Saturated Surface Charging on Micro/Nanoporous Polytetrafluoroethylene for Droplet Manipulation

Abstract

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.

Surface Charge Density Gradient Printing To Drive Droplet Transport: A Numerical Study

“…Once the water droplet rebounded from the superhydrophobic surface, the electric double layer would separate at the solid–liquid interface. The positive charges were taken away by the rebounded droplet, while the negative charges were left to electrify the superhydrophobic surface . The charge amounts Q and SCD ρ Q were evaluated using a coulometer according to the law of conservation of electric charge.…”

“…The positive charges were taken away by the rebounded droplet, while the negative charges were left to electrify the superhydrophobic surface. 40 The charge amounts Q and SCD ρ Q were evaluated using a coulometer according to the law of conservation of electric charge. The SCD gradient K i for every printing case was then calculated by…”

Profile Characteristics and Evolution in Combined Laser and Electrochemical Machining

“…If the laser intensity exceeds a threshold value, the materials in the central region could be removed directly with relatively high efficiency. 20 At the same time, the electrolyte jet could cool the machining zone and flush the melt or plasma away from the side machining gap effectively, as shown in Fig. 1c, which could contribute to the stability of the machining process.…”

Bionic surface diode for droplet steering

“…Coordination between superhydrophobicity and external stimuli, such as light, 17,18 electricity, 19,20 pH, 21 temperature, 22 magnetism, 23 and so forth, is an advisable strategy to achieve such exquisite functions. However, stimulus-responsive superhydrophobic surfaces are not suitable for mild biomedical applications and the engineering aspects that the stimuli cannot access.…”

“…Homogeneous superhydrophobic surfaces exhibit isotropic droplet adhesion, 15,16 thus sophisticated manipulations that necessitate droplet transport on the designed path are restricted. Coordination between superhydrophobicity and external stimuli, such as light, 17,18 electricity, 19,20 pH, 21 temperature, 22 magnetism, 23 and so forth, is an advisable strategy to achieve such exquisite functions. However, stimulus‐responsive superhydrophobic surfaces are not suitable for mild biomedical applications and the engineering aspects that the stimuli cannot access.…”