Spray impaction, retention and adhesion: An introduction to basic characteristics
Authors: Spillman John J. 
Published: November 2006

Droplets, falling under gravity through air that is not moving relative to the target, will impact on any object in their path, while charged droplets will be drawn to objects of earth potential along paths normal to the lines of equipotential; thus near the catching surface, they will move directly towards it. If the air is moving relative to the target, it will tend to move the droplets with it. The greater the drag to mass ratio of a droplet, the more rapidly any initial motion it has through the air will cease, and it will move through the air only very slowly under the effects of gravity and any electromagnetic potential. Air flowing past an object is able to change its path rapidly, but droplets moving with the air are less able to do this. Their ability to avoid impact increases with decrease in droplet size and wind speed, and with increase in the size of the catching surface. Thus small smooth stems catch big droplets in a high wind efficiently, but large smooth branches in a light wind will not catch many small droplets. Artificial cylinders and ribbons are poorer at catching droplets than natural surfaces, which are rarely smooth and often hairy. Hairs or spikes on a surface greatly increase the catch efficiency of droplets carried in the wind.

A droplet several hundred micrometres in diameter is so dominated by gravity that it will fall in a near vertical path even in a moderate wind, impacting on any horizontal surface that obstructs its path. Its chances of reaching a vertical stem are negligible unless it runs off or splashes from a near horizontal surface. Conversely, a small droplet will be carried almost horizontally in any wind and is most likely to impact on vertical surfaces or flapping leaves. It has a much greater chance of getting inside the canopy without being caught because most leaves are near horizontal, and once there, it must rely on the turbulence induced by the wind for transport and impaction on undersurfaces or hairs. Because turbulence is reduced as the droplet nears the ground, it is very difficult to catch droplets on the lower parts of the crop within the canopy. To bounce, a droplet must have enough surplus kinetic energy to rebound clear of the surface, allowing for the energy losses in deforming the droplet in the bounce process; moreover, the surface must not be significantly wetted by the drop. Thus the droplet must be moderate to large in size, must be moving rapidly relative to the surface, and must have a high surface tension to contain it as a droplet, even at its extreme deformation. Surface condition is of great importance; the presence of hairs and the type of roughness affect the probability of maintaining an air film between the surface and the droplet. In general, droplets below 150 μm diameter are unlikely to bounce, but adding small amounts of surfactant to the droplet formulation can increase this size by several times. Any one plant leaf can vary considerably over its area because of age, abrasion and local surface shape. A film of water on a wet surface ensures an air film is maintained and the droplet will bounce.