## 3 Technical Challenges and Recent Efforts to Address Them
### Membrane Wetting  
Membrane wetting is one of the most crucial challenges related to MC operation. Liquid intrusion into membrane pores causes pore wetting in microporous membranes. Membrane wetting, which may be either a partial or full liquid penetration, drastically affects the mass transport property in MCs. In fact, the intruded liquid forms a stagnant layer, a passive boundary inside the membrane pores, resulting in additional mass transfer \begin{table}
\begin{tabular}{l l} \hline \hline
**Mass Transfer Coefficient** & **Parameter** \\ \hline Shell side mass transfer coefficient & \(k_{s}=D\beta\frac{(1-\varphi)}{Z}\left(\frac{d\varphi p}{HZ}\right)^{0.6}\left( \frac{\mu}{pD}\right)^{0.33}\) & \(k_{s}\) : shell side mass transfer coefficient \(\left(\text{m s}^{-1}\right)\) \\ For \(0<\text{Re}<500\) and \(0.04<\varphi<400\) & \(k_{M}\) : membrane mass transfer coefficient \(\left(\text{m s}^{-1}\right)\) \\ \(k_{s}=8D\beta\frac{(1-\varphi)}{Z}\left(\frac{d\varphi}{HZ}\right)\left(\frac{ \mu}{pD}\right)^{0.33}\) & Re : Reynold’s number \\ For \(0.5<\text{Re}<500\) and \(\varphi=0.03\) & \(\text{Gz}\) : Gratz number \\ \(k_{s}=1.25D\frac{(1-\varphi)}{Z}\left(\frac{d\varphi p}{HZ}\right)^{0.93}\left( \frac{\mu}{pD}\right)^{0.33}\) & \(D\) : diffusion coefficient \(\left(\text{m}^{2}\text{ s}^{-1}\right)\) \\ \(k_{T}=1.62\frac{D}{d}\left(\frac{\varphi p}{ZD}\right)^{0.33}\) & \(Z\) : effective module length \(\left(\text{m}\right)\) \\ for \(\text{Gz}>4\) & \(d_{e}\) : hydraulic diameter \(\left(\text{m}\right)\) \\ \hline Porous membranes & \(\mu\) : dynamic viscosity of the fluid \(\left(\text{Pa s}\right)\) \\ \(\text{k}_{M}=\frac{D_{M}/\varepsilon}{\tau\delta}\) & \(\rho\) : liquid density \(\left(\text{kg m}^{-3}\right)\) \\  & \(\varphi\) : packing density \\ \hline Dense membranes & \(\varepsilon\) : membrane porosity \\ \(\text{k}_{M}=\frac{Permability\times Partial\;Press}{Memerate\;Tickness}\) & \(\tau\) : tortuosity \\ \(\delta\) : membrane thickness \(\left(\text{m}\right)\) & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Individual mass transfer correlations for counter-current flow [11; 49; 50]. resistance between the liquid and gas phases. [PERSON] et al. reported that a 20% increase in the overall mass transfer resistance could be induced merely by 5% of pore wetting [53]. In general, membrane wetting can be triggered by a liquid pressure exceeding the critical entry pressure of a membrane (Figure 3a) and/or the capillary condensation (Figure 3b). \[\Delta\mathrm{P}=\frac{2\gamma\cos\theta}{r} \tag{1}\]  
The Laplace-Young equation in Equation (1) adequately describes the wettability of a membrane [54]. According to this equation, the liquid entry pressure (LEP; \(\Delta\mathrm{P}\)) is influenced by the liquid surface tension (\(\gamma\)), the membrane pore radius (r), and the contact angle (\(\theta\)). Thus, the factors that foster membrane wetting are membrane-related (i.e., hydrophobicity, membrane pore size, pore geometry, and microstructure), liquid-related (i.e., surface tension), or operation-related (i.e., liquid flow rate, vacuum pressure). Hence, one should be very careful in controlling those factors to prevent the pore wetting while operating MC systems. Several strategies have been proposed and investigated to mitigate pore wetting by improving membrane-related factors. Several researchers have tried to make superhydrophobic membrane surfaces using fluorinated inorganic nanomaterials such as SiO\({}_{2}\)[55; 56; 57; 58; 59; 60; 61; 62], TiO\({}_{2}\)[55; 63; 64; 65], Al\({}_{2}\)O\({}_{3}\), and ZrO\({}_{2}\)[65]. For instance, [PERSON] et al. successfully fabricated a highly hydrophobic Matrimid(r)-fluorinated silica composite membranes exhibiting high water contact angles (WCA) ranging in 122-124\({}^{\circ}\), leading to an enhanced wetting resistance [59]. The field emission scanning electron microscope (FESEM) images of the membrane surface before and after modification are shown in Figure 4a,b, respectively.