Acoustic levitation represents a technique for suspending material in an aerial medium, counteracting gravitational forces through the application of acoustic radiation pressure generated by high-intensity sound waves.
This phenomenon operates on principles analogous to those employed by acoustic tweezers, leveraging acoustic radiation forces. Nevertheless, acoustic tweezers typically function as small-scale instruments within a fluid environment, where gravitational effects are less pronounced, while acoustic levitation fundamentally aims to counteract gravity. From a technical standpoint, dynamic acoustic levitation can be categorized as a form of acoustophoresis, although this designation is more frequently linked with miniature acoustic tweezers.
Acoustic levitation commonly employs ultrasonic frequencies, rendering the generated sound inaudible to human perception. This practice is primarily necessitated by the substantial sound intensity required to overcome gravitational pull. Despite this, instances of audible frequencies being utilized have been documented.
While multiple methodologies exist for sound generation, the predominant approach involves employing piezoelectric transducers, which are capable of efficiently producing high-amplitude outputs at specified frequencies.
Levitation presents a promising technique for the containerless processing of microchips and other diminutive, fragile components within industrial settings. Containerless processing also finds utility in applications demanding exceptionally high-purity materials or facilitating chemical reactions too aggressive for conventional containment. Although this method poses greater control challenges compared to alternatives like electromagnetic levitation, it offers the distinct benefit of enabling the levitation of non-conductive substances.
Initially static, acoustic levitation has evolved from stationary suspension to encompass the dynamic manipulation of hovering objects, a capability proving valuable in the pharmaceutical and electronics sectors. The initial realization of this dynamic control involved a prototype featuring a chessboard-like arrangement of square acoustic emitters. This system facilitated object movement between squares by progressively reducing the sound intensity from one emitter while simultaneously augmenting it from an adjacent one, thereby enabling the object's virtual 'descent.' Subsequent advancements, particularly the development of phased array transducer boards, have enabled more versatile dynamic control over multiple particles and droplets concurrently.
Recent technological progress has also led to a substantial reduction in the cost associated with this technology. The 'TinyLev' exemplifies this trend, representing an acoustic levitator that can be assembled using readily available, inexpensive off-the-shelf components and a singular 3D-printed frame.
History
Experimental Development
The initial demonstration of acoustic levitation's feasibility occurred during Kundt's Tube experiments in 1866. This experimental setup, conducted within a resonant chamber, illustrated that particles could be aggregated at the nodes of a standing wave through the action of acoustic radiation forces. Nevertheless, the primary objective of the original experiment was to determine wavelengths and, consequently, the speed of sound within a gaseous medium.
The inaugural instance of levitation was demonstrated by Bücks and Muller in 1933, who successfully suspended alcohol droplets between a quartz crystal and a reflector. Subsequent progress was achieved by Hilary St Clair, whose interest in acoustic radiation forces stemmed primarily from their potential application in the agglomeration of dust particles for mining operations. St Clair developed the first electromagnetic apparatus capable of generating the requisite excitation amplitudes for levitation, subsequently achieving the suspension of larger and heavier objects, such as a coin.
Taylor Wang led a research team that extensively utilized acoustic radiation forces as a containment strategy in zero-gravity environments. This team deployed a specialized device aboard the Space Shuttle Challenger mission STS-51-B to examine the behavior of levitated droplets under micro-gravitational conditions. Additional experiments were subsequently performed in 1992 on the United States Microgravity Laboratory 1 (USML-1) and in 1995 on USML-2.
From the 1970s until 2017, the Langevin Horn, comprising a piezoelectric actuator, a metal transmitter, and a reflector, represented the predominant acoustic levitation device. This design, however, necessitated precise calibration of the distance between the transmitter and the reflector, as this separation had to correspond to an exact multiple of the sound wavelength. Such calibration proved challenging because the wavelength fluctuates with the speed of sound, which itself is influenced by environmental variables like temperature and altitude. These devices facilitated substantial research, including investigations into contactless chemistry and the levitation of small biological specimens. Furthermore, multiple Langevin Horns were integrated to achieve continuous planar motion by modulating the sound intensity: decreasing it from one source while augmenting it from an adjacent one, thereby enabling particles to traverse "downhill" within the acoustic potential field.
Recently, a new generation of acoustic levitators, characterized by their use of numerous small, individual piezoelectric transducers, has gained prominence. The inaugural device in this category was the "TinyLev," a single-axis, multi-emitter levitator developed in 2017 by Asier Marzo, Adrian Barnes, and Bruce Drinkwater at the University of Bristol. Key distinctions from the Langevin Horn included the deployment of sound sources from both the top and bottom (instead of a single source and a reflector) and the incorporation of numerous small transducers with parallel excitation, as opposed to a solitary piezoelectric element. This configuration, utilizing two opposing traveling waves instead of a single source and a reflector, allowed for stable levitation even when the vertical separation did not precisely correspond to a multiple of the wavelength. While initially conceived as a cost-reduction strategy, the adoption of multiple small sources also facilitated the development of phased array levitation. Furthermore, the integration of 3D-printed components for the transducer positioning and focusing frame, alongside Arduinos for signal generation, substantially lowered costs and enhanced accessibility. This cost reduction was crucial, aligning with the device's primary objective of democratizing the technology.
This innovative methodology also spurred substantial advancements in levitation techniques employing Phased Array Ultrasonic Transducers (commonly abbreviated as PATs). PATs comprise an assembly of ultrasonic speakers precisely controlled to generate a specific, unified sound field. This control is accomplished by manipulating the relative phase (or delay time) between individual outputs, and occasionally by adjusting their relative magnitudes. In contrast to arrays utilized in non-destructive testing or imaging applications, these levitation arrays operate with a continuous output rather than discrete energy bursts. This continuous operation has enabled both single-sided levitation and the simultaneous manipulation of numerous particles.
An increasingly prevalent method involves utilizing 3D-printed components to introduce the requisite phase delays for levitation, thereby achieving an effect analogous to PATs. This approach offers the advantage of superior spatial resolution compared to phased arrays, enabling the formation of more intricate acoustic fields. Such components are variously termed Acoustic Holograms, Metasurfaces, Delay lines, or Metamaterials. While terminology variations largely stem from the originating design discipline, the fundamental principle underlying all these techniques remains consistent. These components can also be integrated with PATs to achieve dynamic reconfigurability and enhanced sound field resolution. A further benefit is their cost-effectiveness, exemplified by the development of a low-cost ultrasonic tractor beam, for which instructional guides were published.
Despite the emergence of numerous novel manipulation techniques, Langevin Horns continue to be employed in scientific research. Their preference in studies concerning the dynamics of levitated objects stems from their geometric simplicity, which facilitates both simulation and precise control over experimental parameters.
Theoretical
Lord Rayleigh's early 20th-century work primarily focused on the theoretical forces and energy inherent in sound waves. The initial analysis of particles in an acoustic field was conducted by L.V. King in 1934, who calculated the force exerted on incompressible particles. Subsequently, Yosioka and Kawisama extended this research by calculating forces on compressible particles within plane acoustic waves. This progression culminated in Lev P. Gor'kov's generalization of the field into the Gor'kov potential, which remains the fundamental mathematical basis for acoustic levitation today.
The Gor'kov potential is constrained by its underlying assumptions, applying specifically to spheres with a radius significantly smaller than the wavelength, typically limited to one-tenth of the wavelength. While additional analytical solutions exist for simple geometries, extending the analysis to larger or non-spherical objects commonly necessitates the application of numerical methods, particularly the finite element method or the boundary element method. Furthermore, the radiation pressure of sound can be precisely managed through sub-wavelength patterning of an object's surface.
Types of Levitation
Acoustic levitation can be broadly categorized into five distinct types:
- Standing Wave Levitation: This technique traps particles at the nodes of a standing wave, which is generated either by a sound source paired with a reflector (as in the Langevin Horn) or by two independent sets of sources (as in the TinyLev). Its efficacy relies on particles being small relative to the wavelength, typically 10% or less, with a maximum levitated weight generally in the milligram range. Notably, if a particle is excessively small compared to the wavelength, its behavior alters, causing it to migrate to the anti-nodes. These levitators are typically single-axis, confining all particles along a central axis; however, the integration of Phased Array Transducers (PATs) enables dynamic manipulation. This method represents the most robust technique for levitation at distances exceeding a wavelength, owing to the constructive interference produced by the two constituent traveling waves. Forces generated by single beam levitation at a distance are approximately 30 times weaker than those from a simple standing wave.
- Far Field Acoustic Levitation: This method facilitates the levitation of objects larger than the acoustic wavelength by generating a tailored field that matches the object's specific size and shape. This capability allows for the levitation of such objects at distances greater than the wavelength from the source, provided the object is not of high density. Early implementations involved a simple vertical standing wave for disk-shaped objects or a three-transducer configuration for stabilizing spheres. More recent advancements, however, utilize Phased Array Transducers (PATs) and the boundary element method to levitate significantly larger objects over extended distances. The heaviest object successfully lifted by this technique is a 30mm diameter expanded polystyrene sphere weighing 0.6g. The largest object acoustically levitated using PATs positioned above and below the object is an expanded polystyrene octahedron with a 50mm diagonal length and a mass of 0.5g.
- Single Beam Levitation: This technique involves levitating objects at distances greater than a single wavelength from the sources, with access restricted to a single side. The trap design must be specialized, commonly manifesting as a twin trap or a vortex trap, though a bottle trap is also a viable option. The twin trap, being the simplest, creates two high-pressure "tweezers" on opposing sides of the particle. When geometric focusing is employed, this configuration can form a tractor beam using readily available components. Conversely, the vortex trap generates a central "hole" of low pressure. While requiring a more intricate phase field, the vortex trap, unlike the twin trap, can levitate objects larger than the wavelength. In 2019, researchers at the University of Bristol achieved the levitation of the largest object by a tractor beam, a 19.53mm diameter expanded polystyrene ball. This achievement was featured on "The Edge of Science," a BBC Earth production for YouTube Originals presented by Rick Edwards.
- Near Field Levitation: This method involves positioning a substantial, planar object in close proximity to a transducer surface, where it functions as a reflector, enabling levitation on an extremely thin air film. While capable of supporting several kilograms, this technique is limited to elevations of only hundreds of micrometers above the surface. Consequently, from a human perspective, it manifests more as a significant reduction in friction than as true levitation.
- Inverted Near Field Acoustic Levitation: Under specific conditions, the repulsive force responsible for near-field levitation reverses, transforming into an attractive force. In such instances, the transducer can be oriented downwards, facilitating the levitation of an object beneath it. Objects weighing on the milligram scale have been successfully levitated at distances of tens of micrometers. Current investigations indicate this phenomenon occurs when the equivalent radius of the disk is less than 38% of the wavelength.
These broad classifications represent one method for categorizing levitation types, but they are not exhaustive. Ongoing research explores the integration of various techniques to achieve enhanced capabilities, such as the stable levitation of non-axisymmetric objects through the combination of standing wave levitation with a twin-trap system (typically a single-beam levitation method). Furthermore, substantial efforts are dedicated to merging these techniques with 3D-printed phase-shifting components to gain benefits like passive field forming or superior spatial resolution. Control techniques also exhibit considerable diversity; while Phased Array Transducers (PATs) are prevalent, Chladni Plates have also been demonstrated as effective single standing wave sources for manipulating levitated objects by altering their frequency.
Applications
Acoustic levitation primarily finds its applications within scientific research and industrial processes.
Acoustic levitation offers a container-free environment for droplet drying experiments, facilitating the study of liquid evaporation and particle formation. The contactless manipulation of droplets has also garnered substantial interest due to its potential for small-scale, container-free chemistry. Specifically, researchers are keen on mixing multiple droplets using Phased Array Transducers (PATs) to investigate chemical reactions in isolation from conventional containers. Additionally, there is considerable interest in employing small levitated droplets as vessels for protein crystals in X-ray diffraction experiments, aiming to determine crystal structures at atomic resolution, at room temperature, and with high throughput.
Research has also explored the levitation of small living animals, demonstrating no adverse effects on the vitality of species typically found in air. This technique holds potential as a future tool for direct animal study.
Active research is underway in the domain of contactless assembly. Demonstrations include the levitation of surface-mount electrical components and micro-assembly achieved through a combination of acoustic and magnetic fields. Furthermore, commercial interest exists in 3D printing while objects are levitated, exemplified by Boeing's patent filing for this concept.
Acoustic levitation has also been proposed as a method for developing volumetric displays, where light is projected onto a particle that traverses a path to generate an image at a speed imperceptible to the human eye. This capability has already been demonstrated and integrated with audio and haptic feedback originating from the same Phased Array Transducer (PAT).
Acoustic tweezers
- Acoustic tweezers
- Optical levitation
- Radiation pressure
- Electrostatic levitation
- Magnetic levitation
- Aerodynamic levitation
- Buoyancy
References
Live Science – Research on the Levitation of Small Animals
- Live Science – Scientists Levitate Small Animals
- Physics Girl Video – Construction of an Acoustic Levitator and Liquid Levitation Demonstration