Desired Gradient Research Articles

Experimental realization of all-angle negative refraction in acoustic gradient metasurface

In this work, we numerically and experimentally demonstrate that all-angle negative refraction can be obtained with the acoustic gradient metasurface of subwavelength thickness. The coiling labyrinthine structures are utilized to build the desired gradient metasurface, and the apparent negative refraction occurring beyond the critical incident angle has been validated by simulations and experimental measurements, which agrees well with the theoretical predictions given by the revised generalized law of refraction while taking the contribution of the Bragg scattering into account. This work provides the solution to manipulate the acoustic waves and shows good promise in building functional diffractive acoustic elements.

Numerical modelling framework of continuous salt precipitation from super-critical water

ABSTRACTSalt separation at super-critical condition is a promising technology to separate dissolved salts from water by utilizing sharp changes in thermal and physical properties of water close to its critical point in a tube in tube separator. To capture flow complexity and geometric asymmetry, a three-dimensional CFD model of salt separator is developed in Fluent ver 16. Simulation results are compared and validated with experimental work by Schubert et al. [19]. The axial temperature profiles predicted by model at different wall temperature are well in agreement with the reported data [19]. The model provides insight to axial and radial flow field, temperature gradients, and so on within the salt separator. The blurred boundary between super-critical and sub-critical regions is captured by accounting sharp changes in physical properties of water close to critical temperature and pressure. Sensitivity of key process parameters (e.g., vessel wall temperature, feed pre-heat temperature, flow rates and forced cooling in cold region) was carried out to check effect of operating parameters on deviation in performance of salt separator. No pre-heat feed condition (25°C) is best since it ensures no salt deposition in dip tube without affecting the salt separator performance. Optimum wall temperature lies between 390°C to 470°C to avoid salt deposition and maintain desired temperature gradient between hot and cold section. The modelling framework will aid in efficient design and scale up of salt separator.

Direct numerical simulation of a self-similar adverse pressure gradient turbulent boundary layer at the verge of separation

The statistical properties are presented for the direct numerical simulation of a self-similar adverse pressure gradient (APG) turbulent boundary layer (TBL) at the verge of separation. The APG TBL has a momentum thickness-based Reynolds number range from $Re_{\unicode[STIX]{x1D6FF}_{2}}=570$ to 13 800, with a self-similar region from $Re_{\unicode[STIX]{x1D6FF}_{2}}=10\,000$ to 12 300. Within this domain the average non-dimensional pressure gradient parameter $\unicode[STIX]{x1D6FD}=39$, where for a unit density $\unicode[STIX]{x1D6FD}=\unicode[STIX]{x1D6FF}_{1}P_{\!e}^{\prime }/\unicode[STIX]{x1D70F}_{w}$, with $\unicode[STIX]{x1D6FF}_{1}$ the displacement thickness, $\unicode[STIX]{x1D70F}_{w}$ the mean shear stress at the wall and $P_{\!e}^{\prime }$ the far-field pressure gradient. This flow is compared with previous zero pressure gradient and mild APG TBL ($\unicode[STIX]{x1D6FD}=1$) results of similar Reynolds number. All flows are generated via the direct numerical simulation of a TBL on a flat surface with far-field boundary conditions tailored to apply the desired pressure gradient. The conditions for self-similarity, and the appropriate length and velocity scales, are derived. The mean and Reynolds stress profiles are shown to collapse when non-dimensionalised on the basis of these length and velocity scales. As the pressure gradient increases, the extent of the wake region in the mean streamwise velocity profiles increases, whilst the extent of the log-layer and viscous sublayer decreases. The Reynolds stress, production and dissipation profiles of the APG TBL cases exhibit a second outer peak, which becomes more pronounced and more spatially localised with increasing pressure gradient. This outer peak is located at the point of inflection of the mean velocity profiles, and is suggestive of the presence of a shear flow instability. The maximum streamwise velocity variance is located at a wall normal position of $\unicode[STIX]{x1D6FF}_{1}$ of spanwise wavelength of $2\unicode[STIX]{x1D6FF}_{1}$. In summary as the pressure gradient increases the flow has properties less like a zero pressure gradient TBL and more akin to a free shear layer.

Thermal optimization of the continuous casting process using distributed parameter identification approach—controlling the curvature of solid-liquid interface

Thermal optimization of the vertical continuous casting process is considered in the present study. The goal is to find the optimal distribution of the temperature and interfacial heat transfer coefficients corresponding to the primary and secondary cooling systems, in addition to the pulling speed, such that the solidification along the main axis of strand approaches to the unidirectional solidification mode. Unlike many thermal optimization of phase change problems in which the desirable (target) temperature, temperature gradient, or interface position are assumed to be a priori known, a desirable shape feature of the freezing interface (not its explicit position) is assumed to be known in the present study. In fact, the goal is equivalent to attain a nearly flat solid-liquid interface that is featured by its zero mean curvature. The objective functional is defined as a function of the temperature and mean curvature of the freezing interface. The solidus and liquidus iso-contours of the temperature field are used to implicitly determine the solid-liquid mushy region, i.e., the freezing interface. The temperature field is computed by solving a quasi steady-state nonlinear heat conduction equation. A smoothing method is employed to ensure the differentiability of heat equation. The explicit form of first-order necessary optimality conditions is derived. A gradient descent algorithm is introduced to find local solutions of the corresponding optimization problem. Numerical results support the feasibility and success of the presented method.

Generating 2-dimensional concentration gradients of biomolecules using a simple microfluidic design.

This study reports a microfluidic device for generating 2-dimensional concentration gradients of biomolecules along the width and length of a chamber and conventional 1-dimensional gradients along the width of its lateral parallel channels. The gradient profile can be precisely controlled by the applied flow rate. The proposed design is simple and straightforward, has a small footprint size compared to previously reported devices such as tree-shape designs, and for the first time, provides capability of generating desired 2D and 1D gradients, simultaneously. The finite element simulation analysis proves the feasibility of the microfluidic device, and the fluorescently labelled IgG antibody is used to demonstrate generated chemical gradients. This simple microfluidic device can be implemented for a wide range of high-throughput concentration gradient applications such as chemotaxis, drug screening, and organs-on-chips.

Development of a simple static microwell array with uniform cell seeding and a chemical concentration gradient

For personalized screening and therapeutic inventions across many diseases, the drug–dose response for an individual patient is a major unmet need. In this work, we applied a direct strategy to generate a static microwell array for cell culture with uniform cell seeding and to create the desired chemical concentration gradient into this cell array. It is a simple, novel and easily operable device for a high-throughput drug–dose response experiment that complies with the procedures of cell-based drug screening. Only two repeated operating steps suffice for the entire cell-based drug test—an injection of cells or drug into the flow channel and then injection of air flow into the flow channel. This device comprises two PDMS layers: one side forms an air chamber; the other side forms the liquid channel with embedded cavities as the cells culture and reaction region. The concentration of the drug decreases exponentially along the microwell array; the range of the concentration gradient is varied with the rate of air flow, the volume of the drug plug and the initial concentration of the drug. Small variations of concentration were accessed across varied ranges; the IC50 value of DOX in the MDA-MB-231 cell line was thus utilized on this device, precisely and quickly. The IC50 value calculated in this work is consistent with the range published elsewhere. With this device, hundreds of data points per compound drug screening can be tested in one experiment, which will be an essential key to determine customized drug dosage and to make possible personalized medicine.

Optimization of multi-response dynamic systems integrating multiple regression and Taguchi’s dynamic signal-to-noise ratio concept

The principal difference between a dynamic and a static system is that the former includes a signal factor for expressing the intended output while the later does not. Assuming a linear association exists between the response and signal variables, Taguchi offered a two-stage route for optimizing a dynamic system: maximize the dynamic signal-to noise ratio (DSN) and then, change the gradient to the desired gradient by a suitable modification parameter. Some researchers have indicated limitations to Taguchi’s DSN analysis, and advocated alternative approaches for optimization of a dynamic system. However, the Taguchi method as well as these alternative approaches is useful for optimizing a single-response dynamic system only. In realism, the majority of the contemporary manufacturing practices encompass numerous response variables as well as industries demand for developing procedures for optimizing multi-response dynamic system. This paper proposes a novel procedure that integrates multiple regression (MR) technique and Taguchi’s DSN concept to optimize the multi-response dynamic system. In this method, appropriate multiple regression equations according to a chosen model for dynamic system are fitted first based on the observed experimental data and then DSN (called MRDSN) for different response variables are computed using the MRbased predicted values. Finally, weighted MRDSN is considered as the objective function for the optimization. The proposed procedure is investigated with respect to three modelling approaches for the dynamic systems. The results of analysis reveal that the proposed procedure with response modelling approach results in the best optimization performance. It also results in better optimization performance than back-propagation neural network-based approach and data mining-based approach reported by the past researchers.Keywords: multiple responses, multiple regression, weighted dynamic signal-to-noise ratio, performance measure modelling, response function modelling, response modelling, optimization

Restoration for Out-of-Focus Color Image Based on Gradient Profile Sharpness

In color images, out-of-focus problems often occur when different wavelengths of rays are focused at different positions in the focal plane. This occurs because of lenses that have different refractive indices for different wavelengths of light. These color images in turn become blurred, and noticeable colored edges appear around objects. These misaligned edges thus degrade the overall quality of the images. In this study, we propose a restoration algorithm for misaligned edges in color images. This algorithm is based on the assumption that the gradients of color channels are highly correlated such that all edges spatially overlap in the same manner as the desired gradient. The constraint term in least squares optimization is proposed to align edges to match the desired gradient based on the transformation theory of gradient profile sharpness. The proposed constraint term adaptively uses the gradients of the channels with different weights to estimate sharp edges. We also design a new measurement to compute the energy of aligned edges in a color image. The proposed algorithm can be applied to images captured by various sensors in different environments. Experimental results show that the proposed algorithm performs effectively when estimating high-quality color images.

3D printing for the design and fabrication of polymer-based gradient scaffolds.

Additive manufacturing techniques have given tissue engineers the ability to precisely recapitulate the native architecture present within tissue. In addition, these techniques can be leveraged to create scaffolds with both physical and chemical gradients. This work offers insight into several techniques that can be used to generate graded scaffolds, depending on the desired gradient. Furthermore, it outlines methods to determine if the designed gradient was achieved. This review will help to condense the abundance of information that has been published on the creation and characterization of gradient scaffolds and to provide a single review discussing both methods for manufacturing gradient scaffolds and evaluating the establishment of a gradient.

Polydimethylsiloxane-polycarbonate Microfluidic Devices for Cell Migration Studies Under Perpendicular Chemical and Oxygen Gradients

This paper reports a microfluidic device made of polydimethylsiloxane (PDMS) with an embedded polycarbonate (PC) thin film to study cell migration under combinations of chemical and oxygen gradients. Both chemical and oxygen gradients can greatly affect cell migration in vivo; however, due to technical limitations, very little research has been performed to investigate their effects in vitro. The device developed in this research takes advantage of a series of serpentine-shaped channels to generate the desired chemical gradients and exploits a spatially confined chemical reaction method for oxygen gradient generation. The directions of the chemical and oxygen gradients are perpendicular to each other to enable straightforward migration result interpretation. In order to efficiently generate the oxygen gradients with minimal chemical consumption, the embedded PC thin film is utilized as a gas diffusion barrier. The developed microfluidic device can be actuated by syringe pumps and placed into a conventional cell incubator during cell migration experiments to allow for setup simplification and optimized cell culture conditions. In cell experiments, we used the device to study migrations of adenocarcinomic human alveolar basal epithelial cells, A549, under combinations of chemokine (stromal cell-derived factor, SDF-1α) and oxygen gradients. The experimental results show that the device can stably generate perpendicular chemokine and oxygen gradients and is compatible with cells. The migration study results indicate that oxygen gradients may play an essential role in guiding cell migration, and cellular behavior under combinations of gradients cannot be predicted from those under single gradients. The device provides a powerful and practical tool for researchers to study interactions between chemical and oxygen gradients in cell culture, which can promote better cell migration studies in more in vivo-like microenvironments.

Polydimethylsiloxane-polycarbonate Microfluidic Devices for Cell Migration Studies Under Perpendicular Chemical and Oxygen Gradients.

This paper reports a microfluidic device made of polydimethylsiloxane (PDMS) with an embedded polycarbonate (PC) thin film to study cell migration under combinations of chemical and oxygen gradients. Both chemical and oxygen gradients can greatly affect cell migration in vivo; however, due to technical limitations, very little research has been performed to investigate their effects in vitro. The device developed in this research takes advantage of a series of serpentine-shaped channels to generate the desired chemical gradients and exploits a spatially confined chemical reaction method for oxygen gradient generation. The directions of the chemical and oxygen gradients are perpendicular to each other to enable straightforward migration result interpretation. In order to efficiently generate the oxygen gradients with minimal chemical consumption, the embedded PC thin film is utilized as a gas diffusion barrier. The developed microfluidic device can be actuated by syringe pumps and placed into a conventional cell incubator during cell migration experiments to allow for setup simplification and optimized cell culture conditions. In cell experiments, we used the device to study migrations of adenocarcinomic human alveolar basal epithelial cells, A549, under combinations of chemokine (stromal cell-derived factor, SDF-1α) and oxygen gradients. The experimental results show that the device can stably generate perpendicular chemokine and oxygen gradients and is compatible with cells. The migration study results indicate that oxygen gradients may play an essential role in guiding cell migration, and cellular behavior under combinations of gradients cannot be predicted from those under single gradients. The device provides a powerful and practical tool for researchers to study interactions between chemical and oxygen gradients in cell culture, which can promote better cell migration studies in more in vivo-like microenvironments.

New proposed methodology for specific heat capacity determination of materials for thermal energy storage (TES) by DSC

This study presents a methodology to determine the specific heat capacity (Cp) of materials for thermal energy storage (TES) by DSC. These materials have great energy storage capacities, and due to that, important heat flow fluctuations can be observed for each temperature differential, taking more time to reach a desired temperature gradient. Three different DSC methods are considered to be applied in the methodology, and are explained and compared in this study in order to select the most proper one for Cp determination. To perform this study, the Cp of three materials commonly used in sensible TES systems, slate, water, and potassium nitrate (KNO3), is determined. Excellent results with errors lower than 3% are obtained when using the proposed methodology with the areas method. Worse results are obtained with both dynamic and isostep methods, with errors up to 6% and 16% respectively, as a consequence of sensitivity problems during the measurements.

The underdilation of nitinol stents at TIPS implantation: Solution or illusion?

PurposeThis study investigates the behaviour of self-expanding nitinol stents at the time of TIPS-implantation and thereafter. MethodsHundred consecutive patients with cirrhosis receiving a TIPS revision were included. The smallest stent diameter was measured radiologically immediately after implantation and before shunt revision. Accuracy of the measurement was assessed by comparing the nominal stent diameter with the largest stent diameter measured at the time of revision. ResultsPearson correlation between largest measured and nominal diameters was excellent (r=0.952, p<0.001) showing that measurements are accurate. At TIPS implantation all stents were markedly underdilated reaching only 76–92% of their nominal diameter. Smallest measured diameters were similar (8mm) irrespective of the nominal diameter (8, 9, 10mm) of the stent. In addition, smallest diameters of 10mm stents were similar irrespective whether 8, 9 or 10mm balloons were used.During a mean follow-up of 12.7±17.8months (median 3 months, range 1–81) stents expanded by 0.5–1.6mm dependent on the nominal stent size (8, 9, 10mm) and the grade of primary underdilation. No significant difference was found between Viatorr and bare stents. ConclusionsAt TIPS-implantation, the compliance of the surrounding tissue predominantly determines the stent diameter. The nominal size of the stent or the dilatation balloon has little influence. Accurate adjustment of a desired pressure gradient is, therefore, not possible. During follow-up, stents expand towards their nominal diameter questioning the usefulness of underdilation.

A Magneto-Microfluidic System for Investigating the Influence of an Externally Induced Force Gradient in a Collagen Type I ECM on HMVEC Sprouting.

Advances in mechanobiology have suggested that physiological and pathological angiogenesis may be differentiated based on the ways in which the cells interact with the extracellular matrix (ECM) that exhibits partially different mechanical properties. This warrants investigating the regulation of ECM stiffness on cell behavior using angiogenesis assays. In this article, we report the application of the technique of active manipulation of ECM stiffness to study in vitro angiogenic sprouting of human microvascular endothelial cells (HMVECs) in a microfluidic device. Magnetic beads were embedded in the ECM through bioconjugation (between the streptavidin-coated beads and collagen fibers) in order to create a pretension in the ECM when under the influence of an external magnetic field. The advantage of using this magneto-microfluidic system is that the resulting change in the local deformability of the collagen fibers is only apparent to a cell at the pericellular level near the site of an embedded bead, while the global intrinsic material properties of the ECM remain unchanged. The results demonstrate that this system represents an effective tool for inducing noninvasively an external force on cells through the ECM, and suggest the possibility of creating desired stiffness gradients in the ECM for manipulating cell behavior in vitro.

Thermal Vacuum Evaluation of Simulated Spacecraft Radiators with Discretized Emissivity Surface Properties

Incorporating variable emissivity electrochromic devices onto spacecraft radiators offers a unique potential for modulating passive heat rejection, but implementation challenges exist. Although high-/low-end states of these devices are readily achievable, uncertainties remain with reproducibility at intermediate states and also with stability in a thermal vacuum environment. An average intermediate heat rejection rate can be obtained by discretizing the radiator surface into mixed high- and low-emissivity values. We emulated this capability by constructing test articles with two fixed-emissivity states divided into four sections. Coupons were constructed from aluminum 6061 and stainless steel 304 to characterize performance variations resulting from differences in thermal conductivity. Tests were devised to maintain either a constant surface temperature or constant heat flux. The results showed that this approach provided a predictable area-averaged intermediate emissivity regardless of the material’s thermal conductivity or the heat rejection scheme used. When operating in a constant flux mode, however, the stainless steel coupon experienced larger lateral temperature gradients across the surface due to its lower thermal conductivity. Performance of a variable emissivity radiator must, therefore, take into account not only surface properties, but also consider the desired system heat rejection control scheme and localized thermal gradient tolerance.

Full-angle negative reflection realized by a gradient acoustic metasurface

We theoretically demonstrate that full-angle negative reflection can be realized by the gradient acoustic metasurface with a specific surface phase gradient value. A straightforward physical picture is presented here to understand such anomalous phenomena by considering the influence of the non-local effect that originates from the supercell periodicity on the gradient metasurface. Basing on the generalized law of reflection which is modified by a reciprocal lattice vector term, the negative reflection that beyond the critical angle is possible. In this paper, we utilize the coiling-up space structures of deep subwavelength geometrical scale to construct the desired gradient acoustic metasurface and observe the apparent full-angle negative reflection phenomenon. The present work enriches the content of the generalized law of reflection and provide new design methodology for functional acoustic wave modulation devices, such like directional ground acoustic cloaking and acoustic isolation devices.

The coil array method for creating a dynamic imaging volume.

Gradient strength and speed are limited by peripheral nerve stimulation (PNS) thresholds. The coil array method allows the gradient field to be moved across the imaging area. This can help reduce PNS and provide faster imaging for image-guided therapy systems such as the magnetic resonance imaging-guided linear accelerator (MRI-linac). The coil array is designed such that many coils produce magnetic fields, which combine to give the desired gradient profile. The design of the coil array uses two methods: either the singular value decomposition (SVD) of a set of field profiles or the electromagnetic modes of the coil surface. Two whole-body coils and one experimental coil were designed to investigate the method. The field produced by the experimental coil was compared to simulated results. The experimental coil region of uniformity (ROU) was moved along the z axis as shown in simulation. The highest observed field deviation was 16.9% at the edge of the ROU with a shift of 35 mm. The whole-body coils showed a median field deviation across all offsets below 5% with an eight-coil basis when using the SVD design method. Experimental results show the feasibility of a moving imaging region within an MRI with a low number of coils in the array. Magn Reson Med 78:784-793, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

A disposable, continuous-flow polymerase chain reaction device: design, fabrication and evaluation

Polymerase Chain Reaction (PCR) is used to amplify a specific segment of DNA through a thermal cycling protocol. The PCR industry is shifting its focus away from macro-scale systems and towards micro-scale devices because: micro-scale sample sizes require less blood from patients, total reaction times are on the order of minutes opposed to hours, and there are cost advantages as many microfluidic devices are manufactured from inexpensive polymers. Some of the fastest PCR devices use continuous flow, but they have all been built of silicon or glass to allow sufficient heat transfer. This article presents a disposable polycarbonate (PC) device that is capable of achieving real-time, continuous flow PCR in a completely disposable polymer device in less than 13 minutes by thermally cycling the sample through an established temperature gradient in a serpentine channel. The desired temperature gradient was determined through simulations and validated by experiments which showed that PCR was achieved. Practical demonstration included amplification of foot-and-mouth disease virus (FMDV) derived cDNA.

Direct numerical simulation of a self-similar adverse pressure gradient turbulent boundary layer

The statistical properties of a self-similar adverse pressure gradient (APG) turbulent boundary layer (TBL) are presented. The flow is generated via the direct numerical simulation of a TBL on a flat surface with a farfield boundary condition designed to apply the desired pressure gradient. The conditions for self-similarity and appropriate scaling are derived, with the mean profiles, Reynolds stress profiles, and turbulent kinetic energy budgets non-dimensionalised using this scaling. The APG TBL has a momentum thickness based Reynolds number range from Reδ2=300 to 6000, with a self-similar region spanning a Reynolds number range from Reδ2=3500 to 4800. Within this range the non-dimensional pressure gradient parameter β=1. Two-point correlations of each of the velocity components in the streamwise/wall-normal plane are also presented, which illustrate the statistical imprint of the structures in this plane for the APG TBL.

Study on transient beam loading compensation for China ADS proton linac injector II**Supported by National Natural Science Foundation of China (91426303, 11525523)

Significant transient beam loading effects were observed during beam commissioning tests of prototype II of the injector for the accelerator driven sub-critical (ADS) system, which took place at the Institute of Modern Physics, Chinese Academy of Sciences, between October and December 2014. During these tests experiments were performed with continuous wave (CW) operation of the cavities with pulsed beam current, and the system was configured to make use of a prototype digital low level radio frequency (LLRF) controller. The system was originally operated in pulsed mode with a simple proportional plus integral and deviation (PID) feedback control algorithm, which was not able to maintain the desired gradient regulation during pulsed 10 mA beam operations. A unique simple transient beam loading compensation method which made use of a combination of proportional and integral (PI) feedback and feedforward control algorithm was implemented in order to significantly reduce the beam induced transient effect in the cavity gradients. The superconducting cavity field variation was reduced to less than 1.7% after turning on this control algorithm. The design and experimental results of this system are presented in this paper.