Da p05d

diameter with a density of 0.1 g cm 3 has an aerodynamic diameter of ~3 pm.

Gravitational settling

Fine particles falling through the air under the force of gravity do so at a constant velocity such that the resistance of the air balances the mass of the particle. The following equation relates particle diameter, d, and density, p, to terminal velocity pt

To overcome problems of powder flow and agglomeration, porous particles (i.e. particles with a low density) have been developed. Equation (9.24) shows that a particle of 10 pm where g is the gravitational constant and n is the viscosity of the air. For air, n = 1-9 x 10 7 N m 2 s and therefore pt = 2.9 x 106pd2 (m s-1)

Table 9.13 shows the terminal velocities for particles of a range of diameters from 20 to 0.1 pm, reinforcing the importance of small size in preventing removal of particles before entry into the lower reaches of the respiratory tract. In still air, a cloud of powder of about 20 pm diameter takes a few seconds to settle, whereas powder of around 1 pm diameter takes approximately 60 seconds.

Sedimentation

As the particles of drug move with the air in laminar flow in the airways, they fall under the force of gravity a distance equal to ptt,

Table 9.13

Terminal velocity ( of spherical

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