Can a flat chip make selective acoustic tweezers?
A flat spiral-shaped transducer produces a tightly focused swirling sound beam that can grab and move one particle without disturbing its neighbours.
Source
Folding a focalized acoustical vortex on a flat holographic transducer: Miniaturized selective acoustical tweezers
Study at a glance
- Design
- Other — Device experiment: spiral electrodes on a lithium niobate substrate generate a 4.4 MHz focused acoustic vortex; the field is measured by laser vibrometry, compared with angular-spectrum simulations, and used to move particles in a microfluidic chamber.
- N
- No sample of subjects; demonstrations used monodisperse polystyrene particles, including a pattern built from 18 particles.
- Population
- Polystyrene microparticles (radius about 75 micrometres) in a microfluidic chamber
- Outcome
- Measured acoustic intensity and phase in the focal plane; decay of secondary ring intensity; ability to trap and move one particle independently of neighbours
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What they did
The authors derived the shape of electrodes that trace the lines of equal phase of a converging spherical acoustic vortex on a flat surface, in the spirit of a Fresnel lens, giving a spiral between an Archimedes and a Fermat spiral. They printed these electrodes on a piezoelectric chip, drove them at 4.4 MHz so that a vortex beam focused through a glass slide into a microfluidic chamber, and mapped the resulting field with a laser vibrometer. They compared the map with numerical wave-propagation predictions and then used the beam to pick up and move polystyrene particles.
What they found
The measured intensity and phase closely matched the simulations, and the radial profile followed that of a spherical vortex, whose secondary rings fade as one over distance squared rather than one over distance for a cylindrical vortex. This faster fall-off meant the outer rings were too weak to trap particles, so a single particle could be trapped and moved among others as long as they stayed outside the first bright ring. The device was used to arrange 18 particles into a letter pattern.
The limits
What it doesn't show
The paper does not demonstrate true three-dimensional trapping: the authors say this would require a large tank free of reflections, which they did not use. Only one frequency, one vortex order and one kind of particle (low-contrast polystyrene) were tested, and trapping forces were not measured quantitatively. Selectivity is limited by the first ring radius, which here was about 3.3 times the particle radius, so closely spaced particles could not be separated. The authors also hold a patent on the device.
Key terms
- Acoustical vortex
- A helical sound wave whose phase winds around a central line, giving zero pressure at the centre surrounded by a bright ring where particles can be trapped.
- Acoustic radiation force
- The steady force a sound wave exerts on an object because of momentum transfer; it scales with beam intensity.
- Interdigitated transducer (IDT)
- Metal electrodes on a piezoelectric substrate that convert an electrical signal into mechanical vibrations and thus sound waves.
- Fresnel lens principle
- Folding a curved phase surface onto a flat plane in steps so that a flat element can focus a wave.
- Selectivity
- The ability of tweezers to trap one chosen particle without also trapping or moving nearby ones.
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Quiz yourself
What principle lets a flat transducer focus a vortex beam?
Common questions
Why not just use a focused standing wave to trap particles?
Standing waves create many nodes that trap many particles at once, and many common particles and cells are pushed away from a focus, so they cannot pick out a single object.
Why does a spherical vortex give better selectivity than a cylindrical one?
Its outer rings weaken much faster with distance, so only the central trap is strong enough to hold a particle.
What makes the spiral electrodes focus the beam?
The spacing between turns shrinks with radius; an evenly spaced Archimedes spiral would only make a non-focused cylindrical vortex.
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