Holographic surface-acoustic-wave tweezers for functional manipulation of solid or liquid objects

Noninvasive surface-acoustic-wave (SAW) tweezers provide a powerful tool for on-chip object manipulation. However, the piezoelectric materials for generating SAWs are anisotropic with misaligned directions of wave momenta and energy flow, making the conventional interdigital transducer (IDT) inappropriate for perfect SAW control. Here we propose a holographic IDT for phase-velocity engineering, which improves the focusing quality of SAWs with smaller focal size, larger enhancement of intensity, and precise control of focal position. For the holographic IDT, the focal size is approximately 78.2 µm at 40 MHz, which is 36% smaller than the one produced by a circular IDT. About twofold enhancement of focal intensity and focal length of 10λ are further demonstrated for our design. Nonetheless, we show that the holographic SAW tweezers can implement accurate sorting for solid objects of different sizes as well as large active deformation for liquid droplets even at relatively low input power, which have important biomedical applications.

Figure 1

(a) Precise microparticle separation based on holographic IDT. The inset illustrates the modification (time delay Δt_n) between the circular and holographic IDTs. (b) The SAW-based microfluidic device with holographic IDT.

Figure 2

(a) The fabricated SAW device with 12 pairs of straight IDTs for characterizing the anisotropy of phase velocity. (b) The simulated and measured phase velocities at 40 MHz for SAWs propagating in different directions on the ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$ substrate. Normalized intensity distributions of out-of-plane displacements (z component) for (c) the circular IDT and (d) the designed holographic IDT. (e) Quantitative comparison between the displacement intensity distributions on the dashed lines [at approximately 10λ from the electrodes in (c),(d)], which are generated by the circular and holographic IDTs.

Figure 3

(a) Schematic of holographic IDT and LDV systems for measuring the SAW fields. The measured out-of-plane vibration intensity distributions for (b) circular and (c) holographic IDTs. The arrows in (b),(c) mark the positions of maximum intensities for the focusing SAW fields created by the circular and holographic IDTs. (d) The measured FWHMs for SAW focusing by circular and holographic IDTs.

Figure 4

Snapshots of the bright and fluorescent fields for the (a) aligning module and (b) separating module. (c) The relationship between the separation angle of mixed microparticles and input rf power.

Figure 5

The real-time deformation of microdroplets (0.2 µl) driven by the (a) circular IDT and (b) holographic IDT. The diameter D and height H of each droplet are marked for characterizing the deformation scale. Scale bar: 1 mm. (c) The measured deformation scale D/H at different time frames, where the gray region indicates the “on” state of power actuation.

Figure 6

(a) A schematic of the SAW model used in the finite-element solver to calculate the eigenfrequency, where the thickness of unit cell is 6λ, and the pitch of metal fingers is p = λ/2. (b) Simulated phase velocities in each direction for the SAW device.

Figure 7

(a) Schematic for the directions of energy flow and wave momentum for SAWs propagating on the 128° Y-cut ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$ substrate. (b) The power flow angle (Γ) and the anisotropy parameter (γ) in different directions. (c) The curvature radii of IDT fingers versus the propagation angle calculated by Eqs. (3) and (4).

Figure 8

(a) Top view of the holographic IDT and microdroplet. (b) The measured input power that is applied to the circular and holographic IDTs under different input voltages. (c) The measured S11 parameter of the circular and holographic IDTs within the range of 36–44 MHz.