Near-field acoustic manipulation in a confined evanescent Bessel beam

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.