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.
The authors' goal in this study was 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. Developing such a technique required two steps: first, the development of a sythesis routine for making an appropriate polymer and attaching it to a surface, and second, testing the hydrophobicity of the surface.
To make an 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 co-polymer has a polyacrylonitrile (PAN) block and a fluorinated block. The four polymers described in this paper vary by the molecular weights of the two polymers making up the block copolymer, as seen in figure 1 (PAN is the N block, the flourinated block is F).
DMF (dimethylformamide), TFT (trifluorotoluene) Because the PAN block is (...) phobic, while the fluorinated block is (...._ philic), the polymer forms 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 surface gives the surface a bumpy surface 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.
Having modified the surface, the next step in the experiment was to test its hydrophobicity to see if the surface treatment was a success. They used two methods of testing hydrophobicity: the sliding test and the drop test.
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. The authors also used AFM to verify that micelles attached to 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?