Philipp Hahn

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.

On the Use of Meshless Methods in Acoustic Simulations

In this paper, we are investigating the potential and limits of a meshless Lagrangian technique, called Smoothed Particle Hydrodynamics (SPH), as a method for acoustic simulations. Currently the most common techniques for acoustic simulations draw on mesh-based methods such as the Boundary Element Method (BEM), Finite Differences Method (FD) and Finite Element Method (FEM). Though many improvements have been made to each class of methods during the last few years, they still have their weaknesses. Difficulties arise as soon as inhomogeneous media, moving boundaries or aeroacoustic effects are involved. These problems are either particularly hard to describe or cannot be simulated with some of these mesh-based methods. The investigation of SPH for modeling sound propagation is carried out in order to assess its potential in relation to the limitations associated with the existing simulation methods listed above. Simple computational experiments will be carried out for the verification of the new approach and applications on the problems listed above will be discussed.Copyright

A numerically efficient damping model for acoustic resonances in microfluidic cavities

Bulk acoustic wave devices are typically operated in a resonant state to achieve enhanced acoustic amplitudes and high acoustofluidic forces for the manipulation of microparticles. Among other loss mechanisms related to the structural parts of acoustofluidic devices, damping in the fluidic cavity is a crucial factor that limits the attainable acoustic amplitudes. In the analytical part of this study, we quantify all relevant loss mechanisms related to the fluid inside acoustofluidic micro-devices. Subsequently, a numerical analysis of the time-harmonic visco-acoustic and thermo-visco-acoustic equations is carried out to verify the analytical results for 2D and 3D examples. The damping results are fitted into the framework of classical linear acoustics to set up a numerically efficient device model. For this purpose, all damping effects are combined into an acoustofluidic loss factor. Since some components of the acoustofluidic loss factor depend on the acoustic mode shape in the fluid cavity, we propose a two-step simulation procedure. In the first step, the loss factors are deduced from the simulated mode shape. Subsequently, a second simulation is invoked, taking all losses into account. Owing to its computational efficiency, the presented numerical device model is of great relevance for the simulation of acoustofluidic particle manipulation by means of acoustic radiation forces or acoustic streaming. For the first time, accurate 3D simulations of realistic micro-devices for the quantitative prediction of pressure amplitudes and the related acoustofluidic forces become feasible.

Numerical simulation of micro-particle rotation by the acoustic viscous torque

We present the first numerical simulation setup for the calculation of the acoustic viscous torque on arbitrarily shaped micro-particles inside general acoustic fields. Under typical experimental conditions, the particle deformation plays a minor role. Therefore, the particle is modeled as a rigid body which is free to perform any time-harmonic and time-averaged translation and rotation. Applying a perturbation approach, the viscoacoustic field around the particle is resolved to obtain the time-averaged driving forces for a subsequent acoustic streaming simulation. For some acoustic fields, the near-boundary streaming around the fluid-suspended particle induces surface forces on the nonrotating particle that integrate into a non-zero acoustic viscous torque. In the equilibrium state, this torque is compensated by an equal and opposite drag torque due to the particle rotation. The rotation-induced flow field is superimposed on the acoustic streaming field to obtain the total fluid motion around the rotating particle. In this work, we only consider cases within the Rayleigh limit even though the presented numerical model is not strictly limited to this regime. After a validation by analytical solutions, the numerical model is applied to challenging experimental cases. For an arbitrary particle density, we consider particle sizes that can be comparable to the viscous boundary layer thickness. This important regime has not been studied before because it lies beyond the validity limits of the available analytical solutions. The detailed numerical analysis in this work predicts nonintuitive phenomena, including an inversion of the rotation direction. Our numerical model opens the door to explore a wide range of experimentally relevant cases, including non-spherical particle rotation. As a step toward application fields such as micro-robotics, the rotation of a prolate ellipsoid is studied.

Ultrasonic robotics in microfluidic cavities

Ultrasonic standing waves are often used in biomedical applications. It has become quite common to move beads, cells, droplets, and other particles for sorting or biomedical analysis in microfluidic cavities by bulk acoustic waves or by vibrations excited by piezoelectric transducers. The motion of particles is determined by streaming and radiation forces. For the calculation of the radiation forces acting on single particles Gorkov’s potential is considered to be the modeling tool of choice, once the acoustic field and the properties of constituents (fluid and particle density and compressibility, respectively) are known. For the acoustic streaming, predictions can be made numerically. For both aspects large uncertainties exist, due to the complexity of the system and fluid structure interaction at multiple levels. In this paper, first various characterization tools for the acoustic field in the cavity are described. They consist of the interplay between numerical modeling of the device, impedance analys...

Experimental and numerical acoustofluidics in bulk acoustic wave devices at ETH Zurich

Ultrasonic fluid cavity resonances in acoustofluidic micro-devices can be exploited to miniaturize important operations for the handling of beads, cells, droplets, and other particles. With a growing number of experimentally tested unit operations, acoustofluidics holds increasing promise for emerging applications in bio- and microtechnology on lab-on-a-chip systems. We provide an overview of our research activities during the last years with a focus on the latest experimental setups and advances in the numerical simulation. Specifically, we present micro-devices with impedance matched cavity walls that allow a more flexible device design. Further, we show devices for the handling of fluid droplets and report on a method for the direct measurement of the acoustic radiation force on micro-particles. Due to the rapidly growing computational capabilities, numerical simulation has become a valuable tool in acoustofluidics research. We present a numerical model that accurately mimics the boundary layer damping...

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.

A Parallel Boundary Element Algorithm for the Computation of the Acoustic Radiation Forces on Particles in Viscous Fluids

Ultrasonic fields can be used to manipulate particles in fluid suspensions by means of acoustic radiation forces. The physical cause of these forces is the inhomogeneity caused by the particles, resulting in wave scattering and ultimately distortion of the time-harmonic fields. Under certain assumptions, the radiation force can be deduced analytically which provides valuable understanding of the physical processes involved. However, in order to treat more complex situations, numerical solutions are needed. We describe how the Boundary Element Method (BEM) can be used to calculate radiation forces on arbitrarily shaped particles inside viscous fluids. Interactions with other particles or walls are naturally incorporated in these simulation. The details of the BEM algorithm as well as numerical parallelization approaches are discussed. A number of standard problems are solved in order to validate parts of the simulation code by comparison with either analytical solutions or a commercial Finite Element solver.

A novel device allowing for movement and trapping of particles within loop-shaped channels

Resonant excitation of a fluid cavity inside ultrasonic particle manipulation devices leads to standing waves inside the fluid. Acoustic radiation forces, caused by the nonlinear interaction between the time harmonic pressure field and a particle can be used to manipulate particles towards the nodal or anti-nodal planes of the acoustic pressure field. This allows the contactless handling of cells, bacteria or other particles, suggesting a wide range of applications in life science and medical engineering. Most ultrasonic manipulation devices described in the literature utilize reflections at fluid-structure interfaces which create the standing wave. At a given frequency, the nodal planes are fixed since their locations are governed by the geometry of the device. This reduces the suitability of the method for applications that require contactless particle transport over long distances or towards arbitrary positions. In order to overcome the described shortcoming, several methods have been proposed. In this work we introduce a new approach, leveraging circumferential resonances within a loop-shaped fluid waveguide in order to gain full one-dimensional control over the location of nodal planes. Limitations regarding the device geometry and the enclosure materials are discussed and it is described how the position or the velocity of nodal planes can be controlled via amplitude modulation applied on two transducers. Preliminary experimental results illustrate potential applications but they also reveal problems related to the current device design.