## 2 Soiling-Related Pollen Topics
### Pollen Adhesion Forces  
This subsection will address the adhesion of pollen to inert flat surfaces. Its net adhesion force may have three contributors: van der Waals forces, capillary forces, and electrostatic forces. Adhesion experiments performed with several pollen types and flat surfaces showed that the adhesion strength of the pollen grains had a heavy reliance on both the presence or absence of pollenkitt (i.e., a sticky covering substance) and the exine ornamentation (i.e., the external surface morphology). For pollen grains without pollenkitt, the adhesion force primarily consisted in van der Waals forces and was independent of the surface. In contrast, for pollen grains with pollenkitt, both capillary bridges and van der Waals forces were significant; the pollenkitt-related contribution to adhesion was highly species-dependent and the net adhesion force was dependent on the surface hydrophilicity. Besides, the adhesion strength increased along with the effective contact area between the pollen grain and the flat surface [44]. In other words: in the absence of significant electrostatic forces, the pollen adhesion is basically governed by a dry mechanism controlled by the van der Waals forces (as a function of the grain size, the grain shape and the \"surface density\" of the exine ornamentation) and, provided that such pollen contains pollenkitt, also by a wet mechanism controlled by the capillary forces due to the pollenkitt liquid bridges formed between the pollen grain surface and the flat surface [44]. In any case, the resulting pollen adhesion strength is humidity-dependent: while the adhesion force to a hydrophilic surface was found to increase with the relative humidity (RH) up to 70% RH for pollen with and without pollenkitt, the adhesion to a hydrophobic surface was independent of the RH for pollenkitt-free pollen but exhibited a complex dependence on the RH for pollen with pollenkitt [57]. Moreover, the pollenkitt adhesive strength may vary according to its freshness and to the RH: on hydrophilic surfaces (i.e., uncoated glass) a higher RH increased pollen adhesion regardless of the pollen condition (i.e., fresh, aged, without pollenkitt), but on hydrophobic surfaces (i.e., silanized glass) the adherence was significantly lower for fresh pollen than for aged pollen while the ambient humidity had little effect on the pollen adhesion [58].  
In addition, pollen grains are also sensible to electrostatic forces. There is evidence of electrostatically-assisted pollen harvesting by honeybees [59]. Electrostatically-assisted artificial pollination can improve some agricultural productivities [60]. Electrostatic barrier-forming windows have managed to capture airborne pollen in order to provide pollen-free indoor air [61]. Interestingly, recent experiments have confirmed that pollen grains have a small amount of electrostatic charge (either positive or negative, with an average magnitude of 0.8 fC). Once released, pollen grains subjected to wind transport would retain a substantial part of their charge during several minutes before reaching electrostatic equilibrium with the air [62]. Moreover, concerning the aerial transport of pollen, electrostatics could influence it aerodynamically, namely by favoring either the separation or the clumping of the pollen grains [63].