Mode-switching: A new technique for electronically varying the agglomeration position in an acoustic particle manipulator

Acoustic radiation forces offer a means of manipulating particles within a fluid. Much interest in recent years has focussed on the use of radiation forces in microfluidic (or “lab on a chip”) devices. Such devices are well matched to the use of ultrasonic standing waves in which the resonant dimensions of the chamber are smaller than the ultrasonic wavelength in use. However, such devices have typically been limited to moving particles to one or two predetermined planes, whose positions are determined by acoustic pressure nodes/anti-nodes set up in the ultrasonic standing wave. In most cases devices have been designed to move particles to either the centre or (more recently) the side of a flow channel using ultrasonic frequencies that produce a half or quarter wavelength over the channel, respectively.It is demonstrated here that by rapidly switching back and forth between half and quarter wavelength frequencies - mode-switching - a new agglomeration position is established that permits beads to be brought to any arbitrary point between the half and quarter-wave nodes. This new agglomeration position is effectively a position of stable equilibrium. This has many potential applications, particularly in cell sorting and manipulation. It should also enable precise control of agglomeration position to be maintained regardless of manufacturing tolerances, temperature variations, fluid medium characteristics and particle concentration.     

驻波声场中单分散细颗粒的相互作用特性

利用外加声场促进悬浮在气相中的细颗粒发生相互作用,进而引起颗粒的碰撞和凝并,使得颗粒平均粒径增大、数目浓度降低,是控制细颗粒排放的重要技术途径.为探究驻波声场中单分散细颗粒的相互作用,建立包含曳力、重力、声尾流效应的颗粒相互作用模型,采用四阶经典龙格-库塔算法和二阶隐式亚当斯插值算法对模型进行求解.将数值模拟得到的颗粒声波夹带速度和相互作用过程与相应的解析解和实验结果进行对比,验证模型的准确性.进而研究颗粒初始条件和直径对相互作用特性的影响.结果表明,初始时刻颗粒中心连线越接近声波波动方向、颗粒位置越接近波腹点,颗粒间的声尾流效应就越强,颗粒发生碰撞所需要的时间就越短.研究还发现,颗粒直径对颗粒相互作用的影响取决于初始时刻颗粒中心连线偏离声波波动方向的程度.当偏离较小时,颗粒直径越大,颗粒发生碰撞所需要的时间越短;当偏离很大时,直径较小的颗粒能够发生碰撞,而直径较大的颗粒则无法发生碰撞.     

A theoretical study of the feasibility of acoustical tweezers: ray acoustics approach

The optical tweezer has been found to have many biomedical applications in trapping macromolecules and cells. For the trapping mechanism, there has to be a sharp spatial change in axial optical intensity and the particle size must be much greater than the wavelength. Similar phenomenon may exist in acoustics. This work was undertaken to demonstrate theoretically that it is possible to acoustically trap particles near the focal point where most of the acoustic energy is concentrated if certain conditions are met. Acoustic force exerted on a fluid particle in ultrasonic fields is analyzed in a ray acoustics regime where the wavelength of acoustic beam is much smaller than the size of the particle. In order to apply the acoustical tweezer to manipulating macromolecules and cells whose size is in the order of a few microns or less, a prerequisite is that the ultrasound wavelength has to be much smaller than a few microns. In this paper, the analysis is therefore based on the field pattern produced by a strongly focused 100 MHz ultrasonic transducer with Gaussian intensity distribution. For the realization of acoustic trapping, negative axial radiation force has to be generated to pull a particle towards a focus. The fat particle considered for acoustic trapping in this paper has an acoustic impedance of 1.4 MRayls. The magnitude of the acoustic axial radiation force that has been calculated as the size of the fat particle is varied from 8lambda to 14lambda. In addition, both Fresnel coefficients at various positions are also calculated to assess the interaction of reflection and refraction and their relative contribution to the effect of the acoustical tweezer. The simulation results show that the feasibility of the acoustical tweezer depends on both the degree of acoustic impedance mismatch and the degree of focusing relative to the particle size.     

Numerical study of the synergistic effect of cavitation and micro-abrasive particles

In ultrasonic-assisted machining, the synergistic effect of the cavitation effect and micro-abrasive particles plays a crucial role. Studies have focused on the investigation of the micro-abrasive particles, cavitation micro-jets, and cavitation shock waves either individually or in pairs. To investigate the synergy of shock waves and micro-jets generated by cavitation with micro-abrasive particles in ultrasonic-assisted machining, the continuous control equations of a cavitation bubble, shock wave, micro-jet, and micro-abrasive particle influenced by the dimensionless amount (R/R₀), a particle size-velocity–pressure model of the micro-abrasive particle was established. The effects of ultrasonic frequency, sound pressure amplitude, and changes in particle size on micro-abrasive particle velocity and pressure were numerically simulated. At an ultrasonic frequency of 20 kHz and ultrasonic sound pressure of 0.1125 MPa, a smooth spherical SiO₂ micro-abrasive particle (size = 5 µm) was obtained, with a maximum velocity of 190.3–209.4 m/s and pressure of 79.69–89.41 MPa. The results show that in the range of 5–50 μm, smaller particle sizes of the micro-abrasive particles led to greater velocity and pressure. The shock waves, micro-jets, and micro-abrasive particles were all positively affected by the dimensionless amount (R/R₀) of cavitation bubble collapse, the larger the dimensionless quantity, the faster their velocity and the higher their pressure.     

Ultrasonic-trap-enhanced selectivity in capillary electrophoresis

We combine ultrasonic trapping and capillary electrophoresis (CE) with the goal to detect ultra-low concentrations of proteins via size-selective separation and enrichment of antibody-coated latex spheres. An 8.5 MHz standing ultrasonic wave is longitudinally coupled into the sub-100- micro m diam capillary of the CE system. Competition between acoustic and viscous forces result in in-flow separation of micro m-diam spheres according to their size. Experiments separating 2.8- and 2.1- micro m-diam fluorescent latex particles, which model a protein-specific immunocomplex/free particle mixture, indicate a potential improvement of the concentration limit of detection of 10(4) compared to current CE systems. Theoretical calculations show room for further improvement.     

Modelling of particle paths passing through an ultrasonic standing wave

Within an ultrasonic standing wave particles experience acoustic radiation forces causing agglomeration at the nodal planes of the wave. The technique can be used to agglomerate, suspend, or manipulate particles within a flow. To control agglomeration rate it is important to balance forces on the particles and, in the case where a fluid/particle mix flows across the applied acoustic field, it is also necessary to optimise fluid flow rate. To investigate the acoustic and fluid forces in such a system a particle model has been developed, extending an earlier model used to characterise the 1-dimensional field in a layered resonator. In order to simulate fluid drag forces, CFD software has been used to determine the velocity profile of the fluid/particle mix passing through the acoustic device. The profile is then incorporated into a MATLAB model. Based on particle force components, a numerical approach has been used to determine particle paths. Using particle coordinates, both particle concentration across the fluid channel and concentration through multiple outlets are calculated. Such an approach has been used to analyse the operation of a microfluidic flow-through separator, which uses a half wavelength standing wave across the main channel of the device. This causes particles to converge near the axial plane of the channel, delivering high and low particle concentrated flow through two outlets, respectively. By extending the model to analyse particle separation over a frequency range, it is possible to identify the resonant frequencies of the device and associated separation performance. This approach will also be used to improve the geometric design of the microengineered fluid channels, where the particle model can determine the limiting fluid flow rate for separation to occur, the value of which is then applied to a CFD model of the device geometry.     

Electrostatic Charge Measurement and Charge Neutralization of Fine Aerosol Particles during the Generation Process

An aerosol charge analyzer has been constructed to measure the charge distribution of NaCl particles generated in the laboratory. A radioactive electrostatic charge neutralizer utilizing Po‐210 was used to neutralize the electrostatic charge of the particles. The atomization technique was used to generate NaCl particles with diameters of 0.2 to 0.8 μm, while the evaporation and condensation method was adopted to generate particles of 0.01 to 0.2 μm in diameter. The experimental data demonstrates that the absolute average particle charge depends on the particle diameter, and is higher than that calculated by the Boltzmann charge equilibrium for particles within the range of 0.2 to 0.8 μm. The charge increases with decreasing NaCl concentration. When these particles are neutralized using the Po‐210 neutralizer, it is found that the electrostatic charge reaches the Boltzmann charge equilibrium. For 0.01 to 0.2 μm NaCl particles generated using the evaporation and condensation method, test results show that the absolute average particle charge is higher than that calculated by the Boltzmann charge equilibrium for particles larger than 0.03 to 0.05 μm in diameter, while it is lower than that predicted by the Fuchs theory [1], for particles smaller than 0.03 to 0.05 μm. However, after charge neutralization, particles with diameter above 0.05 μm reach the Boltzmann charge equilibrium condition, and the charges for particles with diameters of 0.010 to 0.05 μm, agree well with Fuchs' theory.     

Analysis of spectra of acoustic signals generated by high-power pulsed laser radiation propagating in the atmosphere. 2. Spectra of acoustic signals from laser sparks

Results of theoretical and experimental investigations into the spectra of acoustic signals generated by high-power pulsed laser radiation propagating in the atmosphere in the breakdown regime are presented. Results of analogous investigations for laser breakdown on a single aerosol particle and ensemble of monodispersed particles versus the particle material and size were published in [1]. In the present work, acoustic spectra of discrete laser sparks in the atmosphere are analyzed, and their special features in comparison with acoustic spectra of individual plasma formations, spectra of acoustic signals generated by pulsed laser radiation propagating in the atmosphere in thermooptical regime, and laboratory spectra of continuous laser sparks are discussed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 26–31, December, 2007.     

The role of the particle size reduction and morphological changes of solid substrate in the ultrasound-aided enzymatic hydrolysis of cellulose

The contribution of ultrasound-aided particle size reduction to the efficiency of the subsequent enzymatic hydrolysis and the accompanying morphological changes of bleached cotton and linen powders were investigated. The aqueous suspensions of cellulosic powders were pretreated either with an ultrasonic bath (US-B) or with a horn-type reactor (US-H). Results revealed that the impact of US-H was more pronounced than that of the US-B. Clearly, the linen particles were more sensitive to ultrasonication than cotton. The US-H modified the particle size distribution differently for the cotton and linen powders and reduced the mean size of particles from 49 to 40 µm and from 123 to 63 µm, respectively. A significant increase in the water retention and water sorption capacity was also measured. The smaller particles with increased accessibility were preferably digested in the enzyme treatment, resulting in a considerably higher concentration of reducing sugars and an enrichment of the residual particles with a larger average size (cotton: 47 µm; linen: 66 µm).     

Trapping of microparticles in the near field of an ultrasonic transducer

We are investigating means of handling microparticles in microfluidic systems, in particular localized acoustic trapping of microparticles in a flow-through device. Standing ultrasonic waves were generated across a microfluidic channel by ultrasonic microtransducers integrated in one of the channel walls. Particles in a fluid passing a transducer were drawn to pressure minima in the acoustic field, thereby being trapped and confined at the lateral position of the transducer. The spatial distribution of trapped particles was evaluated and compared with calculated acoustic intensity distributions. The particle trapping was found to be strongly affected by near field pressure variations due to diffraction effects associated with the finite sized transducer element. Since laterally confining radiation forces are proportional to gradients in the acoustic energy density, these near field pressure variations may be used to get strong trapping forces, thus increasing the lateral trapping efficiency of the device. In the experiments, particles were successfully trapped in linear fluid flow rates up to 1mm/s. It is anticipated that acoustic trapping using integrated transducers can be exploited in miniaturised total chemical analysis systems (microTAS), where e.g. microbeads with immobilised antibodies can be trapped in arrays and subjected to minute amounts of sample followed by a reaction, detected using fluorescence.