Acoustofluidics 9: Modelling and applications of planar resonant devices for acoustic particle manipulation

This article introduces the design, construction and applications of planar resonant devices for particle and cell manipulation. These systems rely on the pistonic action of a piezoelectric layer to generate a one dimensional axial variation in acoustic pressure through a system of acoustically tuned layers. The resulting acoustic standing wave is dominated by planar variations in pressure causing particles to migrate to planar pressure nodes (or antinodes depending on particle and fluid properties). The consequences of lateral variations in the fields are discussed, and rules for designing resonators with high energy density within the appropriate layer for a given drive voltage presente

A feasibility study on using inkjet technology, micropumps, and MEMs as fuel injectors for bipropellant rocket engines

Control over drop size distributions, injection rates, and geometrical distribution of fuel and oxidizer sprays in bi-propellant rocket engines has the potential to produce more efficient, more stable, less polluting rocket engines. This control also offers the potential of an engine that can be throttled, working efficiently over a wide range of output thrusts. Inkjet printing technologies, MEMS fuel atomizers, and piezoelectric injectors similar in concept to those used in diesel engines are considered for their potential to yield a new, more active injection scheme for a rocket engine. Inkjets are found to be unable to pump at sufficient pressures, and have possibly dangerous failure modes. Active injection is found to be feasible if high pressure drop along the injector plate are used. A conceptual design is presented and its basic behavior assessed

Acoustic focussing for sedimentation-free high-throughput imaging of microalgae

Microalgae play a key role in aquatic ecology, and methods providing species determination and enumeration can provide critical information about—for instance—harmful algae blooms (HABs) or spreading of invasive species. A crucial step in current methods is the use of sedimentation. This provides the enrichment needed to achieve statistical counts of sometimes rare species within reasonable timeframes, but it comes with the drawback of aggregating the sample. This is a real challenge for computer-aided identification as particle aggregates can often be erroneously classified. In this paper, we propose an alternative method based on flow-through imaging aided by acoustic-focussing, as this provides better input-data for automated counting-methods while simultaneously removing the need for manual sample preparation. We demonstrate that by acoustically focussing microalgae and other particulates in a fast-flowing water sample, it is possible to analyse up to 8 mL sample per minute with sufficient image quality to discriminate the invasive species Ostreopsis ovata from other particulates in samples taken directly from the Mediterranean. We also showcase the ability to achieve sharp images in flow-through at magnifications up to × 50

Effects of surface profile on a boundary-driven acoustic streaming field

Acoustic streaming fields in two-dimensional rectangular enclosures that have structured boundaries are simulated and the effects of surface profile amplitude on a boundary-driven acoustic streaming field are numerically investigated. The standing wave fields in the enclosures are generated by excitation of a boundary and a sine-wave shaped profile on a boundary parallel to the particle oscillations is considered. This surface profile is found to have a large influence on the magnitude of both outer and inner streaming velocities. In terms of streaming pattern, it is found that the number of inner streaming vortices is dependent on the wavelength of profile while this profile has a less significant effect on the outer vortex pattern

Numerical simulation of 3D acoustophoretic motion of microparticles in an acoustofluidic device

Acoustic streaming is typically found in addition to acoustic radiation forces in acoustofluidic devices. Simulation of acoustic streaming is a crucial step for the understanding of its origins, which can provide efficient guidance on creating designs to limit or control this phenomenon. However, most existing methods can only simulate the streaming field in a local area, typically a cross-section of fluid channel. In this work, the three-dimensional (3D) Rayleigh streaming pattern in an acoustofluidic device is simulated and its effects on the movement of microparticles with various sizes are demonstrated. The viability of the simulation of 3D Rayleigh streaming presented here not only can provide better understanding and more comprehensive prediction of experiments in full acoustofluidic devices, but also can offer instructions on the simulation of unusual acoustic streaming patterns, e.g. transducer-plane streamin

Effects of surface profile on a boundary-driven acoustic streaming field

Control of boundary-driven streaming in acoustofluidic systems is vital for various microfluidic applications either to generate it as a positive mechanism (e.g. microfluidic mixing, heat/mass transfer and fluid pumping) or suppressing it as an undesired disturbance (e.g. particle/cell focusing). It has been shown that two-dimensional (2D) and three-dimensional (3D) boundary-driven streaming can be solved from the limiting velocity method as long as the curvature of the surface is small compared to the viscous penetration depth. In this work, acoustic streaming fields in 2D rectangular enclosures that have structured textures, which do not satisfy this condition are numerically studied by full modelling of Reynolds stresses and the effects of surface profile amplitude on a boundary-driven acoustic streaming field are investigated. Specifically, a sine-wave shaped profile on a boundary parallel to the particle oscillations is considered, which is found to have large influences on both the magnitude of acoustic streaming velocities and streaming patterns

Modelling and control of acoustic streaming in standing wave fields

In acoustofluidic particle manipulation and sorting devices streaming flows are typically found in addition to the acoustic radiation forces. Understanding their origins is essential for creating designs to limit or control this phenomenon.In addition to the classical Rayleigh streaming, experimental work from various groups has described ‘unusual’ acoustic streaming, transducer-plane streaming, typically a four-quadrant streaming pattern with the circulation parallel to the transducer face. The cause of this kind of streaming pattern has not been previously explained as it is different from the well-known classical streaming patterns such as Rayleigh streaming[1] and Eckart streaming[2].In this work, both 3D Rayleigh streaming and transducer-plane streaming are investigated using both experimental and numerical methods. Furthermore, acoustic streaming field due to two orthogonal standing wave fields in a microfluidic device is simulated and analysed

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

Acoustofluidic particle steering

Steering micro-objects using acoustic radiation forces is challenging for several reasons: Resonators tend to create fixed force distributions that depend primarily on device geometry, and even when using switching schemes, the forces are hard to predict a priori. In this paper an active approach is developed that measures forces from a range of acoustic resonances during manipulation using a computer controlled feedback loop based in matlab, with a microscope camera for particle imaging. The arrangement uses a planar resonator where the axial radiation force is used to hold particles within a levitation plane. Manipulation is achieved by summing the levitation frequency with an algorithmically chosen second resonance frequency, which creates lateral forces derived from gradients in the kinetic energy density of the acoustic field. Apart from identifying likely resonances, the system does not require a priori knowledge of the structure of the acoustic force field created by each resonance. Manipulation of 10 μm microbeads is demonstrated over 100 s μm. Manipulation times are of order 10 s for paths of 200 μm length. The microfluidic device used in this work is a rectangular glass capillary with a 6 mm wide and 300 μm high fluid chamber.</p