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The geometries of micro-channel play a key role in forming of digital droplets, and can be real-time or effective controlling methodologies. Local convergence regions are designed in the rectangular cross-section channels on PMMA microchips, in which two-phase coaxial jets are introduced by inserting a syringe needle. The two-phase flow (lubricating oil (continuous phase, flow rate  Q c )/deionized water (dispersed phase, flow rate  Q d )) is considered. Two geometric control variables, the relative position (needle displacement  x ) and tapering characteristics (convergence angle  α ), are naturally adopted to discribe such geometry configurations. The micro-flow under the change of these two parameters is mainly studied in this paper. Four kinds of characteristic flow patterns, namely, sausages, slug, dripping and jetting, are found in the experiment, and their occurring parameters and developing dynamic characteristics are discussed. The experiment shows that the increase of inner needle displacement  x  can produce higher frequency and finer droplets, which is in consistent with our previous results obtained in round tube experiments and simulations. While increasing the convergence angle  α , contrarily, takes opposite effects.

Experimental Study on the Effect of Locally Convergent Configurations on the Flow Pattern in PMMA Microchips