Sonic Screwdrivers and Tractor Beams: what the Acoustics Community often gets wrong about gradient forces versus radiation pressure

Gabriel C. Spalding (Lead / Corresponding author), Patrick Dahl, Zhengyi Yang, Peter Glynne-Jones, Michael P. MacDonald, Christine Démoré, Sandy Cochran

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Through discussion of our sonic screwdriver and tractor beam experiments, we aim to highlight the respective advantages of conservative and non-conservative forces. Commonly in acoustic trapping, conservative, gradient-induced mechanisms (e.g., standing waves) are used to manipulate matter. Such situations are reasonably described in terms of potential energy landscapes, an approach also applied to optics, for applications such as cell sorting [MacDonald et al, Nature 426 (2003)]. No such description is possible for radiation pressure, which is non-conservative, a distinction that is sometimes muddled in the literature, although it was made clear even in early work [e.g., King, Proc. R. Soc. Lond. A 147 (1934); Gor’kov, Sov. Phys. Doklady 6, 773 (1962)]. Our “sonic screwdriver” makes use of two non-conservative mechanisms: levitation by radiation pressure and rotation by transfer of azimuthal momentum components [Demore et al., PRL 108 (2012)]. We also note that the term “tractor beam” has often been reserved to describe an effect involving non-conservative forces, and demonstrate an attractive force produced in such an arrangement, even against a net momentum flux [Demore et al., PRL 112 (2014)].
Original languageEnglish
Article number2041
JournalJournal of the Acoustical Society of America
Issue number4
Publication statusPublished - Apr 2016


  • Sound pressure
  • High pressure
  • Acoustic radiation pressure
  • Acoustic standing waves
  • Matter waves


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