AbstractFlow regime maps are plotted and the universal transitional boundary of squeezing regime and dripping regime is gained when both inertial force and viscous force of the dispersed phase are considered. The neck thinning process within the rectangular channel is divided into three stages according to evolutions of the neck width: two‐dimensional shrinking stage, transitional stage, and collapsing stage. Influences of the flow rates of two phases and the dispersed viscosity on evolutions of the neck width at different stages are explored. Different relations between the neck width and the remaining time are found for the earlier two stages. At the third stage, two physical mechanisms are separated by the dispersed viscosity as the neck width decreases according to wn/W ~ τ2/3 under the inertial regime for both water and 40% glycerol, while the neck width reduces linearly as wn ~ 2(0.0709σ/μd)tr under the viscous regime for 55% glycerol.