DEPOSITION PROCESSES 



experimental investigation. With a continuous source emitting during a 



limited time, the area dose will be the same as if the same total quantity of 



particles, Q_05 had been liberated in a number of successive instantaneous 



puffs arriving in a series of greatly fluctuating concentrations. 



We can conveniently express the percentage efficiency of a trapping 



surface as 



^ Trap dose per sq. cm. 



E = T — ^-^1 ^ ^ X 100. 



Area dose per sq. cm. 



This convention expresses the efficiency with which a surface clears the 

 spore-cloud to a height of one centimetre above the surface. Deposition 

 on a surface takes place in several ways, including impaction and sedi- 

 mentation, for sedimentation seldom acts alone. 



Mechanism of Impaction 



When a bluff object such as a cylinder is placed in wind, the oncoming 

 air-stream has to flow around the obstruction, but airborne particles w^ill 

 be carried some distance towards it by their own momentum before they 

 are in turn deflected by the wind flowing around the obstacle (Fig. 9). 



Pig. g. — Streamlines of air and particle trajectories around a cylindrical obstruction 

 (vertical cylinder seen in plan). E = streamlines carrying spores towards cylinder; d = diameter 

 of cylinder; arrows on left show direction of wind of velocity = u. 



If all those particles were impacted whose trajectories in the free wind- 

 stream would have passed through the obstruction, impaction efficiency 

 would be 100 per cent, but apparently in practice it never is. The distance 

 travelled by the particle towards the cylinder before being deflected by the 

 air streamlines flowing around the cylinder is related to both the momen- 

 tum of the particle and the size of the object disturbing the air-flow. 

 Another effect, collection by direct interception, becomes important when 

 the diameter of the particle is an appreciable fraction of the diameter of 



59 



