The use of a capillary rheometer to determine the shear and extensional flow behaviour of nasal spray suspensions.

The rheological profiles of four commercial nasal spray suspensions (Beconase, Flixonase, Nasacort and Nasonex) were compared using rotational viscometry. Two of the nasal sprays (Beconase and Nasonex) were further examined in both shear and extension using a capillary rheometer under conditions similar to those experienced at the spray nozzle (i.e. extremely high shear rates with significant stretching or extensional flow). In rotation, the shear viscosity fell rapidly with increase in shear rate. Plots of the viscosity derived at the lower shear rates in rotation were extrapolated to include the high-shear rate capillary values. At very high shear rates, the shear viscosity of Beconase was higher than that of Nasonex with the cross-over occurring in the extrapolated region at approximately 10,000 s(-1). In the transition region between laminar and turbulent flow (shear rate 6-8 x 10(4) s(-1)) there was a minimum in the shear viscosity to less than that of water for Nasonex and similar to water for Beconase, and a plateau region in extensional viscosity for Beconase but not Nasonex. These anomalies were due to the extensive aeration of both samples when sprayed. Whereas Beconase had de-aerated within 30 min of the experiment, Nasonex had not de-aerated completely after six weeks. The very low viscosity at the shear rates at the nozzle imply that it is unlikely that the low viscosity of the spray on delivery to the nose is a key factor in prolonging its residence time. The extensional viscosity for these rather fluid samples was over 1000-times the shear viscosity (not 3-times as with Newtonian fluids) and both sprays exhibited strain hardening over the range covered. The high extensional stress in the nozzle enables the fluid to form as reasonably sized droplets rather than fine atomized droplets, which rather than settling in the nose, would be prone to redistribution through the normal respiratory function. Both sprays resisted degradation despite the high shear rates and extensional stresses experienced.