Local deformation and coalescence between two equal-sized droplets in a cross-focused microchannel

•Three flow patterns of head-on coalescence of droplets are observed.•Effects of viscosity ratio and surfactant are explored.•Local nipple of droplet is conductive to coalescence.•Influence factors of the nipple are investigated systematically.•A novel model and a predictive equation of the nipple are proposed. The local deformation and the head-on coalescence dynamics of droplets in a cross-focused microchannel are experimentally investigated. The dynamic process of droplets after collision is divided into squeezing stage and decompressing stage, and the flow patterns are divided into squeezing coalescence, decompressing coalescence and non-coalescence based on the moment when the liquid film ruptures. The local nipples caused by the lubrication force in the thin film arise in the decompressing stage and facilitate coalescence. The effects of the dispersed to continuous-phase viscosity ratio and the concentration of surfactant to the dynamic process, operating range of each flow pattern, and the extent of local deformation are investigated, and the predictive equation of the nipple angle is proposed. This research combines the local deformation with coalescence and provides a reference for further understanding the mechanism of droplet coalescence.