Dirk Möller

Acoustofluidics 6: Experimental characterization of ultrasonic particle manipulation devices

Because of uncertainties in material and geometrical parameters in ultrasonic devices, experimental characterization is an indispensable part in their successful application for the manipulation of particles or cells. Its miniaturized size precludes the use of many of the usual tools used for macroscopic systems. Also, a further challenge is the fact that the resulting motion due to the electromechanical actuation has both high frequency and small amplitudes. Contactless methods like laser interferometry are therefore promising methods. In addition, as long as there is strong electromechanical coupling between the transducer and the device also electrical measurements like admittance curves give insight into the frequencies at which the devices might work best. This is the case for example for piezoelectric transducers working at one of their resonance frequencies. Because the devices usually are used in resonant modes, narrow frequency detection methods like lock in amplifiers help to improve the signal to noise ratio. Also many analysis tools have been established in the context of modal analysis, which is based on frequency domain methods. Special emphasis is placed here on the determination of the quality factor Q of the resonator, as Q determines the efficiency of a device.

Multiphysics modelling of the separation of suspended particles via frequency ramping of ultrasonic standing waves

A model was developed to determine the local changes of concentration of particles and the formations of bands induced by a standing acoustic wave field subjected to a sawtooth frequency ramping pattern. The mass transport equation was modified to incorporate the effect of acoustic forces on the concentration of particles. This was achieved by balancing the forces acting on particles. The frequency ramping was implemented as a parametric sweep for the time harmonic frequency response in time steps of 0.1s. The physics phenomena of piezoelectricity, acoustic fields and diffusion of particles were coupled and solved in COMSOL Multiphysics™ (COMSOL AB, Stockholm, Sweden) following a three step approach. The first step solves the governing partial differential equations describing the acoustic field by assuming that the pressure field achieves a pseudo steady state. In the second step, the acoustic radiation force is calculated from the pressure field. The final step allows calculating the locally changing concentration of particles as a function of time by solving the modified equation of particle transport. The diffusivity was calculated as function of concentration following the Garg and Ruthven equation which describes the steep increase of diffusivity when the concentration approaches saturation. However, it was found that this steep increase creates numerical instabilities at high voltages (in the piezoelectricity equations) and high initial particle concentration. The model was simplified to a pseudo one-dimensional case due to computation power limitations. The predicted particle distribution calculated with the model is in good agreement with the experimental data as it follows accurately the movement of the bands in the centre of the chamber.

Towards the automation of micron-sized particle handling by use of acoustic manipulation assisted by microfluidics.

The use of acoustic radiation forces for the manipulation and positioning of micrometer sized particles has shown to be a promising approach. Resonant excitation of a system containing a particle laden fluid filled cavity, can (depending on the mode excited) result in positioning of the particles in parallel lines (1-D) or distinct clumps in a grid formation (2-D) due to the high amplitude standing pressure fields that arise in the fluid. In a broader context, the alignment of particles using acoustic forces can be used to assist manipulation processes which utilise an external mechanical tool, for instance a microgripper. In such a system, particles can be removed sequentially from a line formed by acoustic forces within a microfluidic channel, hence allowing a degree of automation. In order to fully automate the gripping process, the particles must be confined to a repeatable and accurate location in two dimensions (assuming that in the third dimension they sit on the lower surface of the channel). Only in this way it is possible to remove subsequent particles by simply bringing the gripper to a known location and activating its fingers. This combined use of acoustic forces and mechanical gripping requires that one extremity of the channel is open. However, the presence of the liquid-air interface which occurs at this opening, causes the standing pressure field to decay to zero towards the opening. In a volume of liquid in proximity to the interface positioning of particles by acoustic forces is therefore no longer possible. In addition, the longitudinal gradient of the field can cause a drift of particles towards the longitudinal center of the channel at some frequencies, undesirably moving them further away from the interface, and so further from the gripper. As a solution the use of microfluidic flow induced drag forces in addition to the acoustic force potential has been investigated.

Schlieren visualization of ultrasonic standing waves in mm-sized chambers for ultrasonic particle manipulation.

BackgroundFor the design and characterization of ultrasonic particle manipulation devices the pressure field in the fluid cavity is of great interest. The schlieren method provides an optical tool for the visualization of such pressure fields. Due to its purely optical nature this experimental method has got some unique advantages compared to methods like particle tracking.ResultsA vertical schlieren setup and an investigation with the same of a mm-sized chamber used to agglomerate particles are presented here. The schlieren images show a two-dimensional representation of the whole pressure distribution recorded simultaneously with a good resolution in time. The gained description of the pressure field is shown to be in agreement with a numerical simulation. Thermal effects as well as streaming effects are shown.ConclusionsThe results show the great potential of schlieren visualization to investigate ultrasonic particle manipulation devices. Visualized are pressure fields, acoustic streaming, temperature effects and effects caused by fluid volumes of different density.

Novel sample preparation technique for protein crystal X-ray crystallographic analysis combining microfluidics and acoustic manipulation

In order to perform X-ray crystallographic analysis, protein crystals are removed from their growing solution by means of a nylon loop, which is then mounted on a goniometer. As this process is repeated for a large number of crystals, there is a need for automation, especially with regard to the placement on the nylon loop. A novel technique involving the use of acoustic radiation forces and a micro-machined fluidic device is introduced here. After insertion into the micro-machined channel, the crystals are positioned in a row along its centre-line by excitation of a high-frequency standing pressure field, and then moved towards an orifice by applying a flow along the channel, which also ensures spatial separation. Once located in a defined orifice, the single crystals can be removed using a nylon loop. X-ray crystallographic analysis showed that application of ultrasound does not influence the diffraction properties of the crystals.

Chapter 5:Piezoelectricity and Application to the Excitation of Acoustic Fields for Ultrasonic Particle Manipulation

Piezoelectric materials are widely used in the excitation of MHz frequency vibrations in devices for ultrasonic manipulation. An applied electrical voltage is transformed into mechanical stress, strain and displacement. Piezoelectric elements can be used in either a resonant or non-resonant manner. Depending on the desired motion, the piezoelectric longitudinal, transverse or shear effects are exploited. Because of the coupling between electrical and mechanical quantities in the constitutive law, the modelling of devices turns out to be quite complex. In this paper, the general equations that need to be used are delineated. For a one-dimensional actuator, the underlying physics is described, including the consequences resulting for the characterization of devices. For a practical setup used in ultrasonic manipulation, finite element models are used to model the complete system, including piezoelectric excitation, solid motion and acoustic field. It is shown how proper tailoring of transducer and electrodes allows selective excitation of desired modes.

Chapter 20:Experimental Characterization of Ultrasonic Particle Manipulation Devices

Because of uncertainties in the material and geometrical parameters of ultrasonic devices, experimental characterization is an indispensable part of their successful application for the manipulation of particles or cells. Their miniaturized size precludes the use of many of the usual tools used for macroscopic systems. Also, a further challenge is the fact that the resulting motion due to the electromechanical actuation has both high frequency and small amplitudes. Contactless methods such as laser interferometry and schlieren imaging are therefore promising methods. In addition, as long as there is strong electromechanical coupling between the transducer and the device, electrical measurements such as admittance curves give insight into the frequencies at which the devices might work best. This is the case, for example, for piezoelectric transducers working at one of their resonance frequencies. Because the devices are usually used in resonant modes, narrow frequency detection methods such as lock in amplifiers help to improve the signal to noise ratio. Also many analysis tools have been established in the context of modal analysis, which is based on frequency domain methods. Special emphasis is placed here on the determination of the quality factor Q of the resonator, as Q determines the efficiency of a device.