Modeling the separation of water‐in‐oil emulsions in continuously fed gravity settlers using millifluidic experiments

Emulsion separation remains a persistent challenge in chemical and process industries due to the metastable nature of dispersed droplets. In gravity separators, the overall separation rate is governed by the formation of a densely packed zone (DPZ) of deforming and coalescing droplets that mediates between the dispersed and continuous phases. Conventional bottle tests fail to capture the dynamic behavior of such systems under continuous‐flow operation. In this work, a minimal millifluidic platform is developed to emulate gravity‐driven separation under steady inflow, enabling direct observation of DPZ evolution. By independently controlling the emulsion feed rate and coalescence rate through chemical additives, two distinct operating regimes are identified: one yielding a stationary DPZ height and another exhibiting unbounded growth. A stochastic coalescence model captures this transition and quantitatively predicts the critical feed rate and steady‐state DPZ height through dimensionless parameters. This framework advances predictive understanding of emulsion separation in continuous‐flow processes.