# Surface Coatings and Treatments for Controlled Hydrate Formation: A Mini Review
## 3 Hydrate-Public Surface Treatments and Coatings
### Polymeric Coatings  
[PERSON] et al. [42] demonstrated using a chemical vapor deposition (CVD) method that hydrate-phobic bilayer polymeric coatings pDVB/pPFDA of varied thickness on steel and silicon surfaces can lead to a reduction in the ice and hydrate adhesion strength (Figure 2B). Bilayer polymer coatings of a highly dense, crosslinked network on top of a steel substrate covered with a 40-nm, fluorine-rich polymer film exhibited a 10-fold reduction in the hydrate adhesion strength. A significant substrate effect was observed through changes in the roughness values of the polymer films. The roughness in the case of the steel substrate was higher compared to silicon. Similarly, a slight difference in the contact angles was also observed due to increased fluorine concentration and different crystallinity of the 40-nm-thick films compared to the 10-nm one. The authors scanned a wide concentration (0-70 wt.%) of THF solutions and proposed that 19 wt.% should be used as a probe liquid. No statistically significant effect of subcooling and time on the adhesive behavior was observed since the adhesion strengths were comparable irrespective of their formation conditions. After a freezing/hydrate deposition and de-adhesion cycle, the coatings remained functional. However, their long-term durability in aqueous media and humid conditions needs to be explored. These polymeric coatings were also tested for reducing the adhesion strength of ice and cyclopentane (CP) hydrates [43], another widely used model compound for studying natural gas hydrates at atmospheric pressure. The results were similar to what had been observed in the case of THF hydrates. The coated surfaces showed more than a 6-fold reduction in ice and 10-fold reduction in hydrate adhesion strength compared to bare steel and silicone surfaces. The durability of these coatings was assessed through sand erosion tests and repeated adhesion/de-adhesion cycles. The cost associated with the fabrication of these polymeric coatings is low due to the usage of low-cost polymer pDVB beneath a thin layer of pPFDA. The authors claimed that these covalently grafted polymeric coatings possessing the ice-phobic and hydrate-phobic as well as hydrophobic characters will be potential candidates for several industrial applications. [PERSON] et al. [49] once again reported a detailed study on some mechanically robust hydrate and icephobic polymeric coatings that can be stable in harsh subsea or arctic environments with high pressures and low temperatures. They used initiated chemical vapor deposition (iCVD) method to apply rough polymeric films having a bilayer of polyvinylbenzene (pDVB)/poly-perfluorodecylacrylate (pPFDA) on commercial silicon and steel substrates, showing a water contact angle above 150\({}^{\circ}\). Surface roughness of the films was recorded as 178 and 313 nm on silicon and steel substrate, respectively, measured using optical profilometer. After application of the coatings on a rough steel surface, a six-fold reduction in the adhesion strength of the ice and a 10-fold reduction in cyclopentane hydrates' adhesion was observed. Sand erosion, nanoindentation, and nano-scratch tests and multiple adhesion/de-adhesion cycles ensured the robustness and durability of the developed coatings given the fact that they are covalently attached to the substrate surface. The first report where hydrate-phobic coatings were tested in realistic conditions of high pressure was published by [PERSON] et al. [44]. The authors not only assessed the effect of hydrophobic coatings on the methane hydrate formation but also reported that the usage of such coatings can also help to cut down the concentration of LDHis significantly. Theycarried out the tests in separate stainless-steel crystallizers made up of non-coated and coated with hydrophobic perfluoroalkoxy alkane polymer (PFA). The interiors of the crystallizer including baffle, lid, impeller, and thermocouple were also coated with ~1000-mm PFA film except the shaft.