Research ArticleMICROTECHNOLOGY

Acoustic tweezers via sub–time-of-flight regime surface acoustic waves

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Science Advances  13 Jul 2016:
Vol. 2, no. 7, e1600089
DOI: 10.1126/sciadv.1600089
  • Fig. 1 Principle of TOF localized standing waves.

    (A) The opposing sets of IDTs are both actuated with a pulse length shorter than the TOF between them, resulting in traveling waves (TW) that constructively/destructively interfere where these two waves meet. (B) Over multiple pulses, the time-averaged effect of this localized standing wave (SW) yields a number of fixed nodal positions in the substrate, resulting in particle patterning in discrete locations where these pulsed waves intersect. (C) A PDMS channel is used to contain particles suspended in a fluid, which is replaced here with blue dye to better visualize the channel extents.

  • Fig. 2 Simulated and modeled response to SAW pulses on the order of 100 ns.

    (A) The simulated shapes of the transient traveling SAW pulses closely match the model in Eq. 1 for actuation by six–finger pair IDTs, with pulse time periods (tp) of 5T, 10T, and 20T, where T = λSAW/cs, which is the time it takes for a traveling SAW to traverse one SAW wavelength (here simulated with λSAW = 40 μm). (B) The simulated time-averaged substrate displacements closely match the modeled displacement curves for these same finger pair and T values, here showing the convolution ξx(x)*ξx(x) from Eq. 1. (C) The time-averaged nodal positions result from the combination of transient pulses, here showing an intersection sequence using a full 3D simulation during SAW generation (t = 2T), SAW propagation (t = 6T), intersection (t = 12T), and continuing propagation (t = 18T). See movie S2 for time-dependent simulations. (D) The time-averaged minimum displacement locations in (B) correspond to the nodal positions presented here, which show the instantaneous maximum displacements at transient wave intersection. Scale bars, 100 μm.

  • Fig. 3 Experimental particle patterning actuated by continuous and sub-TOF regime pulsed SAW.

    (A) In contrast to conventionally applied acoustic fields, applying pulse lengths with tp < TOF results in the time-averaged standing wave locations only where the two incident waves overlap, here with tp = 50, 100, 200, and 300 ns (approximately 5T, 10T, 20T, and 30T), driven by 40-μm six–finger pair transducers at 96 MHz, visualized with 2-μm particles in a 2-mm-wide channel after 5 s of pulsed excitation (time-averaged applied power = 0.35 W). These patterns correspond to the predicted patterning locations at time-averaged minimum displacement conditions, especially for smaller time-averaged minima (dotted lines correspond to conditions where |ξ| minima are less than 0.2). Continuous patterning is demonstrated in movie S4. (B) During pattern development in a 1-mm-wide channel, 3-μm primary particle patterns result from the combination of the intersecting 100-ns pulsed traveling waves (between dotted lines). a.u., arbitrary units. (C) Mean intensity along the x direction is assayed in the region (for t = 10 s) from movie S1. (D) The appearance of secondary near-field patterning [dash-dot boxes in (B)] results from diffraction in the vicinity of the acoustic impedance discontinuity at the channel wall boundary shown here. Scale bars, 200 μm.

  • Fig. 4 Generation of multiple discrete patterning regions.

    (A) The application of a 75-ns pulse with a 200-ns period, which is less than the minimum time it takes for a complete wave to pass across the 1-mm channel (the channel’s 254-ns TOF plus the 75-ns pulse period), generates symmetric standing wave regions as visualized with patterned 3-μm particles. (B and C) The measured mean intensity across the width of the channel (B), corresponding to the predicted locations of minimum time-averaged displacement (C). Scale bar, 200 μm.

  • Fig. 5 2D patterning control.

    (A) The patterning region in the direction of propagation (the x direction) can be arbitrarily determined through the imposition of a time-delayed pulse, where the pulse traveling from the top of the device is delayed by the value shown. (B) The patterning regions in the y direction (in dashed boxes) can similarly be modified using a SAW device whose wavelength changes along the y direction (here between 40 and 80 μm; device shown in Fig. 1C). All other results are for nonslanted IDT designs. (C) The patterning of nodal positions within the patterning region can also be shifted by imposing a phase shift Δφ to one of the opposing sets of transducers. All particles are 2 μm. Scale bars, 200 μm.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/7/e1600089/DC1

    movie S1. Development of localized acoustic patterning region.

    movie S2. Numerical simulation of pulsed SAW, including 2D and 3D models of the piezoelectric substrate and a model of the acoustofluidic interaction.

    movie S3. Analytical model of sub-TOF regime SAW.

    movie S4. Generation of time-averaged localized patterning regions using 50-, 100-, 200-, and 300-ns pulses.

  • Supplementary Materials

    This PDF file includes:

    • Legends for movies S1 to S4

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    Other Supplementary Material for this manuscript includes the following:

    • movie S1 (.avi format). Development of localized acoustic patterning region.
    • movie S2 (.avi format). Numerical simulation of pulsed SAW, including 2D and 3D models of the piezoelectric substrate and a model of the acoustofluidic interaction.
    • movie S3 (.avi format). Analytical model of sub-TOF regime SAW.
    • movie S4 (.avi format). Generation of time-averaged localized patterning regions using 50-, 100-, 200-, and 300-ns pulses.

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