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Inhaling respiratory drugs is a widespread method to treat a variety of pulmonary diseases. The efficacy of these medications is strongly dependent on the spray cloud characteristics generated by the inhaler device. In the present study, we investigate the atomization of the liquid drug into small inhalable droplets out of a soft mist inhaler. In this device, the drug solution is forced through two converging nozzle channels to generate two laminar micro liquid jets which collide obliquely in the downstream of the nozzles and form a spray cloud. Because of the nozzles' microscopic dimensions and high pressure provided by the device, the jets collide under a high Weber number condition and form a wavy sheet that remains in a so-called impact wave regime. This leads to disintegration of ligaments from the leading edge of the sheet and their further breakup into small droplets. An open-source, geometrical volume of fluid model (VOF) is employed to simulate the jets collision and track the liquid-gas interfaces from the sheet down to the small droplets. A post-processing tool based on connected-component labelling concept is developed to detect the resolved droplets and obtain the droplet sizes. Different design parameters such as the jets velocity, shape, and collision angle as well as the drug solution surface tension were varied to investigate different cases. The impinged sheet properties such frequency of impact waves as well as the cloud properties such as characteristic mean diameter are computed and used as comparison measures between the cases. The simulation results demonstrate that increasing the injection velocities as well as decreasing the surface tension increases the instability of impact waves and the sheet lateral motion which enhances the atomization. Also, the collision angle is found proportional to the backward liquid flow from the sheet region. In addition, the jets cross-section shape is found an essential parameter to control the circumferential dispersion of the droplets for the future design of small inhalers.
