Effect of particle adhesion and interactions on motion by traveling waves on an electric curtain

A discrete-element/boundary-element method is developed for simulation of adhesive particle transport by traveling waves on an electric curtain. The study shows that both wall adhesion and particle–particle collisions have an important influence on particle transport on electric curtains at different wave frequencies. The most significant effect of particle collisions occurs for cases with medium frequencies in which particles with large negative-charge collect in high-concentration bands and move in a synchronous surfing mode, pushing forward particles with lower charge. Cases with higher and lower frequencies exhibited hopping motion, for which adhesion determines the range of non-transported particles.     

基于正交超声驻波场的粒子运动建模和控制

为了获得具有均匀的倒金字塔表面微结构的多晶硅,提出了一种新腐蚀加工方法。通过建立一个相互正交的超声驻波场,来协助硅的化学侵蚀并改善其对光的吸收能力。首先对硅片表面结构进行一些比较,在二维上建立网格状的数学模型。然后建立网格中粒子运动的控制理论模型,并进行仿真验证。最后设计实验用于检验超声驻波对粒子的运动控制效果。多晶硅侵蚀结果说明将超声驻波应用在网格化微加工上是可行、可靠的。     

基于正交超声驻波场的粒子运动建模和控制

为了获得具有均匀的倒金字塔表面微结构的多晶硅,提出了一种新腐蚀加工方法。通过建立一个相互正交的超声驻波场,来协助硅的化学侵蚀并改善其对光的吸收能力。首先对硅片表面结构进行一些比较,在二维上建立网格状的数学模型。然后建立网格中粒子运动的控制理论模型,并进行仿真验证。最后设计实验用于检验超声驻波对粒子的运动控制效果。多晶硅侵蚀结果说明将超声驻波应用在网格化微加工上是可行、可靠的。     

Particle separation in microfluidics using different modal ultrasonic standing waves

Microfluidic technology has great advantages in the precise manipulation of micro and nano particles, and the separation of micro and nano particles based on ultrasonic standing waves has attracted much attention for its high efficiency and simplicity of structure. This paper proposes a device that uses three modes of ultrasonic standing waves to continuously separate particles with positive acoustic contrast factor in microfluidics. Three modes of acoustic standing waves are used simultaneously in different parts of the microchannel. According to the different acoustic radiation force received by the particles, the particles are finally separated to the pressure node lines on both sides and the center of the microchannel. In this separation method, initial hydrodynamic focusing and satisfying various equilibrium constraints during the separation process are the key. Through numerical simulation, the resonance frequency of the interdigital transducer, the distribution of sound pressure in the liquid, and the relationship between the interdigital electrode voltage and the output sound pressure are obtained. Finally, the entire separation process in the microchannel was simulated, and the separation of the two particles was successfully achieved. This work has laid a certain theoretical foundation for the rapid diagnosis of diseases in practical applications.     

Novel wedge-shaped doubly inclined chamber for the flow-through separation of suspended particles

Combined standing and propagating wave modes have previously been successfully used by the authors for simultaneous agglomeration and transportation of small particles suspended in still water. The present study of this method with flowing water, using a 120 x 350 x 13 mm3 agglomerator, confirmed that the proposed method is applicable with suspended SiO2 particles of varying size. The process was found to be most efficient at flow rates below a certain threshold, which varies with particle size. This threshold was found to be 5 ml/s for a particle size of 7.9 microns at an ultrasonic frequency of 2 MHz corresponding to 0.74 mm wavelength. The existence of a maximum particle transport velocity was demonstrated.     

Raman scattering from gas-evaporated silicon small particles

Raman scattering measurements are reported on silicon small particles prepared by gas-evaporation technique. The crystalline structure is also observed for the sample having 70 A particles in average size. Four resolved component modes with Gaussian distribution function are identified with the three usual modes (LA, TO and allowed-TO) and a new surface mode. The surface mode of silicon particle, whose relative integrated intensity decrease with an increase of the particle size, is presented for the first time.     

How waves affect the distribution of particles that float on a liquid surface

We study experimentally how waves affect the distribution of particles that float on a liquid surface. We show that clustering of small particles in a standing wave is a nonlinear effect with the clustering time decreasing as the square of the wave amplitude. In a set of random waves, we show that small floaters concentrate on a multifractal set with caustics.     

Visualization of saltating sand particle movement near a flat ground surface

Movement of natural sand particles (d=200−300 μm) in a simulated atmospheric boundary layer was visualized using a digital high-speed camera. The consecutive particle images recorded at 2000 fps (frame per second) enabled us to observe the particle transport in detail, especially near the flat sand surface. Various modes of sand saltation were identified. The transverse motion of particles, often ignored in previous studies, was also visualized. In addition, instantaneous velocity fields of saltating particles were obtained using a particle tracking velocimetry (PTV), and statistical analysis of saltating particle trajectories was performed. The qualitative and quantitative results of the present study will be useful for understanding the basic physics of transport of saltating sand particles.     

不同振动模式下颗粒分离行为的数值模拟

利用三维离散元法对垂直方向上的直线、圆和椭圆振动模式颗粒分离过程进行了数值模拟研究,对直线振动时上层大颗粒的波动及圆和椭圆振动时出现的聚集、循环等现象的形成机理进行了分析,并讨论了振动强度对各振动模式下颗粒分离形态的影响规律. 研究表明,综合运用空隙填充、侧面驱动的颗粒运动和能量非均匀分布三种机理,并结合颗粒群的速度矢量分布情况能够较好地解释各振动模式下的颗粒分离行为. 振动强度对圆和椭圆振动模式的分离形态具有显著的影响,并在振动强度约为3时,各种振动模式均具有良好的颗粒分离效果和稳定的颗粒运动状态.     

不同振动模式下颗粒分离行为的数值模拟

利用三维离散元法对垂直方向上的直线、圆和椭圆振动模式颗粒分离过程进行了数值模拟研究,对直线振动时上层大颗粒的波动及圆和椭圆振动时出现的聚集、循环等现象的形成机理进行了分析,并讨论了振动强度对各振动模式下颗粒分离形态的影响规律. 研究表明,综合运用空隙填充、侧面驱动的颗粒运动和能量非均匀分布三种机理,并结合颗粒群的速度矢量分布情况能够较好地解释各振动模式下的颗粒分离行为. 振动强度对圆和椭圆振动模式的分离形态具有显著的影响,并在振动强度约为3时,各种振动模式均具有良好的颗粒分离效果和稳定的颗粒运动状态. 关键词： 振动模式 颗粒分离 离散元法 数值模拟     

Nodal patterns of floaters in surface waves

We argue theoretically and demonstrate experimentally that in a standing wave floating particles drift towards the nodes or anti-nodes depending on their hydrophilic or hydrophobic properties. We explain this effect as the breakdown of Archimedes' law by a surface tension, which creates a difference between the masses of the floater and displaced liquid, making the particle effectively inertial. We describe analytically the motion of a small floating particle in a small-amplitude wave and show that the drift appears as a second order effect in wave amplitude. We confirm experimentally that indeed the clustering rate is proportional to the square of the wave amplitude. In the case of surface random waves we show experimentally that the inertial effects significantly change the statistics of floater distribution on a liquid surface. The analysis of particle concentration moments and probability distribution functions shows that particle concentrate on a multi-fractal set with caustics.     

Destabilization of TAE modes by particle anisotropy

Plasmas heated by ICRF produce energetic particle distribution functions which are sharply peaked in pitch-angle. At moderate toroidal mode numbers, this anisotropy is the dominant instability drive when compared with the universal instability drive due to the spatial gradient. The universal drive, acting alone, destabilizes only co-propagating waves (i.e., waves propagating in the same toroidal direction as the diamagnetic flow of the energetic particles), but stabilizes counter-propagating waves (i.e., waves propagating in the toroidal direction opposite to that of the diamagnetic flow of the energetic particles). Nonetheless, in a tokamak, it is possible that particle anisotropy can produce a larger linear growth rate for counter-propagating waves, and provide a mechanism for preferred destabilization of the counter-propagating TAE modes that are sometimes experimentally observed.     

Hopping transport in two-dimensional systems with spin-orbit interaction in external magnetic field

The theory of spin rotation waves (SRWs), representing excitations of a new type arising in twodimensional systems with spin-orbit interaction in an external electric field, has been developed. These intrinsic modes correspond to rotation of the magnetic moment vector in the plane formed by the electric field vector and the normal to the sample plate surface. An experimental method is proposed for detecting SRWs by measuring the frequency dependence of the magnetic susceptibility, which exhibits a resonance at the intrinsic mode frequency. A particular calculation is performed for a hopping conductivity model (for small-size polarons), but it is likely that intrinsic oscillations of the SRW type also take place for the band transport, since their appearance is related to the symmetry of the system.     

On the Mutual Excitation of Embedded Circular Chains of Small Plasmonic Particles

To solve the problems of the repeated scattering of electromagnetic waves at ensembles of dielectric particles taking into account their conductivity, the interaction between collective optical modes of 2D embedded chains of small spherical plasmonic particles, which are characterized by electric dipole coupling, is analytically described on the basis of the quasi-separable T-scattering operator approach. It is assumed that the polarization vector of the electric field of an incident wave is oriented perpendicular to the plane of particle chains. The mutual influence of currents is demonstrated using a regular triangle whose vertices and center of symmetry contain particles and two embedded square chains with eight particles located at their vertices.     

Maximization of Acoustic Levitating Force for a Single-Axis Acoustic Levitation System Using the Finite Element Method

We investigate single-axis acoustic levitation using standing waves to levitate particles freely in a medium bounded by a driver and a reflector. The acoustic pressure at the pressure antinode of the standing wave counteracts the downward gravitational force of the levitating object. The optimal relationship between the air gap and the driving frequency leads to resonance and hence maximization of the levitating force. Slight deviation from the exact resonance condition causes a reduction in acoustic pressure at the pressure antinodes. This results in a significant reduction of the levitating force. The driving frequency is kept constant while the air gap is varied for different conditions. The optimal air gap for maximizing the levitation force is studied for first three resonance modes. Furthermore, a levitating particle is introduced between the driver and the reflector. The dependence of the resonance condition on the size of the levitating particle as well as the position of the particle between the driver and the reflector has also been studied. As the size of the levitating particle increases, the resonance condition also gets modified. Finite element results show a good agreement with the validated results available in the literature. Furthermore, the finite element approach is also used to study the variation of acoustic pressure at the pressure antinode with respect to the size of the reflector. The optimum diameter of the reflector is calculated for maximizing the levitating force for three resonance modes.     

Simple and high efficient optical trapping using a cylindrical lens and a single plane wave of incidence

We suggest a simple and high efficient method for trapping particles in the evanescent field. In this method, a single plane wave is normally incident on the cylindrical surface of a cylindrical lens and then incident on the plane surface of the lens at an angle larger than the critical angle. Multiple reflections of light within the cylindrical lens create two evanescent waves with different directions in the transmitted field. Interference of two evanescent waves comes into being a standing wave which can stably trap particles close to the top of the cylindrical lens. Based on the Rayleigh approximation, we obtain analytical expressions of optical force acting on a Rayleigh particle placed in the vicinity of the lens. We find that the trap stiffness and trap depth is dependent on the radius of the cylindrical lens, wavelength and polarization of light, and incident angle at the lens–liquid interface.     

Nonlinear effects in trapped modes of standing waves on the surface of shallow water

It was shown that traveling waves may coexist with standing waves in a planar infinitely long channel filled by ideal liquid with a free surface. The standing waves are localized near a dynamic inclusion—a massive die on an elastic base. The amplitude of the traveling waves may be turned to zero by appropriately selecting the vibration frequency of the die. The standing waves arise because the vibration eigenfrequencies have a mixed spectrum; that is, the discrete and continuous spectra superpose. Nonlinear effects were observed for the first time when standing waves form in shallow water. In particular, a relationship between the die weight necessary to excite trapped modes, die dimensions, and vibration frequency was derived. It was shown that the nonlinear effects cause double-frequency traveling waves with amplitudes of the next order of smallness. These traveling waves vanish if the die geometry is properly chosen, as for the waves of the zeroth order.     

Guided modes in a rectangular waveguide filled with single-negative materials

Based on classical electromagnetic theory, characteristics of guided modes in a rectangular waveguide filled with a pair of single-negative layers are studied. The results show that only surface waves of TE mode can propagate in this peculiar waveguide, no TM mode in any forms can propagate in it. In addition, TE waveguide modes will be affected by permeability ratio μ₁/μ₂ and dielectric layer thickness ratio P. Finally, from the electric field distribution of TE mode, we find the amplitude and location of the electric field can be adjusted by changing the thickness ratio P.     

The selection of layer thicknesses to control acoustic radiation force profiles in layered resonators

Ultrasonic standing waves can be used to generate radiation forces on particles within a fluid. A number of authors have derived detailed representations of these forces but these are most commonly applied using an approximation to the energy distribution based upon an idealized standing wave within a mode based upon rigid boundaries. An electro-acoustic model of the acoustic energy distribution within a standing wave with arbitrary thickness boundaries has been expanded to model the radiation force on an example particle within the acoustic field. This is used to examine the force profile on a particle at resonances other than those predicted with rigid boundaries, and with pressure nodes at different positions. A simple analytical method for predicting modal conditions for combinations of frequencies and layer thickness characteristics is presented, which predicts that resonances can exist that will produce a pressure node at arbitrary positions in the fluid layer of such a system. This can be used to design resonators that will drive particles to positions other than the center of the fluid layer, including the fluid/solid boundary of the layer, with significant potential applications in sensing systems. Further, the model also predicts conditions for multiple subwavelength resonances within the fluid layer of a single resonator, each resonance having different nodal planes for particle concentration.