2023
Qiang Sun, Evert Klaseboer
The Journal of the Acoustical Society of America 153 (2023) 2855–2866
An analytical solution for the sound and elastic waves generated by a rigid sphere with a shell made of elastic material submerged in an infinite fluid is introduced. The sphere oscillates up and down at a fixed frequency and generates elastic waves (both longitudinal and transverse) in the shell, which are then transmitted to the fluid. The effects of the acoustic boundary layer are included (thus, no implicit arbitrary “slip” on the surface as in the usual fluid acoustic model is present). An example of a 1 mm radius sphere with an elastic shell is analyzed in detail for several conditions to understand the physical phenomena involved in such a system.
Qiang Sun
Advances in Applied Mathematics and Mechanics 15 (2023) 831-851
Localized point sources (monopoles) in an acoustical domain are imple- mented to a three dimensional non-singular Helmholtz boundary element method in the frequency domain. It allows for the straightforward use of higher order sur- face elements on the boundaries of the problem. It will been shown that the effect of the monopole sources ends up on the right hand side of the resulting matrix system. Some carefully selected examples are studied, such as point sources near and within a concentric spherical core-shell scatterer (with theoretical verification), near a curved focusing surface and near a multi-scale and multi-domain acoustic lens.
Sohail Ahmed, Hang Xu, Qiang Sun
Advances in Applied Mathematics and Mechanics 15 (2023) 684-718
In this paper, the natural convection of a complex fluid that contains both nanoparticles and gyrotactic microorganisms in a heated square cavity is considered. The Buongiorno model is applied to descirbe the nanofluid behaviours. Both the top and bottom horizontal walls of the cavity are adiabatic, and there is a temperature difference between the left and right vertical walls. The non-dimensional govern- ing equations are obtained when the stream-vorticity formulation of function is used, which are solved by the recently developed robust Coiflet wavelet homotopy analy- sis method. A rigid verification for the solver is given. Besides, the effects of various physics parameters including the Rayleigh number, the buoyancy ratio parameter, the bioconvection Rayleigh number, the Prandtl number, the Brownian motion parame- ter, the thermophoresis parameter, the heat generation parameter, the Lewis number, the bioconvection Peclet number and the Schmidt number on this complicated nat- ural convection are examined. It is known that natural convection is closely related to our daily life owing to its wide existence in nature and engineering applications. We believe that our work will make a significant contribution to a better understand- ing of the natural convection of a complex fluid in a cavity with suspensions of both inorganic nanoparticles and organic microorganisms.
Samir Eldemrdasha, Giannis Thalassinosb, Amani Alzahrania, Qiang Sun, Ella Walsh, Erin Grant, Hiroshi Abe, Tamar L. Greaves, Takeshi Ohshima, Petr Cigler, Pavel Matejicek, David A. Simpson, Andrew D. Greentree, Gary Bryant, Brant C. Gibson, Philipp Reineck
Carbon 206 (2023) 268–276
Fluorescent nanodiamonds (FNDs) containing nitrogen-vacancy (NV) centers can be used as nanoscale sensors for temperature and electromagnetic fields and find increasing application in many areas of science and technology from biology to quantum metrology. Decreasing the separation between the NV centers and their sensing target often enhances the measurement sensitivity. FND shape strongly affects this distance from NV centers to the particle surface and therefore properties such as brightness and fluorescence spectrum, and can limit sensor applications. Here, we demonstrate that FNDs made from high-pressure high-temperature (HPHT) diamond have predominantly disk-like shapes. Using single-particle atomic force microscopy in combination with ensemble X-ray and light scattering techniques, we show that a typical FND in the 50–150 nm size range has an aspect ratio of three i.e. is three times thinner (e.g. in z) than it is wide (e.g. in the x-y plane). This high aspect ratio of FNDs is important for many quantum sensing measurements as it will enable enhanced sensitivities compared to spherical or other isotropic particle geometries. We investigate FND shape, fluorescence properties, T1 spin relaxation time and T1 fluorescence contrast as functions of particle size and discuss the implications of FND particle shape on quantum sensing applications.
Yutong Chen, Qiang Sun, Xuewei Tu, Liangchen Chen, Weihang Han, Luping Zhang, Xiaofei Duan, Min Liu, Hui Zheng
Journal of Cleaner Production 392 (2023) 136314
Discovery of effective materials for photocatalytic nitrogen fixation under ambient conditions provides new opportunities for sustainable futures. Nitrogenase provides the best example of environmentally friendly nitrogen reduction. Inspired by homocitrate’s specific activating effect on nitrogenase, citric acid was used instead of homocitric acid to prepare citric acid-based bimetallic organic frameworks (MOFs). Metals such as cerium and copper as bimetallic centers were coordinated with citric acid to mimic FeMo bimetallic active center in the M cluster. The results show that the morphology, structure and photocatalytic activity of the catalysts has been significantly affected by the amount of citric acid. The molar ratio of total metal to citric acid of 1.5:20 had the best ammonia evolution rate from air at a normal temperature and pressure (NTP), which was as high as 1027.6 μg g− 1 h− 1. It was further confirmed by the numerically study of the interactions between light and the syn- thesized photocatalytic particles, which can be intuitively seen that CeCu@CA-20 can support more surface charges. This work provides an effective strategy for citrate-based MOF photocatalysts and offers a new idea for nitrogenase biomimetics.
Qiang Sun, Evert Klaseboer
Fluids 8 (2023) 56
The problem of non-unique solutions at fictitious frequencies that can appear in the boundary element method for external acoustic phenomena described by the Helmholtz equation is studied. We propose a method to fully desingularise in an analytical way the otherwise hyper- singular Burton–Miller framework, where the original boundary element method and its normal derivative are combined. The method considerably simplifies the use of higher-order elements, for example, quadratic curved surface elements. The concept is validated using the example of scattering on a rigid sphere and a rigid cube, and its robustness and effectiveness for external sound-wave problems are confirmed.
Qiang Sun, Shuo Li, Taras Plakhotnik, Andrew D. Greentre
Physical Review A 107 (2023) 013720
Nanodiamonds containing luminescent point defects are widely explored for applications in quantum biosens- ing such as nanoscale magnetometry, thermometry, and electrometry. A key challenge in the development of such applications is the large variation in fluorescence properties observed between particles, even when obtained from the same batch or nominally identical fabrication processes. By theoretically modeling the emission of nitrogen-vacancy color centers in spherical nanoparticles, we are able to show that the fluorescence spectrum varies with the exact position of the emitter within the nanoparticle, with noticeable effects seen when the diamond radius, a, is larger than around 100 nm, and significantly modified fluorescence profiles found for larger particles when a = 200 and 300 nm, with negligible effects below a = 100 nm. These results show that the reproducible geometry of point defect position within a narrowly sized batch of diamond crystals is necessary for controlling the emission properties. Our results are useful for understanding the extent to which nanodiamonds can be optimized for biosensing applications.
Liangchen Chen, Yutong Chen, Xuewei Tu, Shouxin Zhu, Can Sun, Luping Zhang, Weihang Han, Xiaofei Duan, Qiang Sun, Hui Zheng
Journal of Colloid and Interface Science 633 (2023) 703-711
Photocatalytic nitrogen fixation opens new opportunities for sustainable and healthier futures, and developing effective and inexpensive photocatalysts is the key. We use the ligand-azomellitic acid (H4abtc) to connect with Fe clusters and Zr clusters to form stable metal-organic frameworks (MOFs) Fe-abtc and Zr-abtc, both of which are responsive to visible lights for nitrogen fixation. It is worth noting that the presence of NN in the ligand makes it respond to visible lights. The tetracarboxyl group
is connected to the metal cluster to form a stable structure. The field-only surface integral method verified that the ligands were successfully applied into the synthesized MOF particles, which expanded the photoresponse range and enhanced the photonic interactions of the synthesized photocatalysts compared with pure MOF particles. This work provides a new idea for the design of cheap and effective MOFs for photocatalytic nitrogen fixation.
Liangchen Chen, Jingxuan Shou, Yutong Chen, Weihang Han, Xuewei Tu, Luping Zhang, Qiang Sun, Jun Cao, Yurong Chang, Hui Zheng
Chemical Engineering Journal 451 (2023) 138592
Photocatalytic nitrogen fixation is an important pathway for carbon neutralization and sustainable development. Inspired by nitrogenase, the participation of molybdenum can effectively activate nitrogen. A novel Ti/Mo composites photocatalyst is designed by sintering the molybdenum acetylacetonate precursor with TiO2. The special carbon-coated hexagonal photocatalyst is obtained which photocatalytic nitrogen fixation performance is enhanced 16 times compared to pure TiO2 at room temperature and ambient pressure. The abundant surface defects in this composite were confirmed to be the key factor for nitrogen fixation. This newly developed Ti/Mo composites provide a simple and effective strategy for photocatalytic nitrogen fixation from air directly under ambient conditions.
2022
Evert Klaseboer, Qiang Sun
Communications in Theoretical Physics 74 (2022) 085003
The famous scientist Hermann von Helmholtz was born 200 years ago. Many complex physical wave phenomena in engineering can effectively be described using one or a set of equations named after him: the Helmholtz equation. Although this has been known for a long time, from a theoretical point of view, the actual numerical implementation has often been hindered by divergence-free and/or curl- free constraints. There is further a need for a numerical method that is accurate, reliable and takes into account radiation conditions at infinity. The classical boundary element method satisfies the last condition, yet one has to deal with singularities in the implementation. We review here how a recently developed singularity-free three-dimensional boundary element framework with superior accuracy can be used to tackle such problems only using one or a few Helmholtz equations with higher order (quadratic) elements which can tackle complex curved shapes. Examples are given for acoustics (a Helmholtz resonator among others) and electromagnetic scattering.
Yuanfang Shen, Jingxuan Shou, Liangchen Chen, Weihang Han, Luping Zhang, Yutong Chen, Xuewei Tu, Shangfu Zhang, Qiang Sun, Yurong Chang, Hui Zheng
Applied Catalysis A, General 643 (2022) 118739
Photocatalytic nitrogen fixation from air directly under sunlight can contribute significantly to carbon neutralization. It is an ideal pathway to replace the industrial Haber Bosch process in future. A Fe-doped layered WO3 photocatalyst containing oxygen vacancies was developed which can fix nitrogen from air directly under sunlight at atmospheric pressure. The iron doping enhances the transport efficiency of photogenerated electrons. The photocatalytic efficiency is around 4 times higher than that of pure WO3. This work provides a simple and cheap strategy for photocatalytic nitrogen fixation from air under mild conditions.
Evert Klaseboer, Qiang Sun
International Journal of Solids and Structures 239-240 (2022) 111448
The analytical solution is given for a vibrating rigid core sphere, oscillating up and down without volume change, situated at the center of an elastic material spherical shell, which in turn is situated inside an infinite (possible different) elastic medium. The solution is based on symmetry considerations and the continuity of the displacement both at the core and the shell-outer medium boundaries as well as the continuity of the stress at the outer edge of the shell. Furthermore, a separation into longitudinal and transverse waves is used. Analysis of the solution shows that a surprisingly complex range of physical phenomena can be observed when the frequency is changed while keeping the material parameters the same, especially when compared to the case of a core without any shell. With a careful choice of materials, shell thickness and vibration frequency, it is possible to filter out most of the longitudinal waves and generate pure tangential waves in the infinite domain (and vice-versa, we can filter out the tangential waves and generate longitudinal waves). When the solution is applied to different frequencies and with the help of a fast Fourier transform (FFT), a pulsed vibration is shown to exhibit the separation of the longitudinal (L) and transverse (T) waves (often called P- and S-waves in earthquake terminology).
Qiang Sun, Evert Klaseboer
Annalen der Physik 534 (2022) 2100397
With the development of condensed-matter physics and nanotechnology, attention has turned to the fields near and on surfaces that result from interactions between electric dipole radiation and mesoscale structures. It is hoped that studying these fields will further the understanding of optical phenomena in nano-optics, quantum mechanics, electromagnetics, and sensing using solid-state photon emitters. Here, a method for implementing dynamic electric and magnetic dipoles in the frequency domain into a non-singular field-only surface integral method is described. It is shown that the effect of dipoles can conveniently be described as a relatively simple term in the integral equations, which fully represents how they drive the fields and interactions. Also, due to the non-singularity, this method can calculate the electric and magnetic fields on the surfaces of objects in both near and far fields with the same accuracy, which makes it an ideal tool to investigate nano-optical phenomena. The derivation of the framework is given and tested against a Mie theory alike formula. Some interesting examples are shown involving the interaction of dipoles with different types of mesoscale structures including parabolic nano-antennas and gold probes.
Siwei Yang, Qiang Sun, Weihang Han, Yuanfang Shen, Zhigang Ni, Shijie Zhang, Liangchen Chen, Luping Zhang, Jun Cao, Hui Zheng
Catalysis Science & Technology 22 (2022) 786
AA simple and highly efficient porous composite via a solvent evaporation method using g-C3N4 and NiSO4 was developed. It can super rapidly remove multiple organic dyes from water including rhodamine B (RhB), indigo carmine (IC), methylene blue (MB) and Congo red (CR) at room temperature and under sunlight. The optimal composite 50%NiCN was obtained by an orthogonal experiment, and it exhibited high removal ability towards the above four organic dyes up to 99.9% within 1 minute, even 96.9% in 2 seconds. The good stability of such a composite has been confirmed after five cycles and its possible degradation pathway was discussed. The dynamics of the photocatalytic degradation reaction were explored based on the structure of the organic dyes and the morphology of the photocatalyst by DFT calculations. In addition, simulation of the interactions between sunlight and the composite was performed by the field-only surface integral method. These two theoretical studies supply the reasonable explanation for this photocatalytic function in depth.
Xiangcheng You, Hang Xu, Qiang Sun
Chaos, Solitons and Fractals 155 (2022) 111725
In this paper, a simple, robust, fast and effective method based on the conserved quantities is developed to approximate and analyse the shape, structure and interaction characters of the solitary waves de- scribed by the Benjamin–Bona–Mahony (BBM) equation. Due to the invariant character of the conserved quantities, there is no need to solve the related complex nonlinear partial differential BBM equation to simulate the interactions between the solitary waves at the most merging instance. Good accuracy of the proposed method has been found when compared with the numerical method for the solitary wave in- teractions with different initial incoming wave shapes. The conserved quantity method developed in this work can serve as an ideal tool to benchmark numerical solvers, to perform the stability analysis, and to analyse the interacting phenomena between solitary waves.
2021
Siwei Yang, Yichao Zhuang, Yuanfang Shen, Weihang Han, Liangchen Chen, Qiang Sun, Di Wu, Hui Zheng
Sustainability 13 (2021) 12669
Contaminated water due to industrial organic dyes presents a significant challenge to sustainability. As a material of green energy, photocatalysts offer an effective and environmentally friendly way to deal with organic dyes for water treatment. A series of simple and highly efficient iron photocatalysts with carbene ligands were developed, which, under the illumination of sunlight, can rapidly degrade multiple organic dyes in water at room temperature, including rhodamine B (RhB), indigo carmine (IC), methyl blue (MB), and congo red (CR). The field-only surface integral method was carried out to determine the absorption spectrum of photocatalyst particles. Under the optimized experimental conditions which were selected by the orthogonal experiments for four dyes, 0.5a@Fe2O3 and 2c@Fe2O3 demonstrated good stability and photocatalytic activity. These two composite materials not only have the ability to remove 98.0% of the degradation in 10 s, but also maintain high reactivity after a few cycles of repeated use.
Shuo Li, Dongbi Bai, Marco Capelli, Qiang Sun, Shahraam Afshar V. David A. Simpson, Scott Foster, Heike Beendorff-Heidepriem, Brant C. Gibson, Andrew D. Greentree
Optics Express 29 (2021) 14425
Diamonds containing the negatively charged nitrogen-vacancy centre are a promising system for room-temperature magnetometry. The combination of nano- and micro-diamond particles with optical fibres provides an option for deploying nitrogen-vacancy magnetometers in harsh and challenging environments. Here we numerically explore the coupling efficiency from nitrogen-vacancy centres within a diamond doped at the core/clad interface across a range of commercially available fibre types so as to inform the design process for a diamond in fibre magnetometers. We determine coupling efficiencies from nitrogen-vacancy centres to the guided modes of a step-index fibre and predict the optically detected magnetic resonance (ODMR) generated by a ensemble of four nitrogen-vacancy centres in this hybrid fibre system. Our results show that the coupling efficiency is enhanced with a high refractive index difference between the fibre core and cladding and depends on the radial position of the nitrogen-vacancy centres in the fibre core. Our ODMR simulations show that due to the preferential coupling of the nitrogen-vacancy emission to the fibre guided modes, certain magnetometry features such as ODMR contrast can be enhanced and lead to improved sensitivity in such diamond-fibre systems, relative to conventional diamond only ensemble geometries.
Qiang Sun, Kishan Dholakia, Andrew D. Greentree
ACS Photonics 8 (2021) 1103–1111
The optical trapping and manipulation of small particles is an important tool for probing fluid properties at the microscale. In particular, microrheology exploits the manipulation and rotation of micron-scale particles to probe local viscosity, especially where these properties may be perturbed as a function of their local environment, for example in the vicinity of cells. To this end, birefringent particles are useful as they can be readily controlled using optically induced forces and torques, and thereby used to probe their local environment. However, the magnitude of optical torques that can be induced in birefringent particles is small, and a function of the particle diameter, meaning that rotational flow cannot readily be probed on length scales much small than the micron level. Here we show modeling that demonstrates that eccentric spherical core–shell nanoparticles can be used to generate considerable optical torques. The eccentricity is a result of the displacement of the center of the core from the shell. Our results show that, for particles ranging from 90 to 180 nm in diameter, we may achieve rotation rates exceeding 800 Hz. This fills a missing size gap in the rotation of microparticles with optical forces. The diameter of particle we may rotate is almost an order of magnitude smaller than the smallest birefringent particles that have been successfully rotated to date. The rotation of eccentric core–shell nanoparticles therefore makes an important contribution to biophotonics and creates new opportunities for rheology in nanoscale environments.
2020
Evert Klaseboer, Qiang Sun, Derek Y. C. Chan
Physics of Fluids 32 (2020) 126105
Analytical solutions in fluid dynamics can be used to elucidate the physics of complex flows and to serve as test cases for numerical models. In this work, we present the analytical solution for the acoustic boundary layer that develops around a rigid sphere executing small amplitude harmonic rectilinear motion in a compressible fluid. The mathematical framework that describes the primary flow is identical to that of wave propagation in linearly elastic solids, with the difference being the appearance of complex instead of real valued wave numbers. The solution reverts to the well-known classical solutions in special limits: the potential flow solution in the thin boundary layer limit, the oscillatory flat plate solution in the limit of large sphere radius, and the Stokes flow solutions in the incompressible limit of infinite sound speed. As a companion analytical result, the steady second order acoustic streaming flow is obtained. This streaming flow is driven by the Reynolds stress tensor that arises from the axisymmetric first order primary flow around such a rigid sphere. These results are obtained with a linearization of the non-linear Navier–Stokes equations valid for small amplitude oscillations of the sphere. The streaming flow obeys a time-averaged Stokes equation with a body force given by the Nyborg model in which the above-mentioned primary flow in a compressible Newtonian fluid is used to estimate the time-averaged body force. Numerical results are presented to explore different regimes of the complex transverse and longitudinal wave numbers that characterize the primary flow.
Yaji Wang, Hang Xu, Qiang Sun
Applied Mathematics and Mechanics 41 (2020) 1735-1746
The Whitham-Broer-Kaup model is widely used to study the tsunami waves. The classical Whitham-Broer-Kaup equations are re-investigated in detail by the generalized projective Riccati-equation method. 20 sets of solutions are obtained of which, to the best of the authors’ knowledge, some have not been reported in literature. Bifurcation analysis of the planar dynamical systems is then used to show different phase portraits of the traveling wave solutions under various parametric conditions.
Muhammad Ishaq, Hang Xu, Qiang Sun
Physics of Fluids 32 (2020) 077109
The interaction of three-dimensional nonlinear high frequency magnetosonic waves in a magnetized plasma is investigated theoretically via the nonlinear Kadomtsev–Petviashvili equation. Though such wave patterns are commonly observed in the solar system and can be generated by magnetic resonance generators, only limited theoretical studies have been performed. We examined the existence of both periodic and solitary solutions of magnetosonic waves by using the modulation instability analysis. The Phillips wave resonance criterion is employed for capturing the periodic wave interaction whose energy conversion is analyzed via Fourier spectra. It is found that more energy is carried by the primary wave relative to that by the higher-order harmonic wave. In addition, it is noted that the rhodonea curve is smooth and closed for rational wavenumbers, but it becomes chaotic to form a dense set for irrational ones. We believe that this work can fill the blanks in the research of magnetosonic wave behaviors in the magnetized plasma.
Siwei Yang, Qiang Sun, Yuanfang Shen, Yixin Hong, Xuewei Tu, Yutong Chen, Hui Zheng
Applied Surface Science 525 (2020) 146559
Developing cheap and efficient photocatalyst is a vigorous pathway for gradually increasing rhodamine B (RhB) pollution problem. Herein, two novel iron-based photocatalysts g-C3N4/ZnO@Fe3O4 (Fe-heterojunction) and 2- amino-5-fluorobenzotrifluoride@Fe3O4 (Fe-organic ligand) were designed and synthesized which exhibit 100% photo-degradative ability with water contaminant RhB under sunlight in about 2 h. They were characterized by SEM, FT-IR, XRD, XPS and BET as well as the analysis of morphological structure and chemical composition. The ESR experiment was also carried out to explore the possible degradation mechanism. The UV-Vis spectrum and band gap energies were examined and calculated. Especially, the Fe-organic ligand photocatalyst shows a specific 3D cockscomb-like structure. This makes its degrading efficiency much improved. The recently developed robust field-only surface integral method was employed to explore the possible mechanism which confirmed with the experimental results. The optimum degradation conditions including ligand ratio, pH and concentration of RhB solution, were screened by orthogonal experiments. Also, the reusability and degradation kinetic equations of the two photocatalysts were built to give a typical reference for a large-scale industrial wastewater treatment as well as for the water sustainable utilization.
Sohail Ahmed, Hang Xu, Qiang Sun
Mathematical Problems in Engineering (2020) 3265143
The homogeneous-heterogeneous reaction in the boundary layer flow of a water-based nanofluid in the stagnation-point region of a plane surface is investigated. The type of small particles explored here is the single-walled carbon nanotubes. The homogeneous nanofluid model is employed for description of behaviours of nanofluids. Here, the homogeneous (bulk) reaction is isothermal cubic autocatalytic, while the heterogeneous (surface) reaction is single, isothermal, and first order. The steady state of this system is analysed in detail, with equal diffusion coefficients being considered for both reactants and autocatalysts. Multiple solutions of the reduced system are captured for some particular sets of physical parameters, which seem to be overlooked in all previous published works with regard to studies of homogeneous-heterogeneous reactions modeled by homogeneous nanofluid models. Besides, we discover the significant limitation of previous conclusion about that the solutions by homogeneous nanofluid flow models can be recovered from those by regular fluids.
Srinivas Mettu, Shunyu Yao, Qiang Sun, Samuel Ronald Lawson, Peter J. Scales, Gregory J. O. Martin, Muthupandian Ashokkumar
Industrial & Engineering Chemistry Research 59 (2020) 7901-7912
Ultrasound standing waves can be used to separate emulsions. So far, they have been applied to oil-in-water emulsions with low continuous phase viscosity. This technique has the potential to be used for novel applications such as separating lipids from algal biomass; however, this requires the methodology to be optimized to process viscous emulsions. We have addressed this issue by studying the effects of bulk phase viscosity (1−23 mPa·s), emulsion droplet size (4.5−20 μm), power (10−54 W/L), and frequency (1 and 2 MHz) of ultrasound on the separation efficiency of model mineral oil-in-water−glycerol-mixture emulsions.
Aneela Bibi, Hang Xu, Qiang Sun, Ioan Pop
International Journal of Numerical Methods for Heat & Fluid Flow 30 (2020) 4083-4101
This study aims to carry out an analysis for flow and heat transfer of a new hybrid nanofluid over a vertical flat surface embedded in a saturated porous medium with anisotropic permeability at high Rayleigh number. Here the hybrid nanofluid is considered as the working fluid, with different kinds of small particles in nanoscale being suspended.
Qiang Sun, Evert Klaseboer, Alex J. Yuffa, Derek Y. C. Chan
Journal of the Optical Society of America A 37 (2020) 284-293
An efficient field-only nonsingular surface integral method to solve Maxwell’s equations for the components of the electric field on the surface of a dielectric scatterer is introduced. In this method, both the vector wave equation and the divergence-free constraint are satisfied inside and outside the scatterer. The divergence-free condition is replaced by an equivalent boundary condition that relates the normal derivatives of the electric field across the surface of the scatterer. Also, the continuity and jump conditions on the electric and magnetic fields are expressed in terms of the electric field across the surface of the scatterer. Together with these boundary conditions, the scalar Helmholtz equation for the components of the electric field inside and outside the scatterer is solved by a fully desingularized surface integral method. Compared with the most popular surface integral methods based on the Stratton–Chu formulation or the Poggio–Miller–Chew–Harrington–Wu–Tsai (PMCHWT) formulation, our method is conceptually simpler and numerically straightforward because there is no need to introduce intermedi- ate quantities such as surface currents, and the use of complicated vector basis functions can be avoided altogether. Also, our method is not affected by numerical issues such as the zero-frequency catastrophe and does not contain integrals with (strong) singularities. To illustrate the robustness and versatility of our method, we show examples in the Rayleigh, Mie, and geometrical optics scattering regimes. Given the symmetry between the electric field and the magnetic field, our theoretical framework can also be used to solve for the magnetic field.
Qiang Sun, Evert Klaseboer, Alex J. Yuffa, Derek Y. C. Chan
Journal of the Optical Society of America A 37 (2020) 276-283
A field-only boundary integral formulation of electromagnetics is derived without the use of surface currents that appear in the Stratton–Chu formulation. For scattering by a perfect electrical conductor (PEC), the components of the electric field are obtained directly from surface integral equation solutions of three scalar Helmholtz equations for the field components. The divergence-free condition is enforced via a boundary condition on the normal com- ponent of the field and its normal derivative. Field values and their normal derivatives at the surface of the PEC are obtained directly from surface integral equations that do not contain divergent kernels. Consequently, high-order elements with fewer degrees of freedom can be used to represent surface features to a higher precision than the traditional planar elements. This theoretical framework is illustrated with numerical examples that provide further physical insight into the role of the surface curvature in scattering problems.
Hui Zheng, Binjing Hu, Qiang Sun, Jun Cao, Fangmin Liu
Journal of Chemical Eduction 97 (2020) 421-426
Pharmaceutical analysis, as the core curriculum of chemistry, chemical engineering, and pharmaceutical engineering, contains broad and in-depth knowledge that leads to massive learning and teaching loads. There are more than 100 analytical methods of medicines in this course. As such, this subject is a big challenge for both students and lecturers. A novel chemical structure teaching (CST) method was developed on the basis of our long-term teaching experience to cope with these challenges. It has been shown in practice that this CST method can significantly unload the stress of students and lecturers simultaneously. The survey about the improvement of students’ interests was carried out and listed in the form of questionnaire. The outcome of CST indicates that it can help students to form abilities of critical and logical thinking, motivate them to discuss with their peers and lecturers, and eventually improve comprehensive abilities such as synthesizing information, thinking logically, and analyzing problems independently as well as the average score. Furthermore, CST can be beneficial for lecturers who teach other relevant curricula in chemical or pharmaceutical engineering to improve the teaching outcome, such as organic chemistry, spectrum analysis, pharmaceutical synthesis, and medicinal chemistry. This CST model can also help students cultivate a life-long learning ability as active learners from the cognitive perspective view.
2019
26. Eliminating the fictitious frequency problem in BEM solutions of the external Helmholtz equation
Evert Klasebeor, Florian D.E. Charlet, Boo-Cheong Khoo, Qiang Sun, Derek, Y. C. Chan
Engineering Analysis with Boundary Elements 109 (2019) 106-116
The problem of the fictitious frequency spectrum resulting from numerical implementations of the boundary element method for the exterior Helmholtz problem is revisited. When the ordinary 3D free space Green’s func- tion is replaced by a modified Green’s function, it is shown that these fictitious frequencies do not necessarily have to correspond to the internal resonance frequency of the object. Together with a recently developed fully desingularized boundary element method that confers superior numerical accuracy, a simple and practical way is proposed for detecting and avoiding these fictitious solutions. The concepts are illustrated with examples of a scattering wave on a rigid sphere.
Evert Klasebeor, Qiang Sun, Derek, Y. C. Chan
Journal of Elasticity 137 (2019) 83-100
The displacement field for three dimensional dynamic elasticity problems in the frequency domain can be decomposed into a sum of a longitudinal and a transversal part known as a Helmholtz decomposition. The Cartesian components of both the longitudinal and transverse fields satisfy scalar Helmholtz equations that can be solved using a desingularized boundary element method (BEM) framework. The curl free longitudinal and divergence free transversal conditions can also be cast as additional scalar Helmholtz equations. When compared to other BEM implementations, the current framework leads to smaller matrix dimensions and a simpler conceptual approach. The numerical implementation of this approach is benchmarked against the 3D elastic wave field generated by a rigid vibrating sphere embedded in an infinite linear elastic medium for which the analytical solution has been derived. Examples of focussed 3D elastic waves generated by a vibrating bowl-shaped rigid object with convex and concave surfaces are also considered. In the static zero frequency limit, the Helmholtz decomposition becomes non-unique, and both the longitudinal and transverse components contain divergent terms that are proportional to the inverse square of the frequency. However, these divergences are equal and opposite so that their sum, that is the displacement field that reflects the physics of the problem, remains finite in the zero frequency limit.
Hang Xu, Huang Huang, Xiao-Hang Xu, Qiang Sun
International Journal of Numerical Methods for Heat & Fluid Flow 29 (2019) 2566-2587
This paper aims to study the heat transfer of nanofluid flow driven by the move of channel walls in a microchannel under the effects of the electrical double layer and slippery properties of channel walls. The distributions of velocity, temperature and nanoparticle volumetric concentration are analyzed under different slip-length. Also, the variation rates of flow velocity, temperature, concentration of nanoparticle, the pressure constant, the local volumetric entropy generation rate and the total cross-sectional entropy generation are analyzed.
Shuaijun Pan, Rui Guo, Joseph J. Richardson, Joseph D. Berry, Quinn A. Besford, Mattias Björnmalm, Gyeongwon Yun, Ruoxi Wu, Zhixing Lin, Qi-Zhi Zhong, Jiajing Zhou, Qiang Sun, Jianhua Li, Yanbing Lu, Zhichao Dong, Margaret Katherine Banks,
Weijian Xu, Jianhui Jiang, Lei Jiang, and Frank Caruso
Advanced Science (2019), 1901846.
Droplet bouncing on repellent solid surfaces (e.g., the lotus leaf effect) is a common phenomenon that has aroused interest in various fields. However, the scenario of a droplet bouncing off another droplet (either identical or distinct chemical composition) while moving on a solid material (i.e., ricocheting droplets, droplet billiards) is scarcely investigated, despite it having fundamental implications in applications including self-cleaning, fluid transport, and heat and mass transfer. Here, the dynamics of bouncing collisions between liquid droplets are investigated using a friction-free platform that ensures ultrahigh locomotion for a wide range of probing liquids. A general prediction on bouncing droplet–droplet contact time is elucidated and bouncing droplet–droplet collision is demonstrated to be an extreme case of droplet bouncing on surfaces. Moreover, the maximum deformation and contact time are highly dependent on the position where the collision occurs (i.e., head-on or off-center collisions), which can now be predicted using parameters (i.e., effective velocity, effective diameter) through the concept of an effective interaction region. The results have potential applications in fields ranging from microfluidics to repellent coatings.
Hang Xu, Qiang Sun
Communications in Theoretical Physics 71 (2019) 903-911
The fully developed mixed convection hybrid nanofluid flow in a vertical microchannel is examined in detail. The simplified hybrid model that omits the nonlinear terms due to the interaction of different nanoparticle volumetric fractions is derived and compared with the existing one. The generalized model describing hybrid nanofluid suspended with multiple kinds of solid particles is suggested. The argument that the corresponding nanofluid solutions obtained by the homogenous model can be recovered from the results of the regular problems through simple arithmetic operations is checked. Solutions in similarity form for this flow problem are formulated by means of a set of similarity variables. The effects of various parameters on important physical quantities are analyzed and discussed.
Binjing Hu, Qiang Sun, Chengyi Zuo, Yunxin Pei, Siwei Yang, Hui Zheng, Fangming Liu
Beilstein Journal of Nanotechnology 10 (2019) 1157–1165
A mild and simple method was developed to synthesize a highly efficient photocatalyst comprised of Ce-doped ZnO rods. The photocatalytic activity was assessed by the degradation of a common dye pollutant found in wastewater, rhodamine B (RhB), using a sunlight simulator.To understand the crystal structure, elemental state, surface morphology and chemical composition, the photocatalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron micro- scope (SEM) and inductively coupled plasma emission spectroscopy (ICP), respectively. The newly developed, robust, field-only surface integral method was employed to explore the relationship between the remarkable catalytic effect and the catalyst shape and porous microstructure. The computational results showed that the dipole-like field covers the entire surface of the rod-like Ce-doped ZnO photocatalyst and is present over the entire range of wavelengths considered. The optimum degradation conditions were determined by orthogonal tests and range analysis, including the concentration of RhB and catalyst, pH value and temperature.
2018
Hang Xu, Ioan Pop, Qiang Sun
Applied Mathematics and Mechanics 39 (2018) 395-408
We develop a mathematical model to describe the flow in a microchannel driven by the upper stretching wall of the channel in the presence of electrokinetic effects. In this model, we avoid imposing any unphysical boundary condition, for instance, the zero electrostatic potential in the middle of the channel. Using the similarity transformation, we employ the homotopy analysis method (HAM) to get the analytical solution of the model. In our approach, the unknown pressure constant and the integral constant related to the electric potential are solved spontaneously by using the proper boundary conditions on the channel walls, which makes our model consistent with the commonly accepted models in the field of fluid mechanics. It is expected that our model can offer a general and proper way to study the flow phenomena in microchannels.
Hongzhan Di, Gregory J.O. Martin, Qiang Sun, Donglin Xie, Dave E. Dunstan
Journal of Membrane Science 555 (2018) 115-124
The detailed structures and distributions of the deposit layer formed during cross-flow filtration of 0.4 μm polystyrene particles has been investigated. A new filtration system was used to directly visualize particle de- position on the membrane surface in a filtration channel in real time. High-resolution and time sequenced images were obtained as well as 3D views of the filtration channel. The structural differences of the deposit layer over a range of solution conditions (pH, ionic strength, and feed concentration) were determined to yield new insight into the fouling mechanism. The time-dependent deposition behaviour was characterized on the basis of surface coverage, deposition volume and normalised deposition volume. The initial deposition process was primarily governed by membrane-particle interactions, while the characteristics of the fully developed deposi- tion layers were mainly affected by particle-particle interactions.
Niazi M. Dilawar Khan, Hang Xu, Qingkai Zhao, Qiang Sun
Zeitschrift für Naturforschung A 73 (2018) 741-751
We consider the fully developed mixed convection flow in a vertical channel driven by an external pressure gradient and buoyancy force. Unlike in previous studies, we employ a new model to formulate the flow problem that guarantees the smoothness and continuity of the distributions of the electrical potential and velocity. This model, solved by the homotopy analysis method, is compatible with the channel flow models commonly used in fluid mechanics.
2017
Evert Klaseboer, Qiang Sun, Derek Y. C. Chan
Applied Optics 56 (2017) 9377-9383
The scattering of electromagnetic pulses is described using a non-singular boundary integral method to solve directly for the field components in the frequency domain, and Fourier transform is then used to obtain the complete space-time behavior. This approach is stable for wavelengths both small and large relative to character- istic length scales. Amplitudes and phases of field values can be obtained accurately on or near material boun- daries. Local field enhancement effects due to multiple scattering of interest to applications in microphotonics are demonstrated.
Qiang Sun, Evert Klaseboer, Derek Y. C. Chan
Physical Review B 95 (2017) 045137
We present a boundary integral formulation of electromagnetic scattering by homogeneous bodies that are characterized by linear constitutive equations in the frequency domain. Our formulation is free of the well-known numerical instability that occurs in the zero-frequency or long-wavelength limit in traditional surface integral solutions of Maxwell’s equations and our numerical results converge uniformly to the static results in the long-wavelength limit. Furthermore, we use a formulation of the scalar Helmholtz equation that is expressed as classically convergent integrals and does not require the evaluation of principal value integrals or any knowledge of the solid angle. Near and far field values can be calculated with equal precision, and multiscale problems in which the scatterers possess characteristic length scales that are both large and small relative to the wavelength can be easily accommodated. From this we obtain results for the scattering and transmission of electromagnetic waves at dielectric boundaries that are valid for any ratio of the local surface curvature to the wave number. This is a generalization of the familiar Fresnel formula and Snell’s law, valid at planar dielectric boundaries, for the scattering and transmission of electromagnetic waves at surfaces of arbitrary curvature. Implementation details are illustrated with scattering by multiple perfect electric conductors as well as dielectric bodies with complex geometries and composition.
Evert Klaseboer, Qiang Sun, Derek Y. C. Chan
IEEE Transactions on Antennas and Propagation, 65 (2017) 972-977
A boundary integral formulation of electromagnetics that involves only the components of E and H is derived without the use of surface currents that appear in the classical Poggio and Miller, Chang and Harrington, and Wu and Tsai formulation. The kernels of the boundary integral equations for E and H are nonsingular so that all field quantities at the surface can be determined to high precision and also geometries with closely spaced surfaces present no numerical difficulties. Quadratic elements can readily be used to represent the surfaces so that the surface integrals can be calculated to higher numerical precision than using planar elements for the same numbers of degrees of freedom.
2016
14. Three-dimensional free bio-convection of nanofluid near stagnation point on general curved isothermal surface
Qingkai Zhao, Hang Xu, Longbin Tao, Ammarah Raees, Qiang Sun
Applied Mathematics and Mechanics 37 (2016) 417-432
In this paper, the three-dimensional nanofluid bio-convection near a stag- nation attachment is studied. with a set of similarity variables, the governing equations embodying the conservation of total mass, momentum, thermal energy, nanoparticles and microorganisms are reduced to a set of fully coupled nonlinear differential equations. The homotopy analysis method (HAM)-finite difference method (FDM) technique is used to obtain exact solutions. The effect of various physical parameters on distribution of the motile microorganisms, and the important physical quantities of practical interests are presented and discussed.
Qiang Sun, Evert Klaseboer, Derek Y. C. Chan
The Journal of Chemical Physics 145 (2016) 054106
This paper presents a re-formulation of the boundary integral method for the Debye-Hückel model of molecular and colloidal electrostatics that removes the mathematical singularities that have to date been accepted as an intrinsic part of the conventional boundary integral equation method. The essence of the present boundary regularized integral equation formulation consists of subtracting a known solution from the conventional boundary integral method in such a way as to cancel out the singularities associated with the Green’s function. This approach better reflects the non-singular physical behavior of the systems on boundaries with the benefits of the following: (i) the surface integrals can be evaluated accurately using quadrature without any need to devise special numerical integration procedures, (ii) being able to use quadratic or spline function surface elements to represent the surface more accurately and the variation of the functions within each element is represented to a consistent level of precision by appropriate interpolation functions, (iii) being able to calculate electric fields, even at boundaries, accurately and directly from the potential without having to solve hypersingular integral equations and this imparts high precision in calculating the Maxwell stress tensor and consequently, intermolecular or colloidal forces, (iv) a reliable way to handle geometric configurations in which different parts of the boundary can be very close together without being affected by numerical instabilities, therefore potentials, fields, and forces between surfaces can be found accurately at surface separations down to near contact, and (v) having the simplicity of a formulation that does not require complex algorithms to handle singularities will result in significant savings in coding effort and in the reduction of opportunities for coding errors. These advantages are illustrated using examples drawn from molecular and colloidal electrostatics.
12. Three-dimensional stagnation flow of a nanofluid containing both nanoparticles and microorganisms on a moving surface with anisotropic slip
Ammarah Raees, Muhammad Raees-ul-Haq, Hang Xu, Qiang Sun
Applied Mathematical Modelling 40 (2016) 4136-4150
We studied the three-dimensional stagnation flow of a Newtonian fluid on a moving plate with anistropic slip to the case of a nanofluid in suspension of both the nanoparticles and microorganisms. The passively controlled nanofluid model is taken into account to describe this problem, which is believed to be physically more realistic than previously commonly used models. The problem is then reduced by a set of similarity transformations to seven coupled nonlinear ordinary equations with coupled boundary conditions. Numerical solutions are given by means of a very efficient finite difference method. The influences of various physical parameters on the distributions of the velocity, the temperature, the nanoparticle volumetric fraction, the density of motile microorganisms profiles, as well as the local skin friction coefficient, the local Nusselt number, the local wall mass flux and the local density of the motile microorganisms are presented and discussed.
2015
Qiang Sun, Evert Klaseboer, Boo-Cheong Khoo, Derek Y. C. Chan
Physics of Fluids 27 (2015) 023102
Single-phase Stokes flow problems with prescribed boundary conditions can be formulated in terms of a boundary regularized integral equation that is completely free of singularities that exist in the traditional formulation. The usual mathematical singularities that arise from using the fundamental solution in the conventional boundary integral method are removed by subtracting a related auxiliary flow field that can be constructed from one of many known fundamental solutions of the Stokes equation. This approach is exact and does not require the introduction of additional cutoff parameters. The numerical implementation of this boundary regularized integral equation formulation affords considerable savings in coding effort with improved numerical accuracy. The high accuracy of this formulation is retained even in problems where parts of the boundaries may almost be in contact.
Qiang Sun, Evert Klaseboer, Boo-Cheong Khoo, Derek Y. C. Chan
Royal Society Open Science 2 (2015) 140520
A boundary integral formulation for the solution of the Helmholtz equation is developed in which all traditional singular behaviour in the boundary integrals is removed analytically. The numerical precision of this approach is illustrated with calculation of the pressure field owing to radiating bodies in acoustic wave problems. This method facilitates the use of higher order surface elements to represent boundaries, resulting in a significant reduction in the problem size with improved precision. Problems with extreme geometric aspect ratios can also be handled without diminished precision. When combined with the CHIEF method, uniqueness of the solution of the exterior acoustic problem is assured without the need to solve hypersingular integrals.
Ammarah Raees, Hang Xu, Qiang Sun, Ioan Pop
Applied Mathematics and Mechanics 36 (2015) 163–178
nalysis of a gravity-induced film flow of a fluid containing both nanoparticles and gyrotactic microorganisms along a convectively heated vertical surface is presented. The Buongiorno model is applied. Two kinds of boundary conditions, the passive and the active boundary conditions, are considered to investigate this film flow phenomenon. Through a set of similarity variables, the ordinary differential equations that describe the conservation of the momentum, the thermal energy, the nanoparticles, and the microor- ganisms are derived and then solved numerically by an efficient finite difference technique. The effects of various physical parameters on the profiles of momentum, thermal energy, nanoparticles, microorganisms, local skin friction, local Nusselt number, local wall mass flux, and local wall motile microorganisms flux are investigated.
2014
Qiang Sun, Ioan Pop
International Journal of Numerical Methods for Heat & Fluid Flow 24 (2014) 2-20
Steady-state free convection heat transfer and fluid flow of Cu-water nanofluid is investigated within a porous tilted right-angle triangular enclosure. The flush mounted heater with finite size is placed on one right-angle wall. The temperature of the inclined wall is lower than the heater, and the rest of walls are adiabatic. The governing equations are obtained based on the Darcy’s law, and the nanofluid model adopted is that by Tiwari and Das. The transformed dimensionless governing equations were solved numerically by finite difference method, and the solution for algebraic equations was obtained through successive under relaxation method.
Qiang Sun, Evert Klaseboer, Boo Cheong Khoo, Derek Y. C. Chan
Engineering Analysis with Boundary Elements 43 (2014) 117–123
A non-singular formulation of the boundary integral method (BIM) is presented for the Laplace equation whereby the well-known singularities that arise from the fundamental solution are eliminated analytically. A key advantage of this approach is that numerical errors that arise due to the proximity of nodes located on osculating boundaries are suppressed. This is particularly relevant in multi-scale problems where high accuracy is required without undue increase in computational cost when the spacing between boundaries become much smaller than their characteristic dimensions. The elimination of the singularities means that standard quadrature can be used to evaluate the surface integrals and this results in about 60% savings in coding effort. The new formulation also affords a numerically robust way to calculate the potential close to the boundaries. Detailed implementations of this approach are illustrated with problems involving osculating boundaries, 2D domains with corners and a wave drag problem in a 3D semi-infinite domain. The explicit formulation of problems with axial symmetry is also given.
2013
Qiang Sun, Evert Klaseboer, Boo Cheong Khoo, Derek Y. C. Chan
Physical Review E 87 (2013) 043009
We study the forces and torques experienced by pill-shaped Janus particles of different aspect ratios where half of the surface obeys the no-slip boundary condition and the other half obeys the Navier slip condition of varying slip lengths. Using a recently developed boundary integral formulation whereby the traditional singular behavior of this approach is removed analytically, we quantify the strength of the forces and torques experienced by such particles in a uniform flow field in the Stokes regime. Depending on the aspect ratio and the slip length, the force transverse to the flow direction can change sign. This is a novel property unique to the Janus nature of the particles.
Qiang Sun, Guo Xiong Wu
International Journal for Numerical Methods in Biomedical Engineering 29 (2013) 309-331
A mathematical model and a numerical solution procedure are developed to simulate flow field through a 3D permeable vessel with multibranches embedded in a solid tumour. The model is based on Poisseuille’s law for the description of the flow through the vessels, Darcy’s law for the fluid field inside the tumour interstitium, and Starling’s law for the flux transmitted across the vascular walls. The solution procedure is based on a coupled method, in which the finite difference method is used for the flow in the vessels and the boundary element method is used for the flow in the tumour. When vessels meet each other at a junction, the pressure continuity and mass conservation are imposed at the junction. Three typical representative struc- tures within the tumour vasculature, symmetrical dichotomous branching, asymmetrical bifurcation with uneven radius of daughter vessels and trifurcation, are investigated in detail as case studies. These results have demonstrated the features of tumour flow environment by the pressure distributions and flow velocity field.
2012
Evert Klaseboer, Qiang Sun, Derek Y. C. Chan
Journal of Fluid Mechanics 696 (2012) 468-478
A formulation of the boundary integral method for solving partial differential equations has been developed whereby the usual weakly singular integral and the Cauchy principal value integral can be removed analytically. The broad applicability of the approach is illustrated with a number of problems of practical interest to fluid and continuum mechanics including the solution of the Laplace equation for potential flow, the Helmholtz equation as well as the equations for Stokes flow and linear elasticity.
2011
Qiang Sun, Ioan Pop
International Journal of Thermal Sciences 50 (2011) 2141-2153
Steady-state free convection heat transfer behavior of nanofluids is investigated numerically inside a right-angle triangular enclosure filled with a porous medium. The flush mounted heater with finite size is placed on the left vertical wall. The temperature of the inclined wall is lower than the heater, and the rest of walls are adiabatic. The transformed dimensionless governing equations were solved by finite difference method and solution for algebraic equations was obtained through Successive Under Relaxation method. It is found that the maximum value of average Nusselt number is obtained by decreasing the enclosure aspect ratio and lowering the heater position with the highest value of Rayleigh number and the largest size of heater. It is further observed that the heat transfer in the cavity is improved with the increasing of solid volume fraction parameter of nanofluids at low Rayleigh number, but opposite effects appear when the Rayleigh number is high.
2005
Qiang Sun
Applied Mathematics and Computation 169 (2005) 355-365
An analytic technique, namely the homotopy analysis method, is applied to solve the nonlinear travelling waves governed by the Klein–Gordon equation. The phase speed and the solution, which are dependent on the amplitude a, are given and valid in the whole region of a.