Nondimensional scaling laws for controlling pharmaceutical spray uniformity: understanding and scale-up

Spatially resolved drop size, drop velocity, and spray volume flux measurements for sprays produced by a commonly used pharmaceutical coating nozzle were performed in this study. Results showed three distinctive spray patterns: Gaussian, homogeneous, and dumbbell shaped. We found that transition from a dumbbell-shaped to a homogeneous pattern is related to the shaping air-induced breakup of already formed drops: depending on the drop size upstream of the location where the shaping air flows meet (i.e., the "junction" point), the drop viscosity, and the magnitude of the shaping air velocity, the shaping air can either pinch the spray or cause additional drop breakup. When the former outweighs the latter, the dumbbell-shaped pattern occurs; the homogeneous pattern is present when the opposite occurs. A corollary to this experimental interpretation is that whether additional drop breakup homogenizes the sprays or pinches, it is related to a Weber number (We) that is calculated using drop sizes upstream of the junction point, drop viscosity and surface tension, and the shaping air velocity at the junction point. With this idea in mind, we propose a We-based scaling method for optimizing the uniformity of air-assist spray patterns.