Novel approaches for quantitative assessments of wetting development in membrane distillation based on optical coherence tomography

Optical coherence tomography (OCT) has been considered as a non-invasive imaging tool to provide real-time, local information of wetting in membrane distillation (MD). However, the follow-up research question is how to quantitatively assess the localized wetting development in order to prevent system failure. This study aims to develop a quantification method based on the changes in the intensity distribution within OCT three-dimensional datasets (volume scan, C-scans). The achieved maps elucidate the wetting depth (e.g., wetting progress) across the membrane area in various cases. Severe wetting with homogeneous and heterogeneous distribution, and even subtle wetting have been quantified successfully. Results indicate that an increase in the volume of wetted membrane (expressed as the wetting ratio) does not necessarily correspond to an increase in the membrane area that is fully wetted (expressed as fully-wetted fraction), revealing the limiting parameter for deterioration in condensate quality. Additionally, the underlying mechanism governing the wetting behavior was also discussed based on the quantified wetting parameters. This OCT-based method would be helpful to investigate wetting not only for MD but also potentially for other membrane processes involving two-phase flow such as gas–liquid membrane contactors and membrane biofilm reactors.