Overcoming the "coffee-stain" effect by compositional Marangoni-flow-assisted drop-drying.

Attempts at depositing uniform films of nanoparticles by drop-drying have been frustrated by the "coffee-stain" effect due to convective macroscopic flow into the contact line. Here, we show that uniform deposition of nanoparticles in aqueous suspensions can be attained easily by drying the droplet in an ethanol vapor atmosphere. This technique allows the particle-laden water droplets to spread on a variety of surfaces such as glass, silicon, mica, PDMS, and even Teflon. Visualization of droplet shape and internal flow shows initial droplet spreading and strong recirculating flow during spreading and shrinkage. The initial spreading is due to a diminishing contact angle from the absorption of ethanol from the vapor at the contact line. During the drying phase, the vapor is saturated in ethanol, leading to preferential evaporation of water at the contact line. This generates a surface tension gradient that drives a strong recirculating flow and homogenizes the nanoparticle concentration. We show that this method can be used for depositing catalyst nanoparticles for the growth of single-walled carbon nanotubes as well as to manufacture plasmonic films of well-spaced, unaggregated gold nanoparticles.