A schematic and a photo of the fabricated wave number–spiral acoustic tweezers are given in Fig. 2 (A and B). Five pairs of IDTs were deposited on an X-cut LiNbO3 substrate (thickness, 500 μm and double side polished) using standard lithography, evaporation, and lift-off techniques (16). As shown in Fig. 2, each IDT covers 30°. In the directions −60° to −30° and 120° to 150°, there are no IDTs, since SAWs are very weak in these directions because of the anisotropic properties of the LiNbO3 substrate. In addition, all the IDTs are divided into two groups. In each group, five IDTs are linked, forming a rosette-shaped IDT cluster, such that only two input channels are needed for powering and controlling all the IDTs.

The disposable microfluidic chamber (fig. S8A) is composed of two layers of PDMS, including a top layer with a microfluidic chamber (height, ~60 μm) and a thin bottom layer (thickness, ~30 μm) for sealing the chamber. The top layer was fabricated through standard soft lithography and mold-replica techniques (fig. S9, A to D). To fabricate the bottom layer, a thin layer of PDMS mixture was coated on a silicon wafer through spin coating (fig. S9E) and then baked at 65°C for 20 min. After that, the top layer was placed on the bottom layer with the open chamber facing downward and baked at 65°C for 40 min to bond the two layers together and seal the chamber (fig. S9F). The sealed chamber was peeled off from the silicon wafer and cut to the desired size for use (fig. S9G).

Before each test, a sealed microfluidic chamber was bonded on the LiNbO3 substrate with IDTs (fig. S8B). The bonding was performed in three steps. First, a drop of ethanol was added on the substrate. Then, the chamber was placed at the desired location. Last, the substrate with a chamber on top was baked at 65°C for 10 min. After these three steps, the chamber was successfully bonded on the substrate but could also be peeled off without damaging the substrate (fig. S8C). Hence, after each test, we removed and disposed the chamber to make it possible to reuse the substrate and avoid cross contamination between different tests. In addition, the energy of SAWs in the substrate could directly transmit into the microfluidic chamber through the thin bottom PDMS layer without using any additional coupling agents such as water, oil, or other ultrasonic couplants.

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