# Preparation and Application of ZIF-8 Thin Layers
## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & & **Water** & & & **Diiodomethane** \\ \hline
**Surface** & **Oil** & \(\theta_{a}\)/**deg** & \(\theta_{a}\)/**deg** & \(\Delta\theta\)/**deg** & \(\theta_{a}\)/**deg** & \(\theta_{a}\)/**deg** & \(\Delta\theta\)/**deg** \\ \hline ZIF-8/PVDF & none & 149.9 \(\pm\) 4.0 & n/a & - & 127.8 \(\pm\) 3.7 & n/a & - \\ (20\%) mono & GPL 100 & 128.8 \(\pm\) 4.0 & n/a & - & 97.1 \(\pm\) 6.3 & n/a & - \\ layer & GPL 105 & 115.1 \(\pm\) 1.4 \(\pm\) 2.3 & 104.1 \(\pm\) 2.3 & 11.0 \(\pm\) 4.9 & 97.4 \(\pm\) 2.8 & 87.7 \(\pm\) 2.7 & 9.7 \(\pm\) 3.9 \\ \cline{2-7} ZIF-8/PVDF & none & 149.2 \(\pm\) 4.5 & n/a & - & 123.1 \(\pm\) 6.9 & n/a & - \\ (20\%) double layer & GPL 100 & 130.9 \(\pm\) 3.0 & n/a & - & 100.4 \(\pm\) 4.9 & n/a & - \\  & GPL 105 & 115.6 \(\pm\) 3.0 & 106.1 \(\pm\) 2.6 & 9.5 \(\pm\) 4.0 & 94.0 \(\pm\) 5.3 & 86.4 \(\pm\) 4.6 & 7.6 \(\pm\) 7.0 \\ \cline{2-7} ZIF-8/PVDF & GPL 105 & 119.3 \(\pm\) 1.2 & 112.6 \(\pm\) 3.1 & 6.7 \(\pm\) 3.3 & - & - & - \\ ZIF-8/PVDF & GPL 105 & 119.4 \(\pm\) 2.4 & 111.6 \(\pm\) 3.7 & 7.8 \(\pm\) 4.4 & - & - & - \\ ZIF-8 & GPL 105 & 116.5 \(\pm\) 1.5 & 111.3 \(\pm\) 2.0 & 5.2 \(\pm\) 2.5 & 94.6 \(\pm\) 1.4 & 87.9 \(\pm\) 2.3 & 6.7 \(\pm\) 2.7 \\ ZIF-8 & GPL 105 & 121.0 \(\pm\) 4.9 & 98.0 \(\pm\) 4.7 & 23.0 \(\pm\) 6.8 & - & - & - \\ PVDF & none & 101.9 \(\pm\) 2.2 & 41.9 \(\pm\) 3.5 & 60.0 \(\pm\) 4.1 & 82.2 \(\pm\) 4.5 & 43.5 \(\pm\) 3.7 & 38.7 \(\pm\) 5.8 \\ HKUST-1 [33] & GPL 105 & 107.1 \(\pm\) 1.3 & 5.7 \(\pm\) 2.7 & 101.4 \(\pm\) 3.0 & & & \\ (e-coating) & \multicolumn{1}{c}{} & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Advancing contact angle \(\theta_{a}\), receding contact angle \(\theta_{e}\) and contact angle hysteresis \(\Delta\theta\) of water and diiodomethane measured on ZIF-8 and PVDF layers. For receding contact angles reported as n/a, droplet pinning occurred and the contact line showed no significant movement. Values for oil-infused HKUST-1 are taken from ref. [33] and included for comparison. be seen from the comparison with the reported contact angle values of HKUST-1 [33]. Compared to the ZIF-8 surfaces, the different surface of the HKUST-1 base structure leads to a slightly lower advancing water contact angle of 107\({}^{\circ}\) but to a significantly higher water contact angle hysteresis of 101\({}^{\circ}\). The advancing and apparent contact angles are mainly governed by the oil layer, while the contact angle hysteresis depends strongly on the properties of the base structure. The underlying mechanism for this has not yet been conclusively clarified, but in our opinion, there are two possible effects can explain these findings. It has been shown before that stable oil infusion is achieved by hemivicking, but surface features of the underlying host layer can penetrate the oil and are then in direct contact with the droplet on top [33]. This creates an inhomogeneous surface that is characterized by a different surface chemistry and morphology as well as different surface tensions. This inhomogeneity leads to contact line pinning [44; 45; 46], which increases the contact angle hysteresis. By adding PVDF to the surface coating, the differences in polarity and surface tension between oil and penetrating spikes can be reduced, leading to a more chemically homogeneous surface. At the same time, the interaction between MOF/PVDF composite and the perfluorated oil is increased, which leads to a better oil infusion of the spikes. This effect can explain the lower contact angle hysteresis on the ZIF-8/PVDF composite layers compared to the electrochemically deposited ZIF-8 samples. The fact that we see no dependence between hysteresis and PVDF content, as well as the fact that we did not see any PVDF-enriched spikes in TEM studies, indicates a second effect that leads to the differences in the contact angle hysteresis. This second effect is directly related to the surface roughness. A smoother host material decreases the amount and size of spikes penetrating the oil.