemes.

Condition for perfect antireflection by optical resonance at material interface

Reflection occurs at an air–material interface. The development of antireflection schemes, which aims to cancel such reflection, is important for a wide variety of applications including solar cells and photodetectors. Recently, it has been demonstrated that a periodic array of resonant subwavelength objects placed at an air–material interface can significantly reduce reflection that otherwise would have occurred at such an interface. Here, we introduce the theoretical condition for complete reflection cancellation in this resonant antireflection scheme. Using both general theoretical arguments and analytical temporal coupled-mode theory formalisms, we show that in order to achieve perfect resonant antireflection, the periodicity of the array needs to be smaller than the free-space wavelength of the incident light for normal incidence, and also the resonances in the subwavelength objects need to radiate into air and the dielectric material in a balanced fashion. Our theory is validated using first-principles full-field electromagnetic simulations of structures operating in the infrared wavelength ranges. For solar cell or photodetector applications, resonant antireflection has the potential for providing a low-cost technique for antireflection that does not require nanofabrication into the absorber materials, which may introduce detrimental effects such as additional surface recombination. Our work here provides theoretical guidance for the practical design of such resonant antireflection schemes.

Novel Application of Neutron Radiography to Forced Convective Drying of Fruit Tissue

Neutron imaging is a promising technique to study drying processes in food engineering as it is a non-intrusive, non-destructive technique, which provides quasi-real-time quantitative information of the water loss during drying and of the internal water distribution, at a high spatial and dynamic resolution. Particularly, the high sensitivity to water is its main advantage for drying studies, despite the limited accessibility to reactor facilities, which produce neutrons. This technique was used to investigate forced convective drying of fruit tissue (pear and apple), placed in a small wind tunnel. Water loss, water distribution in the sample and sample shrinkage were evaluated as a function of time. The water loss, determined quantitatively from neutron radiographs, was underestimated slightly compared to gravimetrical measurements. The overall drying behaviour agreed well with control measurements performed in a climatic chamber and was very similar for both fruit tissues. The corresponding shrinkage behaviour of both tissues was also similar. The large shrinkage, which is characteristic for soft biological materials such as food products, however, hindered post-processing to some extent. From the internal water distribution, the water gradients within the sample, induced by drying, were visualised and were found to predominantly occur at the air–material interface, indicating that the water transport inside the tissue dominated the water loss, instead of the convective exchange with the air flow. Neutron imaging was shown to exhibit unique benefits for studying drying processes of food.

Highly efficient broadband second harmonic generation in a tapered zinc telluride slab using total internal reflection fractional quasi phase matching

We analytically describe broadband second-harmonic generation in a tapered zinc telluride (ZnTe) slab using total internal reflection fractional quasi-phase matching. Refractive index of ZnTe shows minimum deviation over a wide wavelength range resulting in increased coherence length which leads to fractional quasi-phase matching scenario, ensuring highly efficient second harmonic output. The simulated results, considering absorption and reflection losses, indicate 3dB bandwidth of 192.5nm, centered at 8.583μm, with conversion efficiency as high as 19.6% in a 30mm ZnTe slab. The effects of variations in incident angle at air–material interface, length and tapering angle of the slab have also been analyzed.

Performances of the Partition of Unity Finite Element Method for the analysis of two-dimensional interior sound fields with absorbing materials

The paper deals with the numerical simulation of the acoustic field in two-dimensional cavities in which absorbing materials are present. Though Finite Element Method (FEM) could be employed for this purpose, the discretization level required for achieving reasonable accuracy renders the method impractical in the mid-frequency range. To alleviate this limitation, the Partition of Unity Finite Element Method (PUFEM) using plane wave functions has been shown to be very effective for solving short wave Helmholtz problems. In the present work, the method is extended to the computation of the pressure wave field within the absorbing media which is modeled as a bulk-reacting material characterized by a complex-valued and frequency dependent mean density and dynamic compressibility. Lagrange multipliers are used to enforce the transmission conditions at the air–material interface. Performances of the PUFEM are compared with a standard FEM in several examples of practical interests. It is shown that the technique is a good candidate for solving noise control problems at high frequency.

A method for analyzing the microwave absorption properties of magnetic materials

Based on transmission line theory, a complex thickness is introduced to analyze the electromagnetic wave absorption properties of materials with high magnetic loss and low dielectric loss. At each frequency the real part of the complex thickness denotes the thickness of the absorber to reach maximum absorption, while the absolute value of the imaginary part of the normalized complex thickness represents the degree of impedance mismatch between the material and free space. Using this method, the intensity of the reflection loss and the corresponding material thickness can be well predicted and understood. This method describes well the microwave absorption behaviors of PANI/α-Fe and PANI/CIP/Fe3O4 composites. It is found that the electromagnetic energy loss in the material, as well as the interference cancellation of the reflected waves at the air-material interface, contributes to the global minimum of reflection loss for the two composites.

Attractive microwave absorption and the impedance match effect in zinc oxide and carbonyl iron composite

The flower-like ZnO and ZnO/carbonyl-iron composite have been prepared by a sonochemical route and ball-milling process, respectively. For ZnO/carbonyl-iron composite, a reflection loss ( RL) exceeding −20 dB was obtained in a broad frequency range of 8.4–17.9 GHz with a thin thickness of 1.2–2.3 mm. An optimal RL of −61 dB was found at 11.7 GHz for an absorber thickness of 1.91 mm. It is demonstrated that the attractive microwave-absorption properties are a consequence of a proper electro-magnetic impedance match and geometrical cancellation at the air–material interface. In addition, an impedance mismatch function was proposed, which provides an effective method to determine the microwave absorbing properties from the intrinsic materials constants. The calculated value of matching frequency and thickness is well consistent with the experimental data. The method also provides a simple theoretical graphic aid for determining the absorption characteristics and the location of the matching conditions in the frequency domain.

Color selectivity lent to a cholesteric liquid crystal by monitoring interface-induced deformations

The cholesteric liquid crystalline structure is omnipresent in living matter and concerns many applications in optics because of its property of selective light reflection. The color reflected by this structure depends on material parameters such as the molecular chirality or the concentration of chiral dopant, the helical pitch of the twisted structure and the optical indices. In the present publication, we show that the color may be selected simply by varying the annealing time of a cholesteric oligomer film with hybrid anchoring. Experimental and simulated transmittance spectra reveal the color shift. The three-dimensional representation of the structure is provided from the macroscopic scale to the mesoscale by combining complementary imaging techniques (optical microscopy, transmission and scanning electron microscopies, atomic force microscopy) to image the texture in different directions. We show that the color selectivity is due to controlled changes of the orientation of the helical axis with respect to the air–material interface. The optical behavior is described with a single parameter, which is the angle between the film surface normal and the orientation within the film of the helical axis. Potential applications in the field of chiro-optical devices concern self-assembled chiral microreflectors fabricated via a low-cost controllable thermal process. Additionally, the texture formation provides insight to aid the understanding of the exoskeleton morphogenesis of beetles like Chrysina Gloriosa.