A Numerically Efficient Damping Model for Acoustic Resonances in Microfluidic Cavities

AbstractAcoustofluidic damping is a crucial factor that limits the attainable acoustic amplitudes and therefore the effectiveness of acoustofluidic devices. It can be traced back to viscous and thermal dissipation in the bulk and in the boundary layers at cavity walls or suspended particles. However, numerical 3D simulations that include all relevant physics are prohibitively expensive since the acoustic boundary layers need to be resolved. We present a way to incorporate the dissipation effects into a synthetic acoustofluidic loss factor for the use in 3D device simulations. It comes at minimum numerical cost since boundary layers are resolved analytically. Our results and the validity of the physical assumptions we make in the derivation have been verified by analytical and numerical reference solutions. The acoustofluidic loss factor is easily incorporated in device models for a numerically feasible and quantitatively accurate prediction of acoustic amplitudes. In this sense, our work represents the missing link that allows to make not only qualitative but also quantitative predictions of acoustofluidic forces in realistic 3D devices

Strategies for single particle manipulation using acoustic radiation forces and external tools

AbstractThe use of primary acoustic radiation forces has been shown to be a valid technique for the handling of micron sized suspended particles, such as beads or biological cells. These forces arise as a nonlinear effect when an acoustic wave or vibration, which is set up in the fluid by exciting to resonance the system containing the suspension, interacts with the particles. The typical frequencies (upper kHz - lower MHz range) and the periodicity (in the range of hundreds of micrometers) of the acoustic field make this technique particularly suited for the handling of particles within microfluidic systems.A variety of devices for separation, fractionation, trapping and positioning of beads or biological cells, working both in batch or fluid flow mode, have been proposed. With the exception of the ports used to inject or remove the sample or the carrier medium, these systems can be considered as closed systems. Nevertheless, access to the particles with external tools is sometimes needed after acoustic manipulation has been performed. For instance, particles or cells pre-positioned in a sequence along the centerline of a channel using acoustic radiation forces need to be removed from it using a microgripper for further handling. Furthermore, in the field of crystallography research protein crystals have to be placed one by one onto a nylon loop prior to X-ray analysis with synchrotron radiation. This is usually done using the loop to pick up the crystal from the solution where it has been growing with other ones. As this process is sometimes repeated for a large number of crystals there are efforts to automate it. To this purpose it would be advantageous to bring the crystals spatially separated into a known position where they than can be sequentially collected with the loop.Here strategies for single particle manipulation are presented combining the effects of acoustic fields, fluid flow, surface tension and external tools. They are discussed by means of numerical results from FE-simulations of both two and three dimensional models as well as corresponding experiments

Acoustic wood tomography on trees and the challenge of wood heterogeneity

The assessment of tree stability requires information about the location and the geometry of fungal decay or of a cavity in the interior of the trunk. This work aims at specifying which size of decay or cavity can be detected non-destructively by acoustic wood tomography. In the present work, the elastic waves that propagate in a trunk during a tomographic measurement were visualized by numerical simulations. The numerical model enabled to systematically investigate the influence of fungal decay on tomographic measurements neglecting the heterogeneity of wood. The influence of wood heterogeneity was studied in laboratory experiments on trunks. The experiments indicated that the waveforms of the measured signals are by far more sensitive to the natural heterogeneity of trunk wood than the travel times, thereby making waveforms unsuitable for decay detection. Thus, it is recommended to further develop the travel time inversion algorithms for trunks and to neglect the information in waveforms or amplitudes. Fungal decay is detectable if the influence of the decay is distinguishable from the influence of the heterogeneity. It was found from the numerical analysis that the cross-section of a cavity, which is larger than 5% of the total cross-section of the trunk, can be detected by acoustic wood tomograph

Direct 2D measurement of time-averaged forces and pressure amplitudes in acoustophoretic devices using optical trapping

Ultrasonic standing waves are increasingly applied in the manipulation and sorting of micrometer-sized particles in microfluidic cells. To optimize the performance of such devices, it is essential to know the exact forces that the particles experience in the acoustic wave. Although much progress has been made via analytical and numerical modeling, the reliability of these methods relies strongly on the assumptions used, e.g. the boundary conditions. Here, we have combined an acoustic flow cell with an optical laser trap to directly measure the force on a single spherical particle in two dimensions. While performing ultrasonic frequency scans, we measured the time-averaged forces on single particles that were moved with the laser trap through the microfluidic cell. The cell including piezoelectric transducers was modeled with finite element methods. We found that the experimentally obtained forces and the derived pressure fields confirm the predictions from theory and modeling. This novel approach can now be readily expanded to other particle, chamber, and fluid regimes and opens up the possibility of studying the effects of the presence of boundaries, acoustic streaming, and non-linear fluids.ISSN:1473-0197ISSN:1473-018

DETERMINATION OF TURNING PARAMETERS IN CARVED SKIING AND APPLICATION TO A NUMERICAL SKI-BINDING MODEL

Skiing has regained in popularity after the introduction of the caNing technique. The biomechanics of caNing have been investigated in numerous studies. However, a comprehensive study of the behaviour of the ski/binding system taking into account the interactions between athlete, skiing equipment, and snow is still missing. In a first phase of the current study, the forces acting between skier and ski equipment and the evolution of the edging angle during caNing were determined using video analysis and force measurements. Next, the passive snow resistance to a penetrating ski was determined using two specially designed tools. Finally, the determined quantities seNed as boundary conditions for a finite-element simulation of the ski/binding system in the caNing situation. Calculated ski shapes were compared against measured turn radii and good agreement was found. The implemented model is intended to help in the development of improved ski equipment. As such, it can for example be used to study the effect of different skier's actions on the equipment behaviour

Phonon attenuation in the GHz regime : Measurements and simulations with a visco-elastic material model

Aluminum and PMMA thin film samples are investigated regarding their mechanical properties like speed of sound and attenuation. Aluminum is often used as a transducer layer for pump probe laser measurements and different PMMA types have a large importance in the nanoimprinting technique. The measurements are performed on a short pulse laser pump probe setup, where bulk wave packets in the GHz regime are excited and detected using near infrared laser pulses of less than 100 fs duration. This contact-free and non-destructive measurement method is explained. In order to extract the attenuation precisely from the measurements, the entire experimental setup is simulated numerically: The heat distribution and the thermo-elastic wave excitation caused by the laser pulse, the mechanical wave propagation, and the photo-acoustic detection. By means of the visco-elastic modeling of the wave propagation, the simulations are fitted to the measurements by tuning the attenuation parameters in the numerical model. In this way it is possible to extract the attenuation from the measurements. First, two different types of Aluminum on a sapphire substrate are analyzed: Electron beam evaporated Aluminum and sputtered Aluminum, respectively. The thicknesses of the Aluminum films are in the range of 300 nm. It turns out that the attenuation is much higher in the sputtered Aluminum film. Afterwards, PMMA thin films used for nanoimprinting with thicknesses between 300 and 600 nm are analyzed. The PMMA thin films are spincoated onto a Silicon wafer and covered with an Aluminum transducer layer. The very good agreement between the measurements and simulations of the stacked samples allows a reliable determination of the attenuation in the PMMA films in the GHz regime

Rotation of fibers and other non-spherical particles by the acoustic radiation torque

This study is aimed at the theoretical analysis of the acoustic radiation torque and the experimental realization of a controlled rotation of non-spherical particles by ultrasound. A finite element model has been developed and validated to calculate the acoustic radiation torque on a microfiber. The influence of different parameters such as the frequency, fiber size and position in the acoustic field are evaluated. The rotational motion of a non-spherical particle and the resulting drag torque are analyzed as well. This allows for the calculation of the angular velocity of a fiber. Various rotation methods for non-spherical particles with the acoustic radiation torque have been developed, tested experimentally with a microdevice at frequencies in the MHz range and compared to each other. The first method relies on successive change of the wave propagation direction in discrete steps. Three additional rotation methods have been developed which allow for a continuous rotation and alignment at defined orientations. The methods are characterized by the modulation of one single parameter (amplitude, phase or frequency) over time