Insights into the physics of spray coating of SWNT films

Spray coating is a scalable and high-throughput process for fabrication of transparent and conducting coatings (TCCs) composed of single-walled carbon nanotubes (SWNTs). Presently the fundamentals of this process are not well understood. We show that suppression of coalescence of spray droplets by sufficiently rapid heat- and mass-transfer yields homogeneous SWNT films by preventing the formation of ‘coffee stains’ of larger length scale. Such heat and mass transfer is driven by differential evaporation between the top and edges of the drops, whereas thermal and compositional effects on surface tension and buoyancy are weak. Ultrasonic spraying ensures that the droplets are deposited without significant splashing, and delayed splashing at higher Weber number is evidenced. We find that the performance of spray-coated TCCs made from HiPCO SWNTs is limited by bundle diameter rather than length of the constituent SWNTs and bundes. Vapor acid doping with concentrated sulfuric acid roughly doubles the conductivity of the TCCs.