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[Display omitted] • Gelation of chitosan dissolved in an alkaline aqueous solution was investigated. • Ethanol-water mixtures were employed as the coagulant for the APS processes. • Gelling film in the ethanol-containing coagulant was in-situ characterized by OCT. • Film morphology was analyzed by combining OCT and network-based modeling. • Internal rejection of particles was correlated with the morphological formation. Gelation of chitosan dissolved in an alkali/urea aqueous solution promotes the mechanistic diversity of forming a membrane via aqueous phase separation (APS), which is conceptualized to highlight the advantages of using environmentally friendly solvents. When aimed at deepening the knowledge of regulating the structural formation, the alkaline dope of chitosan was employed in this study to investigate how the presence of ethanol in the coagulant could regulate the gelation behavior in APS and thereby change the particle-trapping mechanism. Both the gelling films and the particle-filtering membranes were in-situ characterized by optical coherence tomography (OCT) to compare the film-formation kinetics and the morphological variations in the resulting porous substructures, respectively. It was revealed that the ethanol-containing coagulant could induce the shrinkage of the gelling film and thereby favor the transverse coalescence of polymer-rich phases; that is, increasing the content of ethanol in the coagulant (over a range less than a critical value to avoid film deformation) would give rise to more tortuous substructures (for effectively capturing particles) primarily owing to the lower connectivity in the transverse direction. The theoretical analysis was supported by numerically evaluating the topological analog. This study not only confirms effectiveness of the ethanol-based regulation, but also establishes a paradigm for fabricating membranes with tailored topological characteristics. [ABSTRACT FROM AUTHOR]