Nearly Perfect Durable Superhydrophobic Surfaces Fabricated by a Simple One-Step Plasma Treatment

Fabrication of superhydrophobic surfaces is an area of great interest because it can be applicable to various engineering fields. A simple, safe and inexpensive fabrication process is required to fabricate applicable superhydrophobic surfaces. In this study, we developed a facile fabrication method of nearly perfect superhydrophobic surfaces through plasma treatment with argon and oxygen gases. A polytetrafluoroethylene (PTFE) sheet was selected as a substrate material. We optimized the fabrication parameters to produce superhydrophobic surfaces of superior performance using the Taguchi method. The contact angle of the pristine PTFE surface is approximately 111.0° ± 2.4°, with a sliding angle of 12.3° ± 6.4°. After the plasma treatment, nano-sized spherical tips, which looked like crown-structures, were created. This PTFE sheet exhibits the maximum contact angle of 178.9°, with a sliding angle less than 1°. As a result, this superhydrophobic surface requires a small external force to detach water droplets dripped on the surface. The contact angle of the fabricated superhydrophobic surface is almost retained, even after performing an air-aging test for 80 days and a droplet impacting test for 6 h. This fabrication method can provide superb superhydrophobic surface using simple one-step plasma etching.


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We constructed a L9(3 4 ) orthogonal array table composed of 9 experimental combinations of four main control parameters with different levels based on the Taguchi method (Table S2). When the total amount of gas was 16 sccm, the ratio of argon and oxygen gas was 5:3, the RF power of 100 W was applied for 60 minutes (P2), and the average contact angle was larger than 150°. When the total amount of gas was doubled to 32 sccm, the ratio of argon to oxygen gas was controlled to 5:1 and 5:3, and the RF power was applied at 100 W and 2 / 10 200 W in each run for 180 minutes and 20 minutes (P4 and P5, respectively); a surface having an average contact angle of 150° or more was also obtained. The average sliding angle of the pristine PTFE surface before plasma treatment was larger than 10°. However, the average sliding angle decreased to approximately 6° for the surface with a large contact angle (P2 and P3), and a small sliding angle within 1° was obtained. Figure S2 shows the results of analyzing the effect of each factor on the fabrication of a superhydrophobic surface from the experiments on the 4 main factors, which were based on the Taguchi method. By comparing the S/N ratios of the parameters by the level of the factor, the RF power had the largest effect on the contact angles between the levels, and the plasma exposure time had the next highest level difference. The total amount of gas or the ratio of argon to oxygen gas showed little difference between the levels, and the difference between the two factors was not large. The results of the Taguchi method showed that the best contact angle could be obtained at 16 sccm and 5:3 ratio of argon and oxygen gas. The results also showed that the contact angles could be increased with the plasma exposure time.
The most important factor is RF power, which is further divided into finer RF power levels.
All three parameters except RF power were controlled under the same conditions. to fix the droplet with a volume of 5 µl on the fabricated superhydrophobic surface.
We investigated the effect of plasma exposure time on surface modification toward superhydrophobicity (Fig. S4). The contact angles increased with exposure time, and the sliding angles decreased with time (Fig. S4A). When exposure time was controlled to 60 min, the contact angle of target PTFE surfaces almost reached approximately 150°. However, the sliding angles of the fabricated PTFE surfaces were nearly the same as those of the pristine PTFE surfaces. After plasma treatment for 180 min, the contact angles exceeded 170°, and the sliding angles were less than 1°.
The change of surface wettability may be attributed to chamber temperature. Chamber temperature increased with plasma exposure time, and temperature was nearly approximately 150 °C (Fig. S4B). The surface morphology of the structures of the fabricated PTFE sheets after 1 h of exposure (Fig. S4C) showed nano-sized sharp protrusions around the hollow structure, which is similar to those of fabricated PTFE sheets after 3 h of exposure (Fig. S4D).
However, short fiber structures were formed at the tip of the protrusion on the PTFE sheets under the plasma for 1 h. These structures may increase the adhesion force compared to the case of the best superhydrophobic PTFE sheets.
As a result of optimization of the plasma treatment process to obtain the superhydrophobic surface, the best superhydrophobic surface can be obtained with an RF power of 150 W, with the total amount of reactive gas controlled to 16 sccm, and the amounts of argon and oxygen gases adjusted to 10 sccm and 6 sccm, respectively. Plasma treatment for 3 hours under these conditions can produce the best superhydrophobic PTFE surface. Surface modification was conducted using the excited plasma.     Movie S1. The water droplet rolled down with the carbon particles when it was applied on the fabricated superhydrophobic PTFE surface (right). However, the droplet was remained on the pristine PTFE with dust (left).
Movie S2. Water droplets were perfectly bounced off the fabricated surface during the whole durability test