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 2.35MHz which generates ultrasonic pressure fields in a millimeter-scale fluid-filled chamber. The manipulation capabilities were demonstrated experimentally with 45 and 90-lm-diameter polystyrene spheres. These capabilities bring the dexterity of acoustic tweezers substantially closer to that of optical tweezers

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.

The ability to manipulate dense micrometre-scale objects in fluids is of interest to biosciences with a view to improving analysis techniques and enabling tissue engineering. A method of trapping micrometre-scale particles and manipulating them on a two-dimensional plane is proposed and demonstrated. Phase-controlled counter-propagating waves are used to generate ultrasonic standing waves with arbitrary nodal positions. The acoustic radiation force drives dense particles to pressure nodes. It is shown analytically that a series of point-like traps can be produced in a two-dimensional plane using two orthogonal pairs of counter-propagating waves. These traps can be manipulated by appropriate adjustment of the relative phases. Four 5 MHz transducers (designed to minimize reflection) are used as sources of counter-propagating waves in a water-filled cavity. Polystyrene beads of 10 μm diameter are trapped and manipulated. The relationship between trapped particle positions and the relative phases of the four transducers is measured and shown to agree with analytically derived expressions. The force available is measured by determining the response to a sudden change in field and found to be 30 pN, for a 30 V _pp input, which is in agreement with the predictions of models of the system. A scalable fabrication approach to producing devices is demonstrated.

A method of manipulating microparticles in a liquid using ultrasound is proposed and demonstrated. An ultrasonic standing wave with nodal planes whose positions are controllable by varying the relative phase of two applied sinusoidal signals is generated using a pair of acoustically matched piezoelectric transducers. The resulting acoustic radiation force is used to trap micron scale particles at a series of arbitrary positions (determined by the relative phase) and then move them in a controlled manner. This method is demonstrated experimentally and 5 μm polystyrene particles are trapped and moved in one dimension through 140 μm.