Viscosity effects on nebulisation of aqueous solutions

While the viscosity of sucrose and sodium citrate solutions increased with increasing percentage (w/v) of solute, the surface tension largely remained consistent. Water, sucrose (10–60% w/v) and sodium citrate (7–36% w/v) solutions were nebulised in two air-jet nebulisers (Pari LC; Medix A II) operated at 6 1 min⁻¹ and in an ultrasonic device (Medix Electronic) operated at mid-power setting. A wide variation in nebuliser size and output characteristics existed between the different commercially available models studied. Contrary to atomisation theories for air-jet nebulisation, droplet size was inversely related to solution viscosity (over 1–6 cP). Beyond this critical value droplet size increased as viscosity increased. This anomaly may be due to viscosity acting either directly or through liquid flow-rates. By contrast the ultrasonic device generated droplets of size proportional to viscosity but was unable to nebulise high viscosity fluids (i.e. > 6 cP). The low viscosity solutions offered less resistance to the integral fountain disintegration process, thereby producing smaller droplets and higher outputs.