Superhydrophobic Aluminum Surfaces by Deposition of Micelles of Fluorinated Block Copolymers
-  Superhydrophobic Aluminum Surfaces by Deposition of Micelles of Fluorinated Block Copolymers. Desbief, S., Grignard, B., Detrembleur, C., Rioboo, R., Vaillant, A., Seveno, D., Voue, M., De Coninck, J., Jonas, A.M., Jerome, C., Damman, P., and Lazzaroni, R. Langmuir (2009). doi: 10.1021/la902565y.
Desbief et. al. set out to develop a technique for making large hydrophobic surfaces. While many methods of making hydrophobic surfaces existed previously, the authors saw a need for a method which could be used at large scales. The authors created and tested such a technique which involves sythesizing a diblock copolymer and attaching micelles of it to an aluminum surface.
To make the aluminum surface superhydrophobic, the researchers decided to change both the surface roughness and the surface free energy. The scientists synthesized a block copolymer and deposited micelles of it onto the aluminum. The block copolymer has a polyacrylonitrile (PAN) block and a fluorinated block. The four polymers described in this paper vary only by the molecular weights of the two polymers making up the diblock copolymer, as seen in figure 1 (PAN is labeled N, and fluorinated block is labeled F). Each polymer should result in a different sized micelle and different amounts of fluorine on the surface.
TFT (trifluorotoluene) is a good solvent for forming micelles because the PAN block is TFT-phobic, while the fluorinated block is TFT-philic. The polymer molecules self-assemble into micelles with a core of PAN and an outer layer (corona) of fluorinated blocks. The fluorine molecules on the outside of the micelle aid hydrophobicity by lowering surface energy. Depositing these micelles onto the aluminum gives the surface a bumpy texture on the scale of the size of the micelles (bump diameters= 17nm to 32nm depending on the molecular wight of the polymer). The surface was imaged using AFM as a verification that the process carried out as expected (see figure 3).
Having modified the aluminum surface, the next step in the experiment was to test the fabricated surface's hydrophobicity. Desbief et. al. used two methods for testing hydrophobicity: the sliding test and the drop test. Polymers CP1 and CP4 proved to be the most hydrophobic. Desbief et. al. conclude that they were able to make a surface "which is fully superhydrophobic over areas up to 4 cm<math>^2</math>."
Soft Matter Details
- Experimental Methods:
This experiment makes use of many techniques which I imagine are widely used in soft matter:
-Atomic Force Microscopy (AFM) was used to analyze the roughness of the surface. Figure 3 shows the AFM images which the authors used in their calculation of the Wenzel roughness. Desbief et. al. believe their AFM estimates of Wenzel roughness are unreasonably low. The authors also used AFM to verify that micelles accumulated on the surface as anticipated.
-Proton Nuclear Magnetic Resonance (<math>^1</math>H NMR) was used to determine the molecular weights of the two components of the diblock copolymer. Desbief et. al. created four different diblock copolymers simply by varying the lengths of the the PAN block and the fluorinated block.<math>^1</math>H NMR allowed the scientists to determine the structure of the polymers they created.
-Drop Impact Analysis is one way to test a surface's hydrophobicity. The test requires dropping different sized drops of water onto the surface at different speeds and recording the event with a high-speed camera. Figure 7 shows the fascinating shapes one water drop takes on as it bounces off the hydrophobic aluminum.
-The Sliding Method is an alternate way to test hydrophobicity. Desbief et. al. slid small drops of water across their aluminum surface and monitored them with a Kruss DSA100 contact angle analyzer. The researchers found that to accurately characterize the surface, it was important to slide the drops in two perpendicular directions because the surface had regular striations in one direction.
- Scalable solutions:
The inspiration for this research was finding a hydrophobic treatment that could be scaled up for larger projects. I find this interesting because soft-matter research is often stuck at small scales (energies on the scale of kT, lengths on the scale of capillary lengths for example). Creating micelles of polymers is certainly a soft-matter topic, but having the insight to utilize self-assembled nanostructures to impact macoscopic properties makes this research very interesting.
- Difficult synthesis, really easier than other methods?
The polymer chemistry described in the paper seems very complex. The authors seek an "easily scalable" technique. Is the polymer synthesis simple enough to make large quantities for a commercial application?