The Confluence Point
The key feature of Flow Injection system is the confluence point, where two streams merge at a T formed junction. The advantage of mixing  by T configuration is that it promotes, if well designed, an instant radial mixing resulting in  an even concentration of reagent along the entire length of the injected sample zone. It also allows addition of several reagent in sequence, as it may be required by a given assay protocol.

When two streams of equal flow rates (a+b) merge, mixing takes place at the confluence point. The peak height, recorded by spectrophotometry of a dye injected into colorless carrier stream  at points before and after T-junction show that the concentration of the dye is immediately halved at the confluence point.

The rapid rate of mixing at confluence is promoted by local turbulence, as shown in the photograph below and  visualized by the movie clip. Both images were obtained by merging a blue stream of pH indicator (a- blue in basic form) with a colorless stream of acid (b). Since this acid base reaction is very fast, the rate of formation of the yellow color depicts the rate of mixing.