Abstract
We demonstrate the potential of using evanescent fields, instead of conventional propagating sound fields, to manipulate particles at micro or nano scale. We generate an evanescent acoustic Bessel beam in liquid above a thin, circular, asymmetrically excited plate. In the sub-MHz ultrasound domain, the resulting radiation force causes the particles to assemble at the pressure antinodes along concentric circles corresponding to the Bessel profile. By imposing an axial confinement in the evanescent region, the subwavelength two-plate sandwich system becomes resonant, increasing the radiation force magnitude. Resonances occur for some well-defined gaps for which whole numbers of antinodal circles are observed. Through fine tuning, particles as small as bacteria can be patterned. Further amplification can be obtained by trapping a microbubble in the Bessel beam axis. As we show, this resonant bubble, which acts as an acoustic magnet, can be used to efficiently capture or repel nearby micro-particles.
Highlights
We demonstrate the potential of using evanescent fields, instead of conventional propagating sound fields, to manipulate particles at micro or nano scale
Momentum transfer from radiative electromagnetic and acoustic fields to matter at microscale has been a topic of significant research interest in recent decades
Acoustic tweezers have been developed in the radiative acoustic domain, giving birth to a new branch of physics binding acoustics and microfluidics in the field of acoustofluidics[5]
Summary
We demonstrate the potential of using evanescent fields, instead of conventional propagating sound fields, to manipulate particles at micro or nano scale. BAW and SAW (commonly of leaky Rayleigh type) are emitted at high frequency (in the MHz range) to maximize the associated radiation force And unlike their electromagnetic counterpart, the use of non-radiative acoustic-based forces has attracted little attention to date. This lack seems paradoxical in view of the main features of evanescent waves Regardless of their nature, in a non-radiative field, waves can be confined to an ultra-small region of sub-wavelength dimensions and break the diffraction limit. The flexural plate waves are subsonic interfacial guided waves similar to plasmon waves in the electromagnetic domain In this way, we generate an evanescent Bessel beam of order zero in the liquid above the plate. The microbubble enhancement competes with the near-field radiation force due to the Bessel beam, enabling either to attract or repel the neighboring particles
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