(86b) Interactions and Mechanical Characterization of Spiny Particles

In comparison to smooth particles with simple shapes, the interaction between particles decorated by intricate surface features, such as spines, is not understood well. However, these types of structural features can be used to tailor particle adhesion by adjusting contact area and standoff distance. Plant pollen grains serve as a convenient model and template for such finely structured particles. They are also showing promise as reinforcing fillers for polymers, because sporopollenin, the major component of the pollen exine, is one of the most chemically and thermally stable natural polymers. Meanwhile, the modulus of these particleâ??s spiny ornamentations has a significant influence on particle-particle and particle-surface interactions and the behavior as a filler. However, there are no literature reports of mechanical characterization of individual pollen grains. Here, the interaction between sunflower pollen particles (SFP) was characterized by colloidal probe atomic force microscopy (AFM). Moreover, the mechanical properties of three pollen species were also characterized by AFM, including ragweed, pecan and Kentucky blue grass.

For the interaction between SFPs, the results indicated that the presence of spiny protrusions on a particle surface could either enhance or reduce the interaction force by a factor of 6 times, compared to SFP interaction with a flat substrate. The spiny particle interactions were classified into five distinctive types of force-distance (f-d) curves. These results suggest that the interaction force is highly dependent on the orientation of spines on the surface, which can control whether frictional forces contribute to the observed effective adhesion. A model was developed to compare the effects of pure spine-spine adhesion with spine-spine friction at different orientations.

For the mechanical characterization, the elastic moduli of pollen shells were mapped quantitatively on 5Ã?5 μm² areas. The values of modulus were determined to be between 7~9 GPa for ragweed, 3~7 GPa for pecan, and 8~18 GPa for Kentucky blue grass pollen particles. Saturation at a relative humidity of 90% reduces the Kentucky blue grass pollen modulus to 3~6 GPa after absorbing water. These values are used to place in context recent reports of pollen behavior as a filler. To the best of our knowledge, this is the first time that the interactions between complex spiny microstructured particles have been directly measured by AFM. Moreover, pollen grains have excellent mechanical strength as an environmentally-friendly filler for high-performance composites.