chip generates microbubbles according to a binary pattern. Each microbubble corresponds to a location of zero ultrasound transmission. After the bubble generation is completed, the transducer is turned on and the acoustic wave, at 10MHz, transmits through the SUM and is locally blocked at the pixels that are covered by a microbubble. The remainder of the wavefront propagates into the upper container and diffracts to form the target sound pressure distribution.
The SUM generates a pattern of microbubbles on the surface of the CMOS chip by the electrolysis of water. The microbubble coverage has to be large enough to ensure that the acoustic wave is blocked at the location of the electrode. As the potential difference between the anode and the cathode is constant (5 V), the microbubble volume depends on the time the current flows, from 0.6 to 2.8 ms.
For each acoustic image, it takes around 12 s to write the microbubble hologram, when each pixel is sequentially addressed. Afterward, the transducer is turned on for 15 s, generating ultrasound waves, which are modulated by the SUM and propagate to form the acoustic image in the target plane. This forces the microparticles to aggregate into the corresponding shape. After each assembly step, the transducer is turned off, and a motorized film mechanically “wipes” the microbubbles off the chip surface.
Under each electrode, a CMOS transmission gate connects the electrode to a vertical wire. Outside the electrode array, additional transmission gate switches collect the column wires into eight global wires, which lead to the chip pads and can be accessed from the outside of the chip. Two shift register chains, respectively, for row and column select, are fed by a digital driving signal to control the transmission gate groups. The chip is driven by a commercial microcontroller board (Arduino Mega 2560), which is loaded with the codes for chip electrodes addressing and electrolysis voltage switching. The thickness between the conveyor film and the