Mechanical response of plant leafs to impact loading

Abstract

Agriculture is an economic activity with huge specific weight worldwide that faces great risks like epidemics. One manner epidemics spread from one leaf to another is the ejection of contaminated sessile-droplets due to rain drop impacts. This ejection is produced due to two fragmentation mechanisms: crescent-moon (direct contact between the rain drop and the contaminated drop) and inertial detachment (the rain drop impact on the leaf which elasticity mediates the energy to the contaminated sessile-droplet).

Fragmentation happens because the inertial forces overcome the viscous and surface forces of the fluid itself. In the case of ficus benjamina, the detachment condition may be approximated by inertial forces larger than surface forces during a period larger than the capillary time (~ 7 ms) that take into account the viscous effects. This inertia forces depend on the acceleration developed by the leaf tip, which itself depends on impacting mass, velocity and position.

The response of the leaf to impact loading is mainly characterized by the bending mode of the leaf and, to a lesser extent, by the twisting of the leaf and the bending of the tip.

Finally, it is also shown that due to the reduction of the turgor pressure and mass during the drought process of the leaf, the natural frequency of the leaf decreases.