# Properties of the Iron Bacteria Biofouling on Ni-P-rGO Coating
## 2 Experimental
### Contact Angle and Surface Energy  
Contact angles were obtained by the sessile drop method with the data physics OCA-20 contact angle analyzer, which had an accuracy of \(\pm\)0.25\({}^{\circ}\)[22]. By using distilled water, diiodomethane, and ethylene glycol as probes, every sample was measured six-to-ten times. The relationship between the contact angle of the probe liquids on a solid substrate and the surface energy is described as follows:  
\[\gamma_{L}cos\theta\ =\ \gamma_{s}-\gamma_{SL} \tag{1}\]  
where \(\gamma_{L}\) is the surface tension of the experimental liquids, \(\theta\) is the contact angle, \(\gamma_{S}\) is the solid surface energy, and the \(\gamma_{SL}\) is the solid and liquid interfacial energy. The solid and liquid interfacial energy equation is the following:  
\[\gamma_{SL}\ =\ \gamma_{S}+\gamma_{L}-2(\sqrt{\gamma_{S}^{LW}\gamma_{L}^{LW}}+ \sqrt{\gamma_{S}^{+}\gamma_{L}^{-}}+\sqrt{\gamma_{S}^{-}\gamma_{L}^{+}} \tag{2}\]  
where \(\gamma_{S}^{LW}\) and \(\gamma_{L}^{LW}\) are the Lifshitz-van der waals apolar component of solid surface and the experimental liquid, respectively; \(\gamma_{S}^{+}\) and \(\gamma_{S}^{-}\) are the electron acceptor and the electron donor of the solid surface, respectively; and \(\gamma_{L}^{+}\) and \(\gamma_{L}^{-}\) are the electron acceptor and the electron donor of experimental liquid, respectively.  
By combining Equations (1) and (2), we can obtain:  
\[\gamma_{L}(1+cos\theta)\ =\ 2\Big{(}\sqrt{\gamma_{S}^{LW}\gamma_{L}^{LW}}+ \sqrt{\gamma_{S}^{+}\gamma_{L}^{-}}+\sqrt{\gamma_{S}^{-}\gamma_{L}^{+}}\Big{)} \tag{3}\]  
Surface energy can be seen as the sum of a Lifshitz-van der Waals apolar component \(\gamma_{i}^{LW}\) and a Lewis acid-base polar component \(\gamma_{i}^{AB}\)[23]:  
\[\gamma_{i}^{TOT}\ =\ \gamma_{i}^{LW}+\gamma_{i}^{AB} \tag{4}\]  
\begin{table}
\begin{tabular}{c c} \hline
**Nickel Sulfate** & **25 g/L** \\ \hline Sodium Hypophosphite & 30 g/L \\ Citric Acid & 15 g/L \\ Lactic Acid & 16 g/L \\ Sodium Acetate & 11 g/L \\ Potassium Iodide & 0.01 g/L \\ Graphene & 20–80 mg/L \\ OP-10 & Appropriate \\ pH & 4.8 \\ Temperature & 83 \({}^{\circ}\)C \\ \hline \end{tabular}
\end{table}
Table 1: The electroless bath solution composition and operating conditions.  
The acid-base polar component \(\gamma_{i}^{AB}\) can be obtained by taking advantage of the relationship between the electron donor \(\gamma_{i}^{-}\) and the electron acceptor \(\gamma_{i}^{+}\) subcomponents:  
\[\gamma_{i}^{AB}\ =\ 2\sqrt{\gamma_{i}^{+}\gamma_{i}^{-}} \tag{5}\]  
In order to obtain the surface energy components (\(\gamma_{S}^{LW}\), \(\gamma_{S}^{-}\), and \(\gamma_{S}^{+}\)) of a solid, the contact angles on the solid and the surface tension components (\(\gamma_{L}^{LW}\), \(\gamma_{L}^{-}\) and \(\gamma_{L}^{+}\)) of three selected liquids (distilled water, diiodomethane, and ethylene glycol) must be determined. The surface tension components of the three liquids are reported in Table 2.  
Table 3 reports the contact angles and surface energies among the three selected liquids and the different coating materials. It was found that with the increase of the graphene concentration, the contact angle of H\({}_{2}\)O and C\({}_{2}\)H\({}_{6}\)O\({}_{2}\) increased as well. In contrast, the contact angle of CH\({}_{2}\)I\({}_{2}\) was not influenced by the variation of the graphene concentration. In order to obtain the surface energy of bacteria, the suspension of iron bacteria was centrifuged. The centrifuged iron bacteria were then uniformly coated on a clean glass sheet and dried by air for 1 h. Then, the contact angle of the iron bacteria membrane was measured.