Transition Bands in Gas–Liquid Microchannel Flows: Nonlinear Dynamics, Predictive Modeling, and Implications for Microreactor Design and Process Intensification

Abstract A gas–liquid microreactor can be operated inside a nominally stable flow regime and still produce irreproducible transport and reaction histories when its operating point lies close to an unresolved transition interval. Conventional regime maps are useful for classifying bubbly, Taylor, annular, and parallel flows, but their sharp boundaries can conceal finite transition bands in which interfacial states coexist, switch intermittently, evolve downstream, or depend on the operating history. In these intervals, small changes in local bubble formation or liquid-film renewal can propagate into altered residence time histories, pressure responses, and eventually channel-to-channel maldistribution during numbering-up. Transition bands are therefore more than ambiguous margins in flow pattern maps; they are process-relevant operating intervals that can set practical limits on transport reproducibility, output uniformity, and scale-up reliability. This Review synthesizes the physical mechanisms, characterization methods, and predictive strategies needed to identify and quantify such intervals, with emphasis on transition-width definition, uncertainty-aware prediction, and physics-data-experiment integration. It then translates this transition-band perspective into objective-dependent engineering strategies: avoiding high-uncertainty intervals when stable operation is required and deliberately exploiting bounded interfacial restructuring when transport intensification is beneficial. By connecting interfacial state dynamics with operating window design, process response variability, and scale-up robustness, this review provides a transition-band-aware framework for reliable gas–liquid microreactor operation and process intensification.