Self-propelled drop movement by manipulation of nanoscale adsorbates through molecular self-assembly

We examined reactive wetting and transport phenomena of droplets on chemically patterned surfaces. We employed decahydronaphthalene (DHN) droplets that contained various amounts of an n-alkylamine to reactively wet and move about surfaces that expose a dense packing of carboxylic acid functionalities. The amine compounds adsorb onto this surface and produce one with a lower energy that exposes methyl groups, thereby causing a local surface energy gradient that is sufficient to induce a self-propelled movement of the contacting droplets on the surface. We employed patterning methods (micro-contact printing) to confine the direction of drop movement on these surfaces, thereby allowing direct measurement of fluidic movement and velocity. This ability allowed examination of the relationships between macroscopic droplet behavior and microscopic adsorption events. Specifically, we examined the effects of the force due to the unbalanced surface tension and of drop composition (adsorbate concentration) on the drop velocity, and analyzed these results using a thermodynamic approach.