## 3 Results and Discussion  
### Membrane Morphologies in NIPS vs. VIPS  
Two distinct membrane morphologies were obtained for membranes prepared using NIPS and VIPS processes. In general, VIPS membranes, irrespective of applied RH, show a much higher CA (>125\({}^{\circ}\)) than the NIPS membrane (CA = 90.74\({}^{\circ}\)), as shown in Table 2. This finding regarding VIPS morphology was also reported elsewhere [14]. When calculating the LEP of membranes prepared using both processes, they all range from 103 to 107 kPa, which makes the membranes vulnerable to wetting. This low LEP range, mostly caused by the high BP values for all membranes, disallows the application of high operational pressure (typically up to 200 kPa depending on MD module system and desired cross-flow velocity) that is required to overcome pressure-drop in both the feed and the permeate streams in a module system. Additional pressure is also required to induce sufficient cross-flow velocity for controlling concentration polarization on the membrane surface. The advantage of high CA from the VIPS and low BP from NIPS to achieve high LEP is diminished because of large BP (10-21 \(\upmu\)m) of VIPS membranes and low CA of NIPS membranes. Nevertheless, this finding brings about a potential for disentangling these features in the two processes to gain the advantages of both processes (high CA and low BP). As mentioned at the bottom of Table 2, all the LEP values in this table and throughout this manuscript were calculated using the Cantor-Laplace equation,  
\[\text{LEP}=\frac{-2\gamma\cos\theta}{r_{\text{max}}}\]  
where \(\gamma\) is the surface tension of the wetting liquid (in this case water at 25 \({}^{\circ}\)C, 0.07199 N/m), \(\theta\) is the contact angle between the membrane and the wetting liquid (water) and \(r_{\text{max}}\) is the maximum pore radius of the membrane (i.e., bubble-point pore size). It is worth noting that the assumption in this equation is that pores are perfectly cylindrical with a constant radius of curvature. Since all membranes were prepared from the same material, the difference in the surface morphology can thus be held responsible for the change in surface CA values, as can be seen in Table 2 and Figure 1. It is known that highly hydrophobic surface is a result of the presence of multi-scale surface features consisting of both micro- and nano-scale roughness [7; 15; 16; 17], also known as the lotus leaf effect. The high contact angle on the surface of a lotus leaf is a consequence of such micro- and nano-scale roughness, the micro- and nano-scale roughness can be clearly seen in our case for VIPS-60% and VIPS-80% SEM images, Figure 1. The micron-sized pore mouth most likely contributed to the micro-scale roughness for both VIPS and NIPS membranes. The higher CA of the VIPS membranes originated from the nano-scale roughness from nodule-like morphology (circled features in Figure 1) that most probably originated from small spherulites of PVDF crystals. These can be clearly seen in the VIPS-60% and VIPS-80% SEM images. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Membrane** & **MPS (\(\upmu\)m)** & **BP (\(\upmu\)m)** & **CA** & **LEP (kPa) \({}^{\star}\)** \\ \hline NIPS & 0.08 & 0.38 & 90.7 & 103.91 \\ VIPS (37\% RH) & 7.80 & 12.25 & 136.3 & 106.79 \\ (60\% RH) & 5.85 & 21.19 & 130.9 & 103.56 \\ (80\% RH) & 6.07 & 10.63 & 127.2 & 106.55 \\ \hline \hline \end{tabular} \({}^{\star}\)Calculated using Cantor–Laplace equation. MPS: mean flow pore size; BP: bubble-point pore size; CA: contact angle; LEP: liquid entry pressure; RH: relative humidity.  
\end{table} 
Table 2: Summary of membrane properties prepared using NIPS and vapor-induced phase separation (VIPS) process, prepared under casting thickness of 500 \(\upmu\)m using 12% \(w/w\) PVDF concentration. The formation of more crystalline structure in the VIPS membranes was possible due to the nature of compositional change pathway during the phase inversion process. This pathway can be explained using a three-component system phase diagram (Figure 2).