Saturated Surface Charging on Micro/Nanoporous Polytetrafluoroethylene for Droplet Manipulation

Droplet motion control has important applications in the fields of microfluidic and energy management. In this work, large-area micro/nanoporous superhydrophobic polytetrafluoroethylene (PTFE) surfaces that can be printed with re-writable charges via simple droplet impact were fabricated using carbon dioxide nanosecond pulsed laser ablation. Surface charge density (SCD) was well controlled by Weber numbers, impact cycles of droplets, and structure thickness of solid surfaces. Saturated charging was achieved after only five impacts, which was independent of the Weber number. It is notable that the saturated SCD at each Weber number is 140% larger than the previously reported data. The SCD induces sufficient electric force, which is linearly correlated to the square of the charge, based on Coulomb's law. By taking advantage of the electric force, diverse droplet manipulations including fast droplet transport on the surface with the SCD gradient, seeding of the droplet array, and dynamic droplet mixing on designated spots with high SCD were performed on the micro/nanoporous superhydrophobic PTFE surfaces. Both the fabrication method and droplet manipulation strategy would provide enlightenment for the future design of droplet-based microfluidic devices on biocompatible materials.