Tuning the Coupled-Domain Response for Efficient Ultrasonic Droplet Generation

Acoustic microfluidic devices encompass mechanical, fluidic, and electromechanical domains. Complicated multidomain interactions require the consideration of each individual material domain, as well as coupled behaviors to achieve optimal performance. Herein, we report the co-optimization of components comprising an ultrasonic droplet generator to achieve the high-efficiency liquid atomization for operation in the 0.5-2.5-MHz frequency range. Due to the complexity of the real system, simplified 2-D representations of the device are investigated using an experimentally validated finite element analysis model. Ejection modes (i.e., frequencies at which droplet generation is predicted) are distinguished by maxima in the local pressure at the tips of an array of triangular nozzles. Resonance behaviors of the transducer assembly and fluid-filled chamber are examined to establish optimal geometric combinations concerning the chamber pressure field. The analysis identifies how domain geometries affect pressure field uniformity, broadband operation, and tip pressure amplitude. Lower frequency modes are found to focus the acoustic energy at the expense of field uniformity within the nozzle array. Resonance matching yields a nearly threefold increase in maximum attainable tip pressure amplitude. Significantly, we establish a set of design principles for these complex devices, which resembles a classical half-wave transducer, quarter-wave matching layer, and half-wave chamber layered system.