Chemical influences on adsorption-mediated self-propelled drop movement

We report studies of reactive wetting employing droplets of a nonpolar liquid (decahydronaphthalene) on chemically patterned surfaces. The drops contain an n alkylamine that adsorbs onto surfaces exposing carboxylic acid groups and produces surfaces exposing methyl groups. The change in surface energy that occurs concurrent with the formation of an oriented monomolecular film of alkylamine during this process is sufficient to produce a self-propelled movement of decahydronaphthalene drops on the surface. We employed patterning to direct the movement of the drops on the surface, thereby allowing measurements of the relationships between the macroscopic fluidic behavior of the droplets and microscopic adsorption events. Specifically, we examined the effects of the unbalanced surface-tension force and the influences of adsorbate concentration on drop movement. In this latter case, both kinetic and thermodynamic arguments can be applied to describe the system. We compared the predictions from these two approaches by analyzing data from the present system and those reported by F. Domingues Dos Santos and T. Ondarcuhu [Phys. Rev. Lett. 75(16), 2972 (1975)] that exhibited opposite trends in behavior. The present analysis provides insight into the influence of chemical reaction kinetics on adsorption-mediated drop movement (i.e., reactive wetting).