Primary droplet formation in a microscopic vibrating-mesh nebulizer aperture at ultrasonic frequency: Computational and macroscale experimental modeling

Abstract

Vibrating-mesh nebulizers (VMNs) generate inhalable aerosols of liquid therapeutic agents. The aerosol is generated by vibration of a mesh plate with many microscopic apertures. The objective of this work is to understand the mechanisms of flow through the apertures and the formation of primary aerosol droplets. The flow was simulated computationally using the meshless finite volume particle method with the approximation of axisymmetric single-phase flow. For validation, a large-scale model aperture of 0.5 mm diameter was designed, with liquid properties and oscillation parameters scaled to approximate the Reynolds and Weber numbers of the flow from a microscopic aperture of a VMN device. Computations and experiments show that liquid is pressurized by motion of the aperture plate, and consequently pumped through the aperture. The resulting jet breaks up under Plateau-Rayleigh-like capillary instability. This leads to the formation of multiple droplets on each oscillation cycle of the aperture plate, with droplet diameter on the order of aperture diameter. After primary breakup, some new droplets are formed by coalescence of colliding droplets and jets, and as satellite drops in breakup events. Experiments and computations agree on the structure of the flow. In the VMN device, the oscillation period of the VMN aperture plate is on the order of the time taken for acoustic waves to propagate across the system. Consequently, liquid compressibility plays an important role in pumping. The model may be enhanced in future work to remove the restriction to axisymmetric flow and include interaction of the liquid with the air phase.