Evaporating droplets on oil-wetted surfaces: Suppression of the coffee-stain effect

Significance Evaporating suspension droplets in general leave ring-like stains. For many applications requiring homogeneous deposition, the stains imply a major problem. This in particular holds for inkjet printing, surface patterning, self-assembly, and 3D printing technologies. Here we offer a robust method to suppress the coffee-stain effect by letting particle-laden droplets evaporate on oil-wetted surfaces. Through tuning the surface energy of the droplets by adding surfactants, we can manipulate the contact line behavior, the flow structure, and the wetting state of the thin film surrounding the droplet and accomplish a controlled particle deposition. Our findings may open perspectives for uniform coatings in the abovementioned applications, but are also relevant for other complex liquid droplet evaporation, e.g., for disease transmission or in agriculture.