DEPOSITION PROCESSES 



Strips (ribbon) and — in the field — leaves, stems, and stigmas. The value 

 for E depends also on the shape. The order of decreasing efficiency is 

 probably: spheres, disks, cylinders, and strips (other conditions being 

 equal, including radius or half-width). 



Wind-tunnel results confirm the conclusion of C. N. Davies & 

 Peetz (1956), that under extreme conditions efficiency of impaction can be 

 zero; for example the very small spores of the puff'ball, Lycoperdoti 

 (Calvatiii) giganteum, were not impacted undex any of the conditions 

 tested. The slightly larger spores of the smut fungus, Ustilago perennans, 

 however, showed appreciable efficiencies — but probably because they 

 often travel through the air in clumps which effectively behave as heavier 

 particles than their component single spores. 



The deposit is not uniform over the cylinder, but is densest on the 

 part of the circumference projecting farthest forward into the wind; this 

 is the so-called 'stagnation line'. The density of the deposit falls off" towards 

 the sides of the cylinder, and there is a spore-free zone on the shoulders 

 where the air-flow is more or less tangential to the surface. Usually there 

 is no deposit at the back or downwind side of the cylinder, but with 

 Ly CO podium spores (32 /x diameter) a narrow line of deposit has been 

 observed down the back of cylinders of less than o-i cm. diameter at 

 wind-speeds of i metre per sec. or less. Deposit on the side opposite to the 

 oncoming wind is negligible or zero under most conditions tested. The 

 angle subtended by the deposit on the upwind side was less than 180° in 

 both turbulent and streamline air. Other things being equal, the angle 

 subtended at the centre of the cylinder by the deposit was increased by 

 increasing the wind-speed and by decreasing the cylinder diameter — 

 increasing efficiency of the whole cylinder evidently runs parallel with 

 increase in the angle subtended by the deposit. The low efficiency of wide 

 cylinders and slow winds shows both as a narrower trace, and thinner 

 deposit per unit area. 



The decrease of efficiency with increasing cylinder size was first 

 noticed in field tests. Per unit length, a large cylinder 12 cm. in diameter 

 may collect no more pollen grains or spores than a cylinder i cm. in 

 diameter, and per unit area of surface it may collect many fewer 

 (Gregory, 195 1). 



IMPACTION ON A ROTATING STICKY CYLINDER 



Rotating the cylinder at a peripheral speed comparable with the wind- 

 speed would be expected to reduce the thickness of the boundary layer 

 on the surface, to induce the well-known Magnus effect, and to produce a 

 local rotation of air round the cylinder itself. It was not obvious whether 

 these eflfects would alter impaction efficiency.* 



* This experiment was suggested by Mr. J. R. D. Francis, of the Civil Engineering 

 Department, Imperial College of Science and Technology. I am indebted to him for 

 advice and the loan of equipment for the tests, the results of which have not been 

 published hitherto. 



63 



