DEPOSITION PROCESSES 



If ^ = 90°, then Vs cos ^ = o, and hence the number of particles with 

 trajectories cutting a vertical surface should be independent of the ter- 

 minal velocity of the particles but will depend on the wind-run. (Deposi- 

 tion will in practice be reduced below this value because the air-stream 

 is deflected by the surface itself, and efficiency of collection depends on u 

 (Langmuir & Blodgett, 1949; Gregory, 195 1).) 



D 



Fig. 12. — Diagram illustrating gravity theory of particle deposition. ABCD = trap 

 surface; u = wind velocity; Vg = terminal velocity of particle; ABCDEFGH = rec- 

 tangular skew prism containing particles whose trajectories would bring them to rest 

 on trap surface; 6 = presentation angle; tan a = Vs/u. 



If 6 = 0°, then u sin = o, and deposition under the influence of 

 gravity should depend on the terminal velocity, Vs, so that for a hori- 

 zontal trap the volume of air sampled should be independent of wind- 

 speed, and should depend only on the terminal velocity of the particles. 

 From a cloud of uniform concentration, most trajectories should pass 

 through a surface inclined at an angle d, when tan 9 = u/vs {6 = 45° 

 when Vs = u). 



If the time-mean density of the spore-cloud = x spores per cu. 

 metre, it will be apparent that the area dose A.D. = xVs — from which, 

 if the deposition is by gravity as assumed, the expected area dose = 

 (ioo/u)vs. Comparison of observed results with the expected value will 

 give a convenient test of the validity of the theory. 



Our wind-tunnel experiments show that deposition on a horizontal 

 flat surface is a fairly complex process depending on several factors besides 

 the simple resultant of gravity and wind. The surface studied in greatest 

 detail has been the 76 X 25 X 1-3 mm. glass microscope slide, as this 

 has been extensively used in routine spore trapping. Experiments with 

 other plane surface traps are reported in more detail than given here, by 

 Gregory & Stedman (1953). 



The slide was placed with its long axis at right-angles to the wind and 

 held by clips, placed at the two ends to avoid disturbing the air-flow. 

 Its surface was orientated at various angles to the wind in diflferent experi- 

 ments, the convention adopted being: presentation angle 0° = parallel 

 with the wind; 45° when the leading edge was lower than the trailing 

 E 65 



