3D-printed acoustic metasurface with encapsulated micro-air-bubbles for frequency-selective manipulation.

Acoustic waves provide an effective method for object manipulation in microfluidics, often requiring high-frequency ultrasound in the megahertz range when directly handling microsized objects, which can be costly. Micro-air-bubbles in water offer a solution toward low-cost technologies using low-frequency acoustic waves. Owing to their high compressibility and low elastic modulus, these bubbles can exhibit significant expansion and contraction in response to even kilohertz acoustic waves, leading to resonances with frequencies determined and tuned by air-bubble size. The resonances amplify vibrational amplitude and generate localized turbulence, enabling selective, non-invasive, and high-precision manipulation of microsized objects. However, conventional bubble formation relies on the shear force of the liquid flow and bubble surface tension, facing challenges of instability and random vibration that can impair manipulation precision and performance. To address these issues, we propose a coupled vibration structure with 3D-printed circular microsized air holes encapsulated by a PDMS film. These airholes act as artificial micro-air-bubbles, with their expansion and contraction stabilized by acoustic hard boundaries. The PDMS film further regulates vibration modes through the interaction between air movement and the film's vibration, eliminating randomness. Compared to conventional air-bubbles held by surface tension, these artificial air-bubbles are mechanically stable, allowing for enhanced gas volume changes and stronger forces for object manipulation. We experimentally confirm the stable vibration modes and their frequency-dependent behavior using laser Doppler vibrometry. Precise aggregation, rotation, and separation of micro-objects are demonstrated by adjusting the film's vibration mode. Furthermore, we propose a metasurface design featuring a multi-size microbubble array for frequency-selective manipulation, enabling flexible control of sample trajectory by changing the exciting frequency of an embedded piezoelectric transducer. Our low-frequency acoustic metasurface device offers a versatile, cost-effective solution for drug screening and automated sample handling.