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Chemically Directed Movement of Drops on Self-Assembled Monolayers
Gary C.H. Mo, Jun Yang, K. Isaacson, Seok-Won Lee, Paul E. Laibinis and Daniel Y. Kwok

We report studies of reactive wetting employing droplets of a non-polar liquid (decahydronaph-thalene) on chemically patterned surfaces. The drops contain an n-alkylamine that adsorbs onto surfaces with exposed 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 decahy-dronaphthalene (DHN) drops on the surface. We employed patterning to direct the movement of the drops on the surface, thereby allowing determination of the relationships between the macro-scopic fluidic behavior of the droplets and microscopic adsorption events. Specifically, we examined the effects of the unbalanced surface tension force and the influence of adsorbate concentration on drop movement. In this latter case, both kinetic and thermodynamic arguments are applied to describe the system. We compared the predictions by analyzing data from the present system and those reported by Domingues Dos Santos and Ondarcuhu that exhibited opposite trends in behavior. The present analysis provides insight into the influence of chemical reaction kinetics on adsorption-mediated drop movement. It is concluded that for a system with constant surfactant concentration, the velocity of a drop depends on the volume deposited and an optimal case exists such that the speed is the maximized.

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