Sonic-black-hole acoustic focusing for non-contact atomization of silicone oils up to 2000 cP

Non-contact ultrasonic atomization of highly viscous liquids is limited by the severe acoustic-impedance mismatch between air and liquid and by viscous suppression of free-surface breakup. Here, we propose a sonic-black-hole non-contact atomizer (SBHNA) incorporating a segmented power-law acoustic focusing cavity to enhance the localization of airborne ultrasonic energy near the atomization outlet. The power-law contraction produces a slow-wave compression region near the nozzle-side turning point and strengthens the interaction between the localized field and the subwavelength outlet-reflection boundary. Theoretical analysis and frequency-domain simulations show that the focal-pressure enhancement is governed by the coupled effects of geometric focusing, slow-wave compression, outlet reflection, and full-cavity phase matching rather than by geometric contraction alone. Atomization experiments demonstrate that the SBHNA incorporating the optimized m = 2.2 power-law focusing cavity enables stable non-contact atomization of 2000 cP silicone oil, producing droplets with a Sauter mean diameter of 51.9 μm and a distribution span of 0.356. This capability highlights the potential of SBH acoustic focusing for aerosol generation, coating, spraying, and processing of highly viscous functional liquids.