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

See allHide authors and affiliations

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

    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

    Download PDF

    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.

    Files in this Data Supplement:

Stay Connected to Science Advances

Navigate This Article