Experimental and theoretical studies on neck thinning dynamics of droplets in cross junction microchannels

Abstract

Necking processes of the droplet generation in the cross junction microchannels are experimentally and theoretically investigated, in which deionized water and hexadecane with the surfactant span 80 are used as the dispersed and continuous phases, respectively. Dynamic mechanisms of the neck thinning in two different regimes are revealed in three different microchannels with width ratios of 1, 1/2, and 1/3. Characteristics of the internal flow field are discussed and force estimations are carried out using the velocity information by Micro-PIV to compare the varied roles of the shear force between the two regimes. According to evolutions of the minimum neck width and the thinning rate, the necking process is further divided into different stages and the governing driving force during each stage is confirmed. Effects of the flow rates and the cross-sectional aspect ratio on the necking process as well as the neck profile at different stages are provided in detail. The distinct features of the two regimes in the squeezing stage are well captured by the theoretical estimations of the effective flow rate and the variations of the actual flow rates in different channels are reasonably reflected by the channel width ratio. In the collapsing stage, the quantitative relation between the minimum neck width and the remaining time is constructed to identify the physical mechanism and the self-similar profile is compared to cases in the open environment to reveal the influence of the confinement in microchannels.