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{{Short description|Suspension of objects using sound waves}}
'''Acoustic levitation''' is a method for suspending matter in air against gravity using [[acoustic radiation pressure]] from high intensity [[sound]] waves.<ref name=":1" /><ref>{{cite journal | last1= Andrade | first1= Marco A. B. | last2= Marzo | first2= Asier | last3= Adamowski | first3= Julio C. | title= Acoustic levitation in mid-air: Recent advances, challenges, and future perspectives | journal= Appl. Phys. Lett. | publisher= AIP Publishing | date=2020 | volume= 116 | issn= 0003-6951| doi = 10.1063/5.0012660 | issue = 25|page=250501| bibcode= 2020ApPhL.116y0501A | doi-access= free | hdl= 2454/39386 | hdl-access= free }}</ref>
It works on the same principles as [[acoustic tweezers]] by harnessing acoustic radiation forces. However acoustic tweezers are generally small scale devices which operate in a fluid medium and are less affected by gravity, whereas acoustic levitation is primarily concerned with overcoming gravity. Technically dynamic acoustic levitation is a form of '''acoustophoresis''', though this term is more commonly associated with small scale acoustic tweezers.<ref>{{Citation|last1=Lenshof|first1=Andreas|title=Acoustophoresis|date=2014|encyclopedia=Encyclopedia of Nanotechnology|pages=1–6|editor-last=Bhushan|editor-first=Bharat|publisher=Springer Netherlands|language=en|doi=10.1007/978-94-007-6178-0_423-2|isbn=978-94-007-6178-0|last2=Laurell|first2=Thomas}}</ref>
Typically sound waves at [[Ultrasound|ultrasonic]] frequencies are used<ref>{{Cite web|url=http://otsuka-lab.ee.cit.nihon-u.ac.jp/ken2-e.html|title=Ultrasonic Levitation|date=2006-11-04|archive-url=https://web.archive.org/web/20061104183648/http://otsuka-lab.ee.cit.nihon-u.ac.jp/ken2-e.html|access-date=2020-04-22|archive-date=2006-11-04}}</ref> thus creating no sound audible to humans. This is primarily due to the high intensity of sound required to counteract gravity. However, there have been cases of audible frequencies being used.<ref>{{Cite journal|last1=WANG|first1=T.|last2=SAFFREN|first2=M.|last3=ELLEMAN|first3=D.|date=1974-01-30|title=Acoustic chamber for weightless positioning|journal=12th Aerospace Sciences Meeting|location=Reston, Virginia|publisher=American Institute of Aeronautics and Astronautics|doi=10.2514/6.1974-155}}</ref>[[File:Acoustic Levitation.ogv|thumb|327x327px|A Langevin horn type standing wave acoustic levitator at the [[Argonne National Laboratory]]]] There are various techniques for generating the sound, but the most common is the use of [[Piezoelectric speaker|piezoelectric transducers]] which can efficiently generate high amplitude outputs at the desired frequencies.
[[Levitation (physics)|Levitation]] is a promising method for containerless processing of microchips and other small, delicate objects in industry. Containerless processing may also be used for applications requiring very-high-purity materials or chemical reactions too rigorous to happen in a container. This method is harder to control than others such as [[Electromagnetic force|electromagnetic]] levitation but has the advantage of being able to levitate [[Electrical conductor|nonconducting]] materials.
Although originally static, acoustic levitation has progressed from motionless levitation to dynamic control of hovering objects, an ability useful in the pharmaceutical and electronics industries.<ref name=WashPost20130715>{{cite news |url=http://www.washingtonpost.com/national/health-science/sound-waves-can-be-used-to-levitate-and-move-objects-study-says/2013/07/15/4d808a5e-eb15-11e2-8023-b7f07811d98e_story.html |title=Sound waves can be used to levitate and move objects, study says |last=Kim |first=Meeri |date=July 15, 2013 |newspaper=The Washington Post }}</ref><ref name=":6" /> This dynamic control was first realised with a prototype with a chessboard-like array of square acoustic emitters that move an object from one square to another by slowly lowering the sound intensity emitted from one square while increasing the sound intensity from the other, allowing the object to travel virtually "downhill".<ref name=":6" /> More recently the development of phased array transducer boards have allowed more arbitrary dynamic control of multiple particles and droplets at once.<ref name=":9">{{Cite journal|last1=Marzo|first1=Asier|last2=Drinkwater|first2=Bruce W.|date=2019-01-02|title=Holographic acoustic tweezers|journal=Proceedings of the National Academy of Sciences|language=en|volume=116|issue=1|pages=84–89|doi=10.1073/pnas.1813047115|issn=0027-8424|pmc=6320506|pmid=30559177|bibcode=2019PNAS..116...84M|doi-access=free}}</ref><ref name=":7" /><ref name=":8" />
Recent advancements have also seen the price of the technology decrease significantly. The "TinyLev" is an acoustic levitator which can be constructed with widely available, low-cost off-the-shelf components, and a single 3D printed frame.<ref name=":11">{{Cite web|url=https://www.instructables.com/id/Acoustic-Levitator/|title=Acoustic Levitator: 25 Steps (with Pictures)|date=2018-01-01|archive-url=https://web.archive.org/web/20180101172523/https://www.instructables.com/id/Acoustic-Levitator/|access-date=2020-04-22|archive-date=2018-01-01}}</ref><ref name=":2" />
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The first demonstration of the possibility of acoustic levitation was made in [[Kundt's tube|Kundt's Tube]] experiments in 1866. The experiment in a resonant chamber demonstrated that the particles could be gathered at the nodes of a [[standing wave]] by the acoustic radiation forces. However, the original experiment was conducted with the intention of calculating the [[wavelength]]s and therefore the [[speed of sound]] within a gas.
The first levitation was demonstrated by Bücks and Muller in 1933 who levitated alcohol droplets between a quartz crystal and a reflector.<ref>{{Cite journal|last1=Bücks |first1=Karl |last2=Müller |first2=Hans |date=January 1933|title=Über einige Beobachtungen an schwingenden Piezoquarzen und ihrem Schallfeld |journal=Zeitschrift für Physik |volume=84|issue=1–2|pages=75–86|doi=10.1007/bf01330275|bibcode=1933ZPhy...84...75B |s2cid=120868972 |issn=1434-6001}}</ref> The next advance came from Hilary St Clair, who was interested in acoustic radiation forces primarily for their applications on the agglomeration of dust particles for use in mining applications.<ref name=":0">{{Cite journal|last=Clair|first=Hillary W. St.|date=November 1949|title=Agglomeration of Smoke, Fog, or Dust Particles by Sonic Waves|journal=Industrial & Engineering Chemistry|volume=41|issue=11|pages=2434–2438|doi=10.1021/ie50479a022|issn=0019-7866}}</ref><ref>{{Cite web|url=http://archiveswest.orbiscascade.org/ark:/80444/xv82308|title=Archives West: Hillary W. St. Clair papers, 1896-1997|website=archiveswest.orbiscascade.org|access-date=2020-04-06}}</ref> He created the first electromagnetic device for creating the excitation amplitudes necessary for levitation
[[Taylor Wang]] was the leader of a team which made significant use of acoustic radiation forces as a containment mechanism in zero gravity, taking a device up on the [[Space Shuttle Challenger]] mission [[STS-51-B]] to investigate the behaviour of levitated droplets in micro-gravity.<ref>{{Cite journal|last1=Wang|first1=T. G.|last2=Trinh|first2=E. H.|last3=Croonquist|first3=A. P.|last4=Elleman|first4=D. D.|date=1986-02-03|title=Shapes of rotating free drops: Spacelab experimental results|journal=Physical Review Letters|language=en|volume=56|issue=5|pages=452–455|doi=10.1103/PhysRevLett.56.452|pmid=10033196|bibcode=1986PhRvL..56..452W|issn=0031-9007}}</ref> Further experiments were conducted in 1992 aboard United States Microgravity Laboratory 1 (USML-1),<ref>{{Cite journal|last1=Wang|first1=T. G.|last2=Anilkumar|first2=A. V.|last3=Lee|first3=C. P.|last4=Lin|first4=K. C.|date=1994-10-10|title=Bifurcation of rotating liquid drops: results from USML-1 experiments in Space|url=https://www.cambridge.org/core/product/identifier/S0022112094002612/type/journal_article|journal=Journal of Fluid Mechanics|language=en|volume=276|pages=389–403|doi=10.1017/S0022112094002612|bibcode=1994JFM...276..389W|issn=0022-1120|hdl=2060/19950007805|s2cid=123017388 |hdl-access=free}}</ref> and in 1995 aboard USML-2.<ref>{{Cite web|url=http://www.astronautix.com/w/wang.html|archive-url=https://web.archive.org/web/20161228085315/http://astronautix.com/w/wang.html|url-status=dead|archive-date=December 28, 2016|title=Wang|website=www.astronautix.com|access-date=2020-04-22}}</ref>
The most common levitator from at least the 1970s<ref name=":18">{{Cite journal|last=Whymark|first=R. R.|date=1975-11-01|title=Acoustic field positioning for containerless processing|journal=Ultrasonics|language=en|volume=13|issue=6|pages=251–261|doi=10.1016/0041-624X(75)90072-4|issn=0041-624X}}</ref> until 2017 was the Langevin Horn,<ref name=":4">{{Cite journal|last1=Morris|first1=Robert H.|last2=Dye|first2=Elizabeth R.|last3=Docker|first3=Peter|last4=Newton|first4=Michael I.|date=2019-10-02|title=Beyond the Langevin horn: Transducer arrays for the acoustic levitation of liquid drops|journal=Physics of Fluids|language=en|volume=31|issue=10|pages=101301|doi=10.1063/1.5117335|bibcode=2019PhFl...31j1301M|s2cid=209990197 |issn=1070-6631|url=http://irep.ntu.ac.uk/id/eprint/37905/1/15037_Newton.pdf}}</ref> consisting of a piezo-electric actuator, a metal transmitter and a reflector. However, this required precise tuning of the distance between the transmitter and the reflector as the distance between source and reflector needed to be an exact multiple of the wavelength. This is more difficult than it sounds as the wavelength varies with the [[speed of sound]], which varies with environmental factors such as temperature and altitude. Significant studies have been made with such devices including into contactless chemistry<ref>{{Cite journal|last=Trinh|first=E. H.|date=1985-11-01|title=Compact acoustic levitation device for studies in fluid dynamics and material science in the laboratory and microgravity|journal=Review of Scientific Instruments|volume=56|issue=11|pages=2059–2065|doi=10.1063/1.1138419|bibcode=1985RScI...56.2059T|issn=0034-6748}}</ref><ref>{{Cite journal|last1=Yarin|first1=A. L.|last2=Pfaffenlehner|first2=M.|last3=Tropea|first3=C.|date=February 1998|title=On the acoustic levitation of droplets|url=https://www.cambridge.org/core/journals/journal-of-fluid-mechanics/article/on-the-acoustic-levitation-of-droplets/BF81A648178516BCC23E9DF738FED18A|journal=Journal of Fluid Mechanics|language=en|volume=356|issue=1|pages=65–91|doi=10.1017/S0022112097007829|bibcode=1998JFM...356...65Y|s2cid=123666920 |issn=1469-7645}}</ref> and the levitation of small animals.<ref name="Xie 214102">{{Cite journal|last1=Xie|first1=W. J.|last2=Cao|first2=C. D.|last3=Lü|first3=Y. J.|last4=Hong|first4=Z. Y.|last5=Wei|first5=B.|date=2006-11-20|title=Acoustic method for levitation of small living animals|journal=Applied Physics Letters|volume=89|issue=21|pages=214102|doi=10.1063/1.2396893|bibcode=2006ApPhL..89u4102X|issn=0003-6951}}</ref> A number of these were also combined to create continuous planar motion by reducing the sound intensity from one source whilst increasing that of the adjacent source, allowing the particle to travel "downhill" in the acoustic potential field.<ref name=":6">{{Cite journal|last1=Foresti|first1=Daniele|last2=Nabavi|first2=Majid|last3=Klingauf|first3=Mirko|last4=Ferrari|first4=Aldo|last5=Poulikakos|first5=Dimos|date=2013-07-30|title=Acoustophoretic contactless transport and handling of matter in air|journal=Proceedings of the National Academy of Sciences|language=en|volume=110|issue=31|pages=12549–12554|doi=10.1073/pnas.1301860110|issn=0027-8424|pmid=23858454|pmc=3732964|bibcode=2013PNAS..11012549F|doi-access=free}}</ref>
[[File:TinyLev Acoustic Levitator Diagram.jpg|thumb|A TinyLev acoustic levitator including the electronics and a diagram of the peak pressure field.|420x420px]]
A new generation of acoustic levitators employing a large number of small individual piezoelectric-transducers have recently become more common.<ref>Puranen T., Helander P., Meriläinen A., Maconi G., Penttilä A., Gritsevich M., Kassamakov I., Salmi A., Muinonen K., Hæggström E. Multifrequency acoustic levitation. 2019 IEEE International Ultrasonics Symposium (IUS), DOI: 10.1109/ULTSYM.2019.8926200 https://ieeexplore.ieee.org/abstract/document/8926200</ref> The first of these levitators was a single-axis standing wave levitator called the TinyLev.<ref name=":2">{{Cite journal|last1=Marzo|first1=Asier|last2=Barnes|first2=Adrian|last3=Drinkwater|first3=Bruce W.|date=2017-08-01|title=TinyLev: A multi-emitter single-axis acoustic levitator|journal=Review of Scientific Instruments|volume=88|issue=8|pages=085105|doi=10.1063/1.4989995|pmid=28863691|bibcode=2017RScI...88h5105M|issn=0034-6748|doi-access=free|hdl=1983/0a2d97bb-f39c-482c-943b-6745a0ebc453|hdl-access=free}}</ref><ref name=":11" /> The key differences from the Langevin Horn were the use of sources from both top and bottom (rather than a source and a reflector) and the use of a large number of small transducers with parallel excitation, rather than a single piezoelectric element. The use of two opposing travelling waves, as opposed to a single source and a reflector, meant that levitation was still possible even when the distance between the top and bottom was not a precise multiple of the wavelength. This led to a more robust system which does not require any tuning before operation. The use of multiple small sources was initially designed as a cost saving measure, but also opened the door for phased array levitation, discussed below. The use of [[3D printing|3D printed]] components for the frame which positions and focuses the transducers and [[Arduino]]s as signal generators also significantly reduced the cost whilst increasing the accessibility,<ref>{{Cite web|url=https://www.instructables.com/id/Acoustic-Levitator/ |title=Acoustic Levitator |website=www.instructables.com|access-date=2020-04-06}}</ref> The reduction in cost was particularly important as a principal aim of this device was the democratisation of the technology.<ref>{{Cite web|url=https://www.youtube.com/watch?v=ABjRnSYw-4k|title=I built an acoustic LEVITATOR! Making liquid float on air|last=Cowern|first=Dianna|date=2020-04-23|website=Youtube|access-date=23 April 2020}}</ref>
This new approach also led to significant developments using [[Phased array ultrasonics|Phased Array]] Ultrasonic Transducers<ref name=":7">{{Cite journal|last1=Marzo|first1=Asier|last2=Seah|first2=Sue Ann|last3=Drinkwater|first3=Bruce W.|last4=Sahoo|first4=Deepak Ranjan|last5=Long|first5=Benjamin|last6=Subramanian|first6=Sriram|date=2015-10-27|title=Holographic acoustic elements for manipulation of levitated objects|journal=Nature Communications|language=en|volume=6|issue=1|pages=8661|doi=10.1038/ncomms9661|pmid=26505138|pmc=4627579|bibcode=2015NatCo...6.8661M|issn=2041-1723}}</ref><ref name=":9" /> (often referred to as PATs) for levitation. Phased Array Ultrasonic Transducers are a collection of ultrasonic speakers which are controlled to create a single desired sound field. This is achieved by controlling the relative [[Phase (waves)|phase]] (i.e. the delay time) between each output, and sometimes the relative output magnitudes. Unlike their counterparts in the [[Ultrasonic testing|non-destructive testing]] or [[Medical ultrasound|imaging]] fields, these arrays will use a continuous output, as opposed to short bursts of energy. This has enabled single sided levitation<ref name=":7" /> as well as manipulation of large numbers of particles simultaneously.<ref name=":9" />
Another approach which is growing in popularity is the use of 3D-printed components to apply the phase delays necessary for levitation, creating a similar effect to the PATs but with the advantage that they can have a higher spatial resolution than the phased array, allowing more complex fields to be formed.<ref name=":10">{{Cite conference |title=Soundbender |book-title=UIST '18: Proceedings of the 31st Annual ACM Symposium on User Interface Software and Technology |date=October 2018 |pages=247–259 |language=en|doi=10.1145/3242587.3242590 |doi-access=free |url=http://sro.sussex.ac.uk/id/eprint/78305/1/SoundBender_UIST2018_pdf2.pdf}}</ref> These are sometimes referred to as Acoustic Holograms,<ref name=":12">{{Cite journal|last1=Melde|first1=Kai|last2=Mark|first2=Andrew G.|last3=Qiu|first3=Tian|last4=Fischer|first4=Peer|date=September 2016|title=Holograms for acoustics|url=https://www.nature.com/articles/nature19755|journal=Nature|language=en|volume=537|issue=7621|pages=518–522|doi=10.1038/nature19755|pmid=27652563|bibcode=2016Natur.537..518M|s2cid=4403584|issn=1476-4687}}</ref> Metasurfaces,<ref>{{Cite journal|last1=Tian|first1=Zhenhua|last2=Shen|first2=Chen|last3=Li|first3=Junfei|last4=Reit|first4=Eric|last5=Gu|first5=Yuyang|last6=Fu|first6=Hai|last7=Cummer|first7=Steven A.|last8=Huang|first8=Tony Jun|date=March 2019|title=Programmable Acoustic Metasurfaces|journal=Advanced Functional Materials|language=en|volume=29|issue=13|pages=1808489|doi=10.1002/adfm.201808489|issn=1616-301X|pmc=6527353|pmid=31123431}}</ref> Delay lines<ref name=":13">{{Cite journal|last1=Marzo|first1=A.|last2=Ghobrial|first2=A.|last3=Cox|first3=L.|last4=Caleap|first4=M.|last5=Croxford|first5=A.|last6=Drinkwater|first6=B. W.|date=2017-01-02|title=Realization of compact tractor beams using acoustic delay-lines|journal=Applied Physics Letters|language=en|volume=110|issue=1|pages=014102|doi=10.1063/1.4972407|bibcode=2017ApPhL.110a4102M|issn=0003-6951|doi-access=free|hdl=1983/d0bdf9dd-cd7d-4302-9742-87bcb0d82006|hdl-access=free}}</ref> or Metamaterials.<ref name=":14">{{Cite journal|last1=Polychronopoulos|first1=Spyros|last2=Memoli|first2=Gianluca|date=December 2020|title=Acoustic levitation with optimized reflective metamaterials|journal=Scientific Reports|language=en|volume=10|issue=1|pages=4254|doi=10.1038/s41598-020-60978-4|issn=2045-2322|pmc=7060201|pmid=32144310|bibcode=2020NatSR..10.4254P}}</ref><ref>{{Cite
Although many new techniques for manipulation have been developed, Langevin Horns are still used in research. They are often favoured for research into the dynamics of levitated objects due to the simplicity of their geometry and subsequent ease of simulation<ref name=":16" /> and control of experimental factors.<ref>{{Cite journal|last1=Andrade|first1=Marco A. B.|last2=Polychronopoulos|first2=Spyros|last3=Memoli|first3=Gianluca|last4=Marzo|first4=Asier|date=2019-03-01|title=Experimental investigation of the particle oscillation instability in a single-axis acoustic levitator|journal=AIP Advances|volume=9|issue=3|pages=035020|doi=10.1063/1.5078948|bibcode=2019AIPA....9c5020A|doi-access=free|hdl=2454/35368|hdl-access=free}}</ref>
=== Theoretical ===
[[John William Strutt, 3rd Baron Rayleigh|Lord Rayleigh]] developed theories about the pressure force associated with sound waves in the early 1900s,<ref>{{Cite journal|last=Rayleigh|first=Lord|date=March 1902|title=XXXIV. On the pressure of vibrations|journal=The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science|volume=3|issue=15|pages=338–346|doi=10.1080/14786440209462769|issn=1941-5982|url=https://zenodo.org/record/2466116}}</ref><ref>{{Cite journal|last=Rayleigh|first=Lord|date=September 1905|title=XLII. On the momentum and pressure of gaseous vibrations, and on the connexion with the virial theorem|journal=The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science|volume=10|issue=57|pages=364–374|doi=10.1080/14786440509463381|issn=1941-5982|url=https://zenodo.org/record/1856546}}</ref> however this work was primarily based around the theoretical forces and energy contained within a sound wave. The first analysis of particles was conducted by L.V. King in 1934, who calculated the force on incompressible particles in an acoustic field.<ref>{{Cite journal|date=1934-11-15|title=On the acoustic radiation pressure on spheres|journal=Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences|volume=147|issue=861|pages=212–240|doi=10.1098/rspa.1934.0215|bibcode=1934RSPSA.147..212K|issn=0080-4630|last1=King|first1=Louis V.|doi-access=free}}</ref> This was followed by Yosioka and Kawisama, who calculated the forces on compressible particles in plane acoustic waves.<ref>{{Cite journal|last1=Rajabi|first1=Majid|last2=Behzad|first2=Mehdi|date=2014-03-01|title=Point Source Stimulated Acoustic Radiation of Cylindrical Shells: Resonance and Background Fields|journal=Acta Acustica United with Acustica|volume=100|issue=2|pages=215–225|doi=10.3813/aaa.918701|issn=1610-1928}}</ref> This was followed by Lev P. Gor'kov's work which generalised the field into the Gor'kov potential,<ref>{{Cite journal|date=May 1961|title=Soviet Physics—Doklady|journal=Physics Today|volume=14|issue=5|pages=47|doi=10.1063/1.3057553|issn=0031-9228}}</ref> the mathematical foundation for acoustic levitation which is still in widespread use today.
The Gor'kov potential is limited by its assumptions to spheres with a radius significantly less than the wavelength,<ref>{{Cite journal|last=Bruus|first=Henrik|date=2012|title=Acoustofluidics 7: The acoustic radiation force on small particles|url=http://xlink.rsc.org/?DOI=c2lc21068a|journal=Lab on a Chip|language=en|volume=12|issue=6|pages=1014–21|doi=10.1039/c2lc21068a|pmid=22349937|issn=1473-0197}}</ref> the typical limit is considered to be one
== Types of Levitation ==
Acoustic levitation can broadly be divided into five different categories:<ref name=":1">{{Cite journal|last1=Andrade|first1=Marco A. B.|last2=Pérez|first2=Nicolás|last3=Adamowski|first3=Julio C.|date=2018-04-01|title=Review of Progress in Acoustic Levitation|journal=Brazilian Journal of Physics|language=en|volume=48|issue=2|pages=190–213|doi=10.1007/s13538-017-0552-6|bibcode=2018BrJPh..48..190A|s2cid=125461009|issn=1678-4448}}</ref>
#'''Standing Wave Levitation:''' Particles are trapped at the nodes of a [[standing wave]], formed by either a sound source and reflector (in the case of the Langevin Horn) or two sets of sources (in the case of the TinyLev). This relies upon the particles being small relative to the wavelength, typically in the region of 10% or less, and the maximum levitated weight is usually in the order of a few milligrams.<ref name=":1" /> It is also worth noting that if the particle is too small relative to the wavelength then it will behave differently and travel to the anti-nodes.<ref>{{Cite journal|last1=Habibi|first1=Ruhollah|last2=Devendran|first2=Citsabehsan|last3=Neild|first3=Adrian|date=2017|title=Trapping and patterning of large particles and cells in a 1D ultrasonic standing wave|url=http://xlink.rsc.org/?DOI=C7LC00640C|journal=Lab on a Chip|language=en|volume=17|issue=19|pages=3279–3290|doi=10.1039/C7LC00640C|pmid=28840206|issn=1473-0197}}</ref> Typically these levitators are single-axis, meaning that all particles are trapped along a single, central axis of the levitator. However, with the use of PATs they can also be dynamic. This is the strongest technique for levitation at a distance greater than a wavelength due to the constructive interference from the two travelling waves which form it. The forces from single beam levitation at a distance are 30 times weaker than a simple standing wave.<ref name=":17">{{Cite journal|last1=Cox|first1=L.|last2=Croxford|first2=A.|last3=Drinkwater|first3=B. W.|last4=Marzo|first4=A.|date=2018-07-30|title=Acoustic Lock: Position and orientation trapping of non-spherical sub-wavelength particles in mid-air using a single-axis acoustic levitator|journal=Applied Physics Letters|language=en|volume=113|issue=5|pages=054101|doi=10.1063/1.5042518|bibcode=2018ApPhL.113e4101C|issn=0003-6951|url=https://research-information.bris.ac.uk/en/publications/acoustic-lock(a18b40a6-d392-4e69-847c-26bfea65f352).html|hdl=1983/a18b40a6-d392-4e69-847c-26bfea65f352|s2cid=126387250 |hdl-access=free}}</ref>[[File:Acoustic Virtual Vortices Levitating Mie Particle.jpg|thumb|A single beam acoustic levitator using a vortex trap to levitate an expanded polystyrene particle approximately twice the size of the wavelength. The vortices are alternated rapidly in direction to avoid spinning the particle to the point of instability.<ref name="Marzo 044301">{{Cite journal|last1=Marzo|first1=Asier|last2=Caleap|first2=Mihai|last3=Drinkwater|first3=Bruce W.|date=2018-01-22|title=Acoustic Virtual Vortices with Tunable Orbital Angular Momentum for Trapping of Mie Particles|journal=Physical Review Letters|language=en|volume=120|issue=4|pages=044301|doi=10.1103/PhysRevLett.120.044301|pmid=29437423|bibcode=2018PhRvL.120d4301M|issn=0031-9007|doi-access=free|hdl=1983/681ab143-7d53-4642-a859-8f0364394174|hdl-access=free}}</ref> Here 450 transducers at 40kHz are used.|alt=|383x383px]]
# '''Far Field Acoustic Levitation:''' Larger than wavelength objects are levitated by generating a field which
#'''Single Beam Levitation:''' Levitation of objects at a distance greater than a single wavelength from the sources with access only from a single side. In this case the trap must be specially designed, and usually takes the form of a twin trap or a vortex trap, although a third trap type called a bottle trap is also possible. The twin trap is the simplest of these possibilities which forms two high pressure "tweezers" on either side of the particle.<ref name=":7" /> If geometric focusing is used then this can be used to build a tractor beam with commonly available parts.<ref name=":13" /><ref name=":15" /> The vortex trap creates a "hole" of low pressure in the centre. It requires a more complex phase field but, unlike the twin trap, can be used to lift larger than wavelength objects.<ref name="Marzo 044301"/> In 2019 the largest object ever lifted by a tractor beam was done so at the [[University of Bristol]] and shown on "The Edge of Science
#'''Near Field Levitation:''' A large, planar object is placed very close to the transducer surface and acts as a reflector, allowing it to levitate on a very thin film of air. This technique is capable of lifting several kilograms, but cannot go higher than hundreds of micrometers above the surface.<ref>{{Cite journal|last1=Ueha|first1=Sadayuki|last2=Hashimoto|first2=Yoshiki|last3=Koike|first3=Yoshikazu|date=2000-03-01|title=Non-contact transportation using near-field acoustic levitation|url=http://www.sciencedirect.com/science/article/pii/S0041624X99000529|journal=Ultrasonics|language=en|volume=38|issue=1|pages=26–32|doi=10.1016/S0041-624X(99)00052-9|pmid=10829622|issn=0041-624X}}</ref> As such on a human scale it appears more as a huge reduction in friction, rather than as levitation.
#'''Inverted Near Field Acoustic Levitation:''' Under certain conditions the repulsive force which produces near field levitation inverts and become an attractive force. In this case the transducer can be pointed downwards and the set up will levitate the object will be levitated below it. The object will be levitated at a distance of tens of micrometers and objects in the milligram scale have been levitated. Current research suggests it occurs where the equivalent radius of the disk is less than 38% of the wavelength <ref name=":3" />
These broad classifications are a single way of sorting the types of levitation, but they are not definitive. Further work is being conducted on combining techniques to obtain greater abilities, such as the stable levitation of non axis-symmetric objects by combining standing wave levitation with a twin-trap (typically a single beam levitation technique).<ref name=":17" /> There is also a significant amount of work into combining these techniques with 3D printed phase shifting components for advantages such as passive field forming<ref name=":12" /><ref name=":13" /><ref name=":14" /> or higher spatial resolution.<ref name=":12" /><ref name=":10" /> There is also significant variation in the control techniques. Whilst PATs are common it has also been shown that [[Chladni plates|Chladni Plates]] can be used as a single standing wave source to manipulate levitated objects by changing the frequency.<ref>{{Cite journal|last1=Wijaya|first1=Harri|last2=Latifi|first2=Kourosh|last3=Zhou|first3=Quan|date=April 2019|title=Two-Dimensional Manipulation in Mid-Air Using a Single Transducer Acoustic Levitator|journal=Micromachines|language=en|volume=10|issue=4|pages=257|doi=10.3390/mi10040257|pmid=31003415|pmc=6523525|doi-access=free}}</ref>
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[[File:Doerr2020 Paper Peptide Isolation via Spray Drying.jpg|thumb|(Left) Images of acoustically levitated droplets during liquid evaporation and particle formation. (Right) [[X-ray microtomography]] gives insights into the final particle 3D structure.<ref>{{cite journal |last1=Doerr |first1=Frederik |last2=Burns |first2=Lee |last3=Lee |first3=Becky |last4=Hinds|first4=Jeremy |last5=Davis-Harrison|first5=Rebecca |last6=Frank |first6=Scott |last7=Florence |first7=Alastair |title=Peptide Isolation via Spray Drying: Particle Formation, Process Design and Implementation for the Production of Spray Dried Glucagon |journal=Pharmaceutical Research |date=2020 |volume=37 |issue=12 |page=255 |doi=10.1007/s11095-020-02942-5 |pmid=33319329 |pmc=7736029 |doi-access=free}}</ref>|alt=|376x376px]]
Acoustic levitation provides a container-less environment for droplet drying experiments to study liquid evaporation and particle formation.<ref>{{cite journal |last1=Doerr |first1=Frederik |last2=Oswald |first2=Iain |last3=Florence |first3=Alastair |title=Quantitative investigation of particle formation of a model pharmaceutical formulation using single droplet evaporation experiments and X-ray tomography |journal=Advanced Powder Technology |date=2018 |volume=29 |issue=12 |
<ref>{{cite journal |last1=Sloth |first1=Jakob |last2=Kiil |first2=Søren |last3=Jensen |first3=Anker |last4=Andersen |first4=Sune |last5=Jørgensen |first5=Kåre |last6=Schiffter |first6=Heiko |last7=Lee|first7=Geoffrey |title=Model based analysis of the drying of a single solution droplet in an ultrasonic levitator |journal=Chemical Engineering Science |date=2006|volume=61 |issue=8 |
The contactless manipulation of droplets has also gained significant interest as it promises small scale contactless chemistry <ref name=":4" /> There is particular interest in the mixing of multiple droplets using PATs so that chemical reactions can be studied in isolation from containers.<ref>{{Cite journal|last1=Watanabe|first1=Ayumu|last2=Hasegawa|first2=Koji|last3=Abe|first3=Yutaka|date=December 2018|title=Contactless Fluid Manipulation in Air: Droplet Coalescence and Active Mixing by Acoustic Levitation|journal=Scientific Reports|language=en|volume=8|issue=1|pages=10221|doi=10.1038/s41598-018-28451-5|issn=2045-2322|pmc=6033947|pmid=29977060|bibcode=2018NatSR...810221W}}</ref><ref name=":8">{{Cite journal|last1=Andrade|first1=Marco A. B.|last2=Camargo|first2=Thales S. A.|last3=Marzo|first3=Asier|date=2018-12-01|title=Automatic contactless injection, transportation, merging, and ejection of droplets with a multifocal point acoustic levitator|journal=Review of Scientific Instruments|volume=89|issue=12|pages=125105|doi=10.1063/1.5063715|pmid=30599572|bibcode=2018RScI...89l5105A |hdl=2454/33737|s2cid=58578863 |issn=0034-6748|hdl-access=free}}</ref> There is also interest in using
The levitation of small living animals has also been studied, and the vitality of animals which typically exist in air was not affected.<ref name="Xie 214102"/> In the future it could be used as a tool to study the animals themselves.
There is active research in the field of contactless assembly. The levitation of [[Surface-mount technology|surface mount]] electrical components has been demonstrated<ref name=":2" /><ref name=":3" /> as has micro-assembly with a combination of acoustic and magnetic fields.<ref>{{Cite journal|last1=Youssefi|first1=Omid|last2=Diller|first2=Eric|date=April 2019|title=Contactless Robotic Micromanipulation in Air Using a Magneto-Acoustic System|journal=IEEE Robotics and Automation Letters|volume=4|issue=2|pages=1580–1586|doi=10.1109/LRA.2019.2896444|s2cid=67872033|issn=2377-3766}}</ref> There is also commercial interest in 3D printing whilst levitated, with [[Boeing]] filing a patent on the concept.<ref>{{Cite patent|title=Free-Form Spatial 3-D Printing Using Part Levitation|gdate=2014-07-29|url=https://patents.google.com/patent/US20160031156A1/en}}</ref>
Acoustic levitation has also been proposed as a technique for creating a [[volumetric display]], with light projected onto a particle, which moves along the path to create the image faster than the eye can process. This has already proved possible<ref>{{Cite journal|last1=Fushimi|first1=Tatsuki|last2=Marzo|first2=Asier|last3=Drinkwater|first3=Bruce W.|last4=Hill|first4=Thomas L.|date=2019-08-05|title=Acoustophoretic volumetric displays using a fast-moving levitated particle|journal=Applied Physics Letters|volume=115|issue=6|pages=064101|doi=10.1063/1.5113467|bibcode=2019ApPhL.115f4101F|hdl=2454/36412|s2cid=201271065 |issn=0003-6951|url=https://research-information.bris.ac.uk/ws/files/204902388/Acoustophoretic_Volumetric_Displays_using_a_Fast_Moving_Levitated_Particle.pdf|hdl-access=free}}</ref> and has been brought together with audio and haptic feedback from the same PAT.<ref>{{Cite journal|last1=Hirayama|first1=Ryuji|last2=Martinez Plasencia|first2=Diego|last3=Masuda|first3=Nobuyuki|last4=Subramanian|first4=Sriram|date=November 2019|title=A volumetric display for visual, tactile and audio presentation using acoustic trapping|url=https://www.nature.com/articles/s41586-019-1739-5|journal=Nature|language=en|volume=575|issue=7782|pages=320–323|doi=10.1038/s41586-019-1739-5|pmid=31723288|bibcode=2019Natur.575..320H|s2cid=207986492|issn=1476-4687}}</ref>
[[File:Acoustophoretic Volumetric Display.jpg|thumb|An acoustophoretic volumetric display where a small expanded polystyrene particle is rapidly moved with light projected onto it to produce the image of a 'stop sign'. This is a composite image taken over 20 seconds.<ref>{{Cite journal|last1=Fushimi|first1=Tatsuki|last2=Marzo|first2=Asier|last3=Drinkwater|first3=Bruce W.|last4=Hill|first4=Thomas L.|date=2019-08-05|title=Acoustophoretic volumetric displays using a fast-moving levitated particle|journal=Applied Physics Letters|language=en|volume=115|issue=6|pages=064101|doi=10.1063/1.5113467|bibcode=2019ApPhL.115f4101F|hdl=2454/36412|s2cid=201271065 |issn=0003-6951|url=https://research-information.bris.ac.uk/ws/files/204902388/Acoustophoretic_Volumetric_Displays_using_a_Fast_Moving_Levitated_Particle.pdf|hdl-access=free}}</ref>|alt=|379x379px]]
==See also==
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* [[Aerodynamic levitation]]
* [[Buoyancy]]
{{clear}}
==References==
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==External links==
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