Role of wetting state in improved prediction of wetting resistance in membrane distillation

Membrane wetting remains a critical limitation in membrane distillation (MD), yet contact angle ( θ ) measurements that are commonly used to assess wetting resistance are defined and interpreted inconsistently in the MD literature. In this study, we characterize the surface properties and wetting resistance of three commercially available polytetrafluoroethylene membrane distillation (MD) membranes and clarify the physical meaning of commonly reported contact angles. By adapting an established method to quantify wetting-state transitions, we show that conventional contact angle measurements ( θ m , conv ) correspond to a Cassie-Baxter wetting state. We further demonstrate that the transition to a Wenzel wetting state occurs at applied pressures of ~270–290 Pa, well below typical MD operating pressures, indicating that these hydrophobic membranes operate in a Wenzel wetting state during MD. To more accurately characterize wetting resistance during MD, we introduce methods to calculate a pore-scale Wenzel state contact angle, θ W ∗ , based on measured membrane roughness and intrinsic contact angle ( θ 0 ). The resulting θ W ∗ values are unique: θ W ∗ is significantly greater (by 12–17%) than θ 0 and significantly less (by 11–17%) than θ m , conv . Importantly, the different definitions of contact angle lead to large discrepancies (20–185%) in predicted liquid entry pressure (LEP). Using electrochemical impedance spectroscopy to precisely measure LEP, we show that theoretical LEP values based on θ m , conv overestimate wetting resistance, while those based on θ 0 underestimate it. In contrast, LEP predictions based on θ W ∗ are in closest agreement with experimental measurements for all three membranes. This indicates that θ W ∗ best represents contact angles at pore entrances under MD operating conditions. This work provides new methods and physically meaningful metrics (i.e., θ W ∗ and theoretical LEP based on θ W ∗ ) for predicting wetting resistance, with direct implications for membrane design and operation.