Effect of Processing Conditions on the Formation and Microstructure of Electrospray Printed Polymer Films

An electrospray uses high electric potential to atomize a liquid suspension. The droplets evaporate in-flight, leaving behind dry solute particles that can be printed on a target substrate. We report on the use of electrospray to print thin films of polytetrafluoroethylene (PTFE) on silicon substrates. The coupled effects of ambient humidity (50%, 10%), suspension conductivity (9, 25, 85 μS/cm), suspension volatility (20%, 60%; alcohol in suspension), and flow rate (0.5, 1 μL/min) on the growth and microstructure of the films is reported. The suspension conductivity governed the net electric charge of the emitted droplets and particles, while humidity and volatility influenced the in-flight droplet dynamics. Films printed in dry air were denser with a well-defined boundary compared to films printed in humid air. Semi-spherical polymer aggregates were delivered to the substrate when the suspension conductivity was low. Suspensions with higher conductivity produced films with a woven structure of interconnected particle groups. Similarly, sprays utilizing a more volatile suspension produced films with a woven structure, with strands of solute linking groups of aggregates. The use of higher flow rates increased the particle packing density in the printed film.