A. Grinenko

Independent trapping and manipulation of microparticles using dexterous acoustic tweezers

An electronically controlled acoustic tweezer was used to demonstrate two acoustic manipulation phenomena: superposition of Bessel functions to allow independent manipulation of multiple particles and the use of higher-order Bessel functions to trap particles in larger regions than is possible with first-order traps. The acoustic tweezers consist of a circular 64-element ultrasonic array operating at 2.35 MHz which generates ultrasonic pressure fields in a millimeter-scale fluid-filled chamber. The manipulation capabilities were demonstrated experimentally with 45 and 90-μm-diameter polystyrene spheres. These capabilities bring the dexterity of acoustic tweezers substantially closer to that of optical tweezers.

Dexterous manipulation of microparticles using Bessel-function acoustic pressure fields

We show that Bessel-function acoustic pressure fields can be used to trap and controllably position microparticles. A circular, 16-element ultrasound array generates and manipulates an acoustic field within a chamber, trapping microparticles and agglomerates. Changes in the phase of the sinusoidal signals applied to the array elements result in the movement of the Bessel-function pressure field and hence the microparticles. This demonstrates ultrasonic manipulation analogous to holographic optical tweezers. The manipulation limits of the device are explained by the existence of unwanted resonances within the manipulation chamber.

Efficient counter-propagating wave acoustic micro-particle manipulation

A simple acoustic system consisting of a pair of parallel singe layered piezoelectric transducers submerged in a fluid used to form standing waves by a superposition of two counter-propagating waves is reported. The nodal positions of the standing wave are controlled by applying a variable phase difference to the transducers. This system was used to manipulate polystyrene micro-beads trapped at the nodal positions of the standing wave. The demonstrated good manipulation capability of the system is based on a lowering of the reflection coefficient in a narrow frequency band near the through-thickness resonance of the transducer plates.

Proof of principle study of ultrasonic particle manipulation by a circular array device

A feasibility study of a circular ultrasonic array device for acoustic particle manipulation is presented. A general approach based on Greens function is developed to analyse the underlying properties of a circular acoustic array. It allows the size of a controllable device area as a function of the number of array elements to be established and the array excitation required to produce a desired field distribution to be determined. A set of quantitative parameters characterizing the complexity of the pressure landscape is suggested, and relation to the number of array elements is found. Next, a finite-element model of a physically realizable circular piezo-acoustic array device is employed to demonstrate that the trapping capability can be achieved in practice.

Acoustic radiation force analysis using finite difference time domain method

Acoustic radiation force exerted by standing waves on particles is analyzed using a finite difference time domain Lagrangian method. This method allows the acoustic radiation force to be obtained directly from the solution of nonlinear fluid equations, without any assumptions on size or geometry of the particles, boundary conditions, or acoustic field amplitude. The model converges to analytical results in the limit of small particle radii and low field amplitudes, where assumptions within the analytical models apply. Good agreement with analytical and numerical models based on solutions of linear scattering problems is observed for compressible particles, whereas some disagreement is detected when the compressibility of the particles decreases.

Tunable beam shaping with a phased array acousto-optic modulator

We demonstrate the generation of Bessel beams using an acousto-optic array based on a liquid filled cavity surrounded by a cylindrical multi-element ultrasound transducer array. Conversion of a Gaussian laser mode into a Bessel beam with tunable order and position is shown. Also higher-order Bessel beams up to the fourth order are successfully generated with experimental results very closely matching simulations.

Two-dimensional manipulation of microparticles using phase-controllable ultrasonic standing waves

The ability to trap, and then manipulate, micro-particles in a fluid, is of interest as a research tool in the biosciences. Applications include tissue engineering, particle sorting and improving alignment with bio-sensors. This paper relates to the use of phase-controllable counter-propagating ultrasonic waves to generate a standing wave with pressure nodes whose positions are determined by the relative phases of the component counter-propagating travelling waves. As dense (relative to the fluid) particles are forced to nodes in the pressure field this allows particles to be trapped at particular points and moved to arbitrary positions. Counter-propagating waves are generated using pairs of opposing transducers, matched and backed to minimise reflection. Using one pair of transducers allows particles to be trapped and manipulated in one dimension. Using two pairs of transducers, positioned orthogonally, and adjusting the relative phases appropriately, allows trapping and manipulation in two dimensions. T...