Research Article

Acoustic trapping of microbubbles in complex environments and controlled payload release

Diego Baresch and Valeria Garbin
PNAS July 7, 2020 117 (27) 15490-15496; first published June 22, 2020; https://doi.org/10.1073/pnas.2003569117

  1. Edited by William R. Schowalter, Princeton University, Princeton, NJ, and approved May 21, 2020 (received for review February 26, 2020)

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Figures

  • Fig. 1.
    Fig. 1.

    Vortex beam trap for microbubbles. (A) The simulated pressure field, |p|, and phase, arg(p), of the trapping vortex beam are shown in the transverse plane (x,y). The phase variation results in a helicoidal wavefront in propagation direction, z, where the pressure must vanish. This is confirmed in B, where experimental scans of the pressure field along x for three different axial positions relative to the focal plane z=0, 1.5, and 3  mm are shown in the absence of the bubble. (C) The total pressure field surrounding the bubble scattering the beam is simulated in the propagation plane (x,z). (D) The same total field seen in the transverse plane (x,y) . It gives rise to the net pushing force Fz for a bubble centered on the vortex core (x=y=0). (E) Same as D for a bubble shifted by a distance x=0.5λ. The total field illustrates how the bubble oscillations distort the incident beam and give rise to a strong lateral trapping force attracting the bubble back toward the vortex core.

  • Fig. 2.
    Fig. 2.

    Pushing acoustic force and scattered field. (A and B) Force measurements by balancing the acoustic pushing force, Fz, and bubble buoyancy, FB. (A) Measurements (solid circles) and model (solid lines) for bubbles in the range a=25 to 500 μm as a function of the distance to the trapping beam focus (z=0). (B) Interpolated values of the measured pushing force (solid circles) at z=4  mm, normalized by the bubble buoyancy, for p0=0.7  MPa, compared to the acoustic force model alone (black solid line) and with the addition of a Stokes drag, Fs=6πμavz, originating from an acoustic streaming flow (vz=8  mm/s; see main text). (C) Direct hydrophone measure of the far-field scattering form function represented on a polar plot (see SI Appendix for more details). Model (solid lines) and measured (solid circles) diagrams for two independant bubbles are normalized with their respective value for θ=90○.

  • Fig. 3.
    Fig. 3.

    Trap robustness through opaque-to-light elastic layers. (A) Three different elastic layers were made of (Left to Right) a 0.5% agarose hydrogel with 1-μm fluorescent polystyrene beads, a 2% hydrogel, and a thick block of tofu. (Scale bar, 10 mm.) (B) Time-lapse photography of a bubble trapped and maneuvered with the trapping beam propagating through the elastic layer. (Scale bar, 100 μm.) (C) Force measurements with the elastic layers positioned on the propagation path between the trapping beam and the bubble.

  • Fig. 4.
    Fig. 4.

    In situ bubble dynamics, payload release, and material transport guided by an acoustical trap. (A) Bubble positioned at a distance d=200±20 μm from a distant wall. (B) Driving bubble oscillations (Δa) with a secondary source of ultrasound (frequency f′=28.2  kHz) as a function of bubble radius at rest, a. Shown are experimental points (circles) obtained for a “free bubble” far from any boundary (black) and a bubble at a distance d from a hydrogel wall with 3% (dark gray) and 4% (light gray) concentration of agarose. Black curves are the best fits to the data. In all curves, the bubble radius a is normalized by the theoretical Minnaert resonant size aM=115 μm. (C–F) Particle-coated bubbles as a model for in situ ultrasonic payload delivery. In the micrograph of a particle-coated bubble in C, the bubble interface is covered by 1 μm polystyrene particles. (Scale bar, 50 μm.) (D and E) Photographs and sketches of observed experimental particle release events. The bubbles are covered with 0.5 μm polystyrene particles. (D, i) Bidirectional release with particle plume toward the adjacent wall (surface mode n=5). (D, ii) Multidirectional release with six petal-like delivery sites (surface mode n=6). (D, iii) Bidirectional release with no particle plume toward the adjacent wall (surface mode in transition). (F) Experimental recording of the release event in D, ii during 2,000 acoustic cycles (Movie S5). (G) Subsequent transport of polystyrene particles toward the adjacent wall by bubble-generated microstreaming flows (Upper part of the photograph). Particles are also transported toward the bulk by the millimeter-scale flow generated by the trapping beam itself. (D, F, and G) Approximative bubble radius a=100 μm.

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Acoustic trapping of microbubbles in complex environments and controlled payload release
Diego Baresch, Valeria Garbin
Proceedings of the National Academy of Sciences Jul 2020, 117 (27) 15490-15496; DOI: 10.1073/pnas.2003569117
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