One-Step Method Produces Robust Superhydrophobic Surfaces

By MTI Admin posted 27 days ago


Potential Application for Polymers to Reduce Permeation of Water

Engineered surfaces with extreme wetting properties have gained increased attention because of their wide range of applications from microfluidic devices to macroscale smart coatings1

Polymer interfacial engineering via surface sanding. (a) Schematic of surface sanding for various polymer surface modification. (b) An illustration of surface modification by sanding, repeated three times.

Superhydrophobic surfaces, in particular, have garnered sustained interest because of their extensive applications in the fields of self-cleaning, anti-icing and drag reduction systems. However, previous research usually involves complex fabrication strategies to modify the surface wettability. Additionally, traditional micro-to-nano fractal structures and organic coatings are fragile, susceptible to wear, and decay over time, resulting in inevitable exposure of subsurface regions and rapid degradation of the intrinsic wetting properties.

Some strategies, such as chemical grafting with stronger covalent bonds, fabricating armored hierarchical structures or nanocomposite coatings, and the addition of a sacrificial top layer, had been developed to increase the robustness of the superhydrophobic samples in terms of mechanical and temporal stability. However, these techniques either fall short of achieving mechanical and time-dependent stability or involve complex and specialized fabrication procedures, which slows down their commercialization.

Here, a one-step sand-in method is proposed, to produce superhydrophobic coating layers directly atop various substrates with distinct initial wettability2. The resulting surfaces exhibit an apparent contact angle (CA) up to ~163.8° and hysteresis <5° with enhanced temporal and mechanical stability. The superhydrophobic surfaces are shown to be stable after 18 months of storage in a humid environment and can withstand 100 Scotch tape-peeling tests. The surface wettability can be adjusted to range from being hydrophilic to superhydrophobic by modifying the sand-in conditions and powder additives, which expands the spectrum of possible applications for the sand-in process.

Sand-In Strategy to Build Robust Modified Surfaces
A typical sand-in method (Figure 1) entails sanding the substrates for ~1 min by vitrified abradants (sandpaper) to introduce surface roughness, followed by depositing
powder atop the substrates and sanding the substrates again for ~3 min. The sanded-in surface is then cleared of any extra powder. This process is repeated  with newly added powder to guarantee homogeneity. To disrupt heterogeneity, care must be taken to change sanding direction often. Sanding protocols or machine-assisted sanding could further improve the homogeneity of the surface.

We compare the hydrophobicity of three different polymer substrates: polytetrafluoroethylene (PTFE, Teflon) — intrinsically hydrophobic; high density

Surface hydrophilicity tests for unsanded and sanded polymer surfaces, including PP, HDPE, and PTFE. The CPC-FG sanded surfaces become superhydrophobic, while CB-FG(O) sanded surfaces are hydrophilic. The grit size of sandpaper is 220. The error bars reflect the standard deviations from at least 3 individual measurements. The optical images at the right side show the static droplets atop PTFE-CPCFG, PTFE and PTFE-CBFG(O).

polyethylene (HDPE) and polypropylene (PP) — intrinsically slightly hydrophilic. These substrates are sanded with two types of powder additives: hydrophobic calcined petroleum coke derived flash graphene (CPC-FG) and hydrophilic carbon black flash graphene (CB-FG). It is observed that when using CPC-FG to sand the surface, the sanded surface can be superhydrophobic, while when using CB-FG, the surface becomes more hydrophilic than the substrate (Figure 2). This result demonstrates that we can tune the surface wettability over a wide range for various substrates.

Mechanism of Sand-in Method
The morphologies and structures of the tribofilms are further analyzed to gain a better understanding of the surface modification mechanism of the sanding treatment. A profilometer is used to measure surface roughness at the microscale, and atomic force microscopy is utilized to measure surface roughness at the nanoscale. It is noted that:

  • Direct sand-in treatment contributes to the micro-roughness
  • Powder additives contribute to the nano-roughness

In order to account for the contribution of roughness from the microscale and nanoscale, we modify the Cassie-Baxter model (Figure 4). It is proved that the experimental results follow a modified Cassie-Baxter model3, as shown in eq 1.

Equation 1

Temporal and Mechanical Stability of Sand-In Surfaces
In practical applications, the robustness of the superhydrophobic surfaces is of major importance. The wettability of surfaces can be affected by oxidation and aging
after being exposed to air, especially when using organic coatings. Additionally, surfaces can suffer severe degradation due to mechanical abrasion when in contact with
other objects. Therefore, the durability and mechanical stability of various sand-in surfaces were assessed (Figure 5).

The contact angle of various surfaces treated by the sand-in method was measured after different periods of time. Experimental results show no obvious change in
wettability after 18 months. Samples also showed great thermal stability, as the apparent CA remained constant after being exposed to heat. This result is reasonable,
since the vapor pressure of carbon is very low at room temperature and the thermal decomposition temperature is high, at ~600 °C. Additionally, transparent tape is used to test the mechanical stability, and it is observed that the surface can withstand 100 times transparent tape exfoliation. On the other hand, experimental results show that UV-radiation can indeed gradually decrease the CA of the surfaces. This can be explained by graphene’s surface oxygen functionalization, which gradually increases its hydrophilicity. Finally, it is observed that sanded surfaces show good anti-icing performance. The time it takes for water to freeze increases by 2.6% and
the adhesion of ice decreases by ~40%, even at ~-40 °C.

In conclusion, a one-step, solvent and water-free sand-in method to prepare robust superhydrophobic surfaces directly atop various substrates has been developed.
The surface wettability can be tuned from hydrophilic to superhydrophobic by varying the powder additives and sanding conditions. The sand-in method induces surface modification and the formation of tribofilms. It is revealed that both microstructures and nanoscale asperities contribute to the robust superhydrophobic features of sand-in surfaces. Additionally, it is demonstrated that the sand-in surfaces show good temporal and mechanical stability. These findings could guide the design of surfaces with controlled wettability, and the large-scale application of this technique across a variety of surfaces. 

[1] Wang, D.; Sun, Q.; Hokkanen, M. J.; Zhang, C.; Lin, F. -Y.; Liu, Q.; Zhu, S. -P.; Zhou, T.; Chang, Q.; He, B.; Zhou, Q.; Chen, L.; Wang, Z.; Ras R. H. A.; Deng, X. Design of Robust Superhydrophobic Surfaces. Nature 2020, 582, 55-59.

[2] Chen, W., Wang, W., Luong, D. X., Li, J. T., Granja, V., Advincula, P. A., Higgs, C.F. & Tour, J. M. (2022). Robust Superhydrophobic Surfaces via the Sand-In
Method. ACS Applied Materials & Interfaces, 14(30), 35053-35063.

[3] Cassie, A. B. D.; Baxter, S. Wettability of Porous Surfaces. Trans. Faraday Soc. 1944, 40, 546-551.

This article was originally published in MTI CONNECT 2023, Issue 1. Download for the complete article with all figures.