# Surface Coatings and Treatments for Controlled Hydrate Formation: A Mini Review
## 2 Hydrate Adhesion and Deposition  
It was envisaged from several works that hydrate deposition occurs at the pipeline wall. One study [63] suggested that if sufficient water is available, the hydrate particles will start growing either in the liquid phase or on a suitable seed present in the liquid phase, close to the gas/liquid interface, or at the pipeline wall. In another study [64] it was found that hydrate agglomeration was less profound compared to the stickiness between hydrate particles, hydrate particles to the wall, or a combination of the two effects. They suggested that wettability of the pipeline wall plays an important role in determining the stickiness of the hydrate particles, which are hydrophilic in nature. In the case of a hydrophobic surface, the formation of a water bridge can be reduced, which may influence the hydrate nucleation and deposition.  
Later, [PERSON] et al. [65] investigated the possibility of hydrate deposition on pipeline walls since it is the coldest point in the system and contains nucleation sites. A micromechanical force (MMF) apparatus was used to measure the strength of adhesion force of cyclopentane (CP) hydrate to a carbon steel substrate. The micromechanical force (MMF) measurement is a central technique that has been used to measure the strength of adhesive forces between the solid substrate samples and the hydrate particles in a majority of the works presented in this review. The working principal of the MMF apparatus is presented briefly in Figure 1, which can be consulted in detail elsewhere [65; 66; 67; 68]. It was found that the adhesion forces between the hydrate-hydrate particles were higher than the forces between the hydrate and carbon steel in a water-free environment. It was proposed that capillary forces between hydrate and steel were dominating but not enough for the hydrates to deposit on the wall.  
[PERSON] and [PERSON] [69] critically scrutinized the research revolving around various interfacial phenomena involved in gas hydrates' systems and pointed out that solid/solid interfacial behavior, especially adhesion of hydrate to the pipeline wall, is a crucial factor in flow assurance. This idea was later experimentally corroborated [70] and the need to address this issue using chemical and physical methods has been emphasized.  
\begin{table}
\begin{tabular}{l l l l} \hline \hline
**Hydrate Formers** & **Substrates** & **Coatings** & **Experimental Conditions** & **References** \\ \hline Tetrahydrofuran & -X0 pipeline steel & -Fluoro-coating (F-coating) & \\ (19.0 wt.\% in water) \& -X80 pipeline steel & -Polydimethylsiloxane (PDMS) & Atmospheric Pressure, \\ Methane & -Zirconia (ZrO\({}_{2}\)) & -Hexagonal boron nitride (HBN) & \(-3\) °C; \\  & -Tin plate & -Hydrophobic fuming SiO\({}_{2}\) (HB-630, diameter \(\leq\) 300 nm) & \(-10\) °C, 82 MPa \\ \hline Cyclopentane & -X90 pipeline steel & -Copper oxide (CuO) layer modified with & Atmospheric pressure, \\  & & & 1 °C \\ \hline Cyclopentane & -X80 pipeline steel & -Cerium oxide (CeO\({}_{2}\))/polydopamine (pDA) & Atmospheric pressure, n.a. (0 °C \textgreater{} T \textgreater{} 7.7 \text{\text{\textdegree}}\)C & [54] \\ \hline \hline \end{tabular}
\end{table}
Table 1: _Cont._