## 1 Introduction  
The membrane distillation (MD) process is driven by the vapor pressure difference between a hot and a cold stream in the feed and permeate side of a membrane module, respectively. This driving force is created by the temperature difference between the hot and cold streams. Both streams are separated by a membrane that ideally features the following properties: high hydrophobicity, high porosity, low tortuosity, and low thickness. In addition to low mass transfer resistance, an MD membrane should be able to withstand a certain liquid entry pressure (LEP) to avoid wetting [1]. Theoretical LEP of a membrane is a function of maximum pore size (also called bubble point, BP) and surface hydrophobicity, expressed in terms of the surface contact angle (CA) with water. LEP can be estimated using the Cantor-Laplace equation [1]. Polyvinylidene fluoride (PVDF) is an attractive MD membrane material, which can be made into membranes via phase inversion. It has low surface energy, good thermal stability and low conductivity. In the phase inversion process, a thermodynamically-stable polymer solution (mixture of polymer and its solvent) is brought to instability by several means: non-solvent-induced phase separation (NIPS), vapor-induced phase separation (VIPS) or temperature-induced phase separation (TIPS). All of these have been applied to prepare PVDF-based MD membranes [2; 3; 4]. The NIPS process is the most popular and has been used to prepare different types of membranes, including reverse osmosis, nanofiltration, ultrafiltration, microfiltration and MD [5; 6]. When used to produce PVDF membranes in a strong solvent/non-solvent system, typically, the NIPS process is characterized by a relatively fast mass transfer rate (instantaneous demixing, relative to VIPS) leading to asymmetric structure (dense top layer, supported by a more porous layer) [5], narrow pore size distribution (PSD), relatively low mean flow pore size (MPS) and maximum pore size (BP) very close to MPS; all are desired properties of an MD membrane. However, under those same conditions, NIPS also produces a membrane with a fairly smooth surface (low roughness) leading to a low surface energy (high CA), which promotes low LEP and thus membrane wetting. Increasing the CA of PVDF membranes has been demonstrated by applying weak non-solvent (ethanol) in a single bath system [7] and weak non-solvent (ethanol) as the first bath followed by strong non-solvent (water) in the second bath of a dual-bath coagulation system [8]. These methods increased the CA significantly (up to 153\({}^{\circ}\)) but the resulting membranes had low porosity originating from a dense structure as a result of a delayed demixing that led to a high mass-transfer resistance. Superhydrophobic PVDF membranes can also be obtained via the VIPS process, but feature large BP that diminishes the LEP [8]. A study which investigated the phase inversion mechanisms that led to the formation of a highly hydrophobic surface [3] emphasized the importance of solution composition pathway when crossing both binodal and spinodal lines in the three-phase diagram. Depending on the route of the composition changes, porous and spherulitic structures resulting from PVDF crystallization resulted in a hydrophobic surface, while net-like morphology as a result of spinodal-decomposition (SD) resulted in a superhydrophobic surface [8]. The latter can be produced under low PVDF concentrations (<5%), high relative humidity (RH) and high vapor temperature. However, too-low PVDF concentrations produce membranes that are vulnerable to wetting and low-salt rejections (SR). At such low concentrations, there is a lack of mechanical stability in the membrane and the anchoring of the membrane on the support layer becomes problematic. To avoid this problem, [PERSON] et al.