Dendrimer pattern formation in evaporating drops: Solvent, size, and concentration effects

Atomic force microscopy studies comparing the behavior of G2-50%C ₁₂, G3-50%C₁₂, and G4-50%C₁₂ dendrimers in two solvent alcohols (pentanol and hexanol) on mica substrates confirm that the detailed morphologies of condensed dendrimer ring structures resulting from microdroplet evaporation depend on both the solute evaporation rate and the dendrimer generation. Pentanol evaporates at a rate 3 times that of hexanol, and thus the flux of molecules to the growing ring is expected to be significantly higher in pentanol than hexanol. In the dilute regime (0.1 wt %) the growth of highly stratified (monomolecular height terraces) and periodically "scalloped" dendrimer rings is ubiquitous in these systems. The instability wavelength of the scalloped rings is proportional to the width of the ring with the proportionality constant depending strongly on solvent, 4.8 for pentanol and 2.3 for hexanol. A short wavelength undulation is also noted in all of the dilute ring structures at the front edge of the lower terraces. The wavelength of this secondary instability again depends on solvent (in this case, shorter in pentanol than hexanol). The influence of dendrimer generation on ring structure primarily reflects the increase in dendrimer density with generation number. The evolution of G2-50%C₁₂-pentanol rings as a function of dendrimer concentration is also described.